Protein and Energy Utilization by Ruminants at Pasture...PROTEIN AND ENERGY USE BY RUMINANTS 279 Table 1. Frequency of distribution (%] of live weight gain in the wet season in northern

D P Poppi and S R McLennanProtein and energy utilization by ruminants at pasture.

1995, 73:278-290.J ANIM SCI

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Protein and Energy Utilization by Ruminants at Pasture1

Dennis P. Poppi*,2 and Stuart R. McLennant

*Department of Agriculture, The University of Queensland, Q. 4072, Brisbane, Australia and ‘Queensland Department of Primary Industries, Animal Research Institute,

Yeerongpilly, Q . 4105, Brisbane, Australia

ABSTRACT: Low live weight gain of cattle in the wet season of tropical areas was identified as a major limitation to achieving annual growth rates from tropical pasture systems sufficient to meet new market specifications of young animals of high carcass weight. Both protein and energy are limiting nutrients for growth. Net transfer of feed protein to the intestines is often not complete, and losses occur with grasses and legumes when CP content exceeds 210 g of CPkg of digestible OM. This protein loss is important because a collation of experimental data indicated that cattle consuming low- and high-quality pasture and silage-based diets all responded to extra protein. The response was less for the higher-quality forage. The role of legumes in supplying this protein was inves- tigated and, unless legumes can increase total DM1 by at least 30%, they will not supply suffkient intestinal protein to increase live weight gain by about 300 g/d. The problem with legumes and some grasses is the loss of protein from the rumen, and increasing energy

supply to the rumen, either through improved digestibility or energy supplements, is a strategy that could be used to reduce this. Strategies to increase the proportion of escape protein would be successful, but incorporation of lowly degradable protein fractions into legumes may be more difficult because of the level of expression of these protein fractions required for a significant live weight gain response. Cattle entering the wet season usually exhibit compensatory growth and are exposed to high ambient temperatures and often to high humidity. Intestinal protein above that stipulated in feeding standards may be beneficial in these circumstances, and more emphasis should be placed on the ability of legumes to supply protein postruminally. At present the protein delivery capacity of agronomically competitive legumes seems to be inadequate for the higher growth rates required in production systems, and supplements of energy and protein will be needed to achieve these higher targets until new cultivars appear.

Key Words: Ruminants, Protein, Energy Consumption, Pastures, Legumes

Introduction

Ruminants in the tropics and sub-tropics experience marked seasonal fluctuations in feed supply and pasture quality. This results in a seasonal pattern of wet season live weight gain and dry season live weight loss until animals reach a marketable weight, which occurs at an age 2 3.5 yr depending on climatic conditions and soil type. In northern Australia it is not unusual for 4- t o 6-yr-old steers to be marketed. This system is successful for supplying manufacturing beef but the higher-priced markets require animals no older than 2 to 3 yr of age with carcass weights

‘Presented at a symposium titled “Utilization of Protein and Energy in Forages” at the ASAS 85th Annu. Mtg., Spokane, WA.

‘To whom correspondence should be addressed. Received November 29, 1993. Accepted July 29, 1994.

J. Anim. Sci. 1995. 73:278-290

exceeding 300 kg. Such requirements pose new challenges for our pasture-based production systems, particularly in their ability to supply nutrients that limit the high growth rates required. This review will identify the nutrient constraints to live weight gain and assess some opportunities to overcome these, with particular reference to the ability of legumes to supply adequate protein.

Wet Season Live Weight Gain

Much research has gone into reducing the marked loss of live weight that occurs during the dry season. This has targeted supplementation (e.g., using urea/ molasses [Winks, 19841), using alternative feeds (e.g., oats, leucaena [Winter et al., 19911), and reducing maintenance requirements (e.g., with trenbolone ace- tate [Hunter et al., 19931).

These have all been successful to varying degrees and at differing economic costs. What has emerged is

278

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PROTEIN AND ENERGY USE BY RUMINANTS 279

Table 1. Frequency of distribution (%] of live weight gain in the wet season in northern Australiaa

Growth rate, gid

Forages < 200 200-400 400-600 600-800 800-1,000 > 1,000

Native pasture ( N P Sown grass ( S G ) NP + legume SG + legume

6 19 27 36 10 2 17 28 17 11 11 16 3 3 8 48 16 22 8 15 30 30 9 8

aAdapted from Winter et al. (1991).

that live weight loss can be maintained or reduced to within 10% of the weight of animals entering the dry season. The pattern of live weight change in the tropical production system now resembles more closely that seen in a temperate pasture system. Neverthe- less, during the growth period in the tropical system, animals gain less than during the equivalent period in temperate systems. Low wet season live weight gain from warm-season grasses and legumes is now emerg- ing as the major constraint to increasing annual live weight gain and reducing age of slaughter of cattle. Winter et al. (1991) recently reviewed live weight gain experiments in northern Australia (Table 1 ) and concluded that average wet season live weight gain is about 700 g/d, whereas target live weight gain over the spring period is 1 to 1.4 kg/d for cattle on temperate pastures in New Zealand (Nicol and Nicoll, 1987). The strategies to improve wet season live weight gain are to increase the duration of the quality pasture supply or t o improve the quality of the pasture. It is this latter aspect we wish to examine here. Animals coming into the wet season in northern Australia also experience two other factors that affect nutrient requirement and live weight gain: compensatory growth and climatic stress. These will also be examined in this review.

Protein-Energy Relationships

Live weight gain depends mainly on the supply of amino acids ( A A ) and energy-yielding substrates delivered to the tissues, up to the genetic limit for protein synthesis, which is probably never reached for animals consuming pasture. The supply of amino acids depends on the protein content of the diet, its net transfer through the rumen to the intestines as undegraded plant protein and microbial protein, and its absorption from the small intestine. The deposition of protein depends on the efficiency of use of absorbed protein, which is dependent on the availability of non- protein energy-yielding substrates and limiting essential amino acids.

Various aspects of protein supply from fresh pasture (as distinct from dried pasture or hay) have been reviewed previously (Beever and Siddons, 1986; Poppi, 1990) and data from temperate pastures collated (Figure 1). Few data are available from

tropical pastures other than low-quality, protein- deficient forages typical of the dry season (e.g., Hunter, 1988). What emerges from the data on temperate pasture is that there is complete net transfer of ingested protein to the intestines as microbial, undegraded, and endogenous protein when dietary protein content is below approximately 160 g of CPkg of OM. This value arises from data on temperate pasture of high digestibility and a unit of protein to available energy (e.g., grams of CPI kilogram of digestible OM) would be more universal in its application. By using the relationships derived for cattle and sheep, it can more confidently be stated that losses of protein or incomplete net transfer will occur when CP content exceeds approximately 210 g of CPkg of digestible OM ( DOM) (derived from Beever et al., 1986a,b; TJlyatt et al., 1988; Cruickshank et al., 1992). Such a value represents a degradable CP/ available energy relationship for ruminal microbes of

., 0 10 20 30 40 50

Diet N, g/kg DM

Figure 1. The supply of non-ammonia N entering the small intestine [g of nonammonia N [NAN]/kg of DM intake) in relation to the N content of the diet (g of N/kg of DM). The line (-1 represents equality, (0) grasses, (e) legumes (from data collated by Poppi (1990) for non- tannin species: Egan, 1974; MacRae and Ulyatt, 1974; Ulyatt and Egan, 1979; Corbett et al., 1982; Verite et al., 1984; Beever et al., 1985; Beever et al., 1986a,b; Beever et al., 1987; Ulyatt et al., 1988; Cruickshank et al., 1992).




280 POPPI AND

Table 2. Dietary CP levels (g/kg of DM) above which losses in the net transfer of ingested protein

to intestinal protein (as microbial, undegraded, and endogenous protein) will occura

OM digestibility

Crude protein, &g of DM

.80

.70

.60

.50

151 132 113 94

aThis is calculated from experimental evidence that net transfer decreases below unity when CP content rises above approximately 210 g of C P k g of digestible OM (Cruickshank et al., 1992) and

DM). assuming 900 g of OMkg of DM (e.g., 210 x .8 x .9 = 151 g of CPkg

13.3, 11.9, or 9.3 g of CPimegajoule ME for potential protein degradabilities of 1.0, .9, and .7, respectively, and can be compared to yields of 9 to 11 g of microbial CP/megajoule of fermentable ME (AFRC, 1992). This value is derived from plants containing no tannin. Presumably, for plants containing condensed tannin, the CP percentage at which incomplete net transfer occurs would be much higher because of the lower degradability of the plant protein.

This value (210 g of CPkg of DOM) can be used to identify situations in which significant losses of ingested protein might occur (Table 2). Tropical grasses in the wet season range in digestibility from approximately 55 to 65% and are unlikely to exceed the critical CP value, but many tropical legumes exceed this value and losses may be expected. This can be extrapolated from results of Minson (1990; Table 31, in which the mean values indicate that both temperate and tropical legumes are likely to lose significant amounts of protein from the rumen due to microbial degradation and absorption of ammonia. It is important to note that the higher values where losses occur are generally seen in the grazing situation or where fresh pasture is consumed and there is high leaf intake of high protein solubility.

The results of a recent experiment (Higgins et al., 1992) illustrate some of these principles (Table 4). Cattle grazing glenn joint vetch (Aeschynomene americana) lost about 50% of the feed CP in net transfer to the intestines (as microbial and undegraded protein), whereas there was complete net transfer of the protein from signal grass ( Brachiaria decumbens) . This could be deduced from data in Table 2 and gives confidence in their use. Of further interest is that if the grams of duodenal CPkilogram of DOM is taken as an indica- tor of the proteinlenergy ratio of absorbed nutrients, then this is very similar for grasses and legumes and quite unlike the difference seen in CP content (Table 4). The peripheral tissues of the animal would not be able to discern whether the animal was grazing grass or legume. These proteidenergy ratios from Table 4 are lower than values derived for temperate pastures

MCLENNAN

Table 3. Mean values of CP content (g/kg of DM) and DM digestibility of temperate grasses and

legumes and tropical grasses and legumesa

Mean CP Mean DM content

digestibility g k g of DM

Temperate grass .67 117 Temperate legume .61 175b Tropical grass .54 92 Tropical legume .57 165b

aAdapted from Minson (1990). bForage types from which incomplete net transfer of protein

across the rumen would be expected from Table 2.

(210 to 260 g of CPkg of DOM) and lower than that used to promote lean growth (200 to 340 g of CPkg of DOM; Poppi, 1990). The ability to alter body composition of animals at pasture will depend on obtaining high proteidenergy ratios of absorbed nutrients (Poppi, 1990).

The significance of these results for grassflegume pastures can be seen in Table 5 . A proportion of the diet as legume will markedly increase the CP content of the diet, but it may not increase intestinal protein supply per unit of DM1 or the proteidenergy ratio of absorbed products. This is especially so where the legume has a low digestibility and provides insufficient energy for the microbes to utilize the degraded protein (e.g., glenn joint vetch). Tannin-containing plants such as leucaena (Leucaena Eeucocephala) are a marked exception to this because of the lower degradability of the plant protein. Although leucaena has a lower CP content, including it in the diet would have an effect similar to incorporating a high-quality temperate legume such as white clover (Trifolium repens) in the diet (Table 5 ) . These values are by necessity theoretical; such comparisons on mixed diets apparently have not been examined experimentally. In Figure 2, the calculated intestinal protein supply/ kilogram of DM1 is plotted against the CP content of the plant for two different digestibilities using the

Table 4. Protein transactions across the rumen of cattle grazing glenn joint vetch (A. americana)

or signal grass (B. decumbens)"

Glenn Signal joint

Item grass vetch

OM digestibility .58 .64 Extrusa CP, g k g OM 104 223 Nitrogen intake, g k g BW-l.d-' .47 .79 Extrusa CP, g k g of digestible OM 180 349 Duodenal NANiN intake 1.04 .57 Duodenal CP, g k g of digestible OM 150 159

"Adapted from Higgins et al. (1992).




PROTEIN AND ENERGY USE BY RUMINANTS 28 1

Table 5. The effect of a mixed grass (7O%)/legume (30%) diet on estimated intestinal protein supply”

SG (70%) SG (70%) + SG (70%) + Signal + vetch leucaena white clover

Item grass (SG) (30%) (30%) (30%)

Diet CP content (g/kg of OM) 104 140 138 163 OM digestibility .58 .60 .63 .65 Intestinal protein supply, &g of OM intake 90 94 115 114

aValues are estimated by simple proportion and using values derived for intestinal protein supply,’kg DM1 for signal grass and vetch (Higgins et al. 19921, leucaena (Bamaulin et al., 1984) and white clover (Beever et al., 1985; Cruickshank et al., 1992) and digestibility values for vetch of .64, leucaena .76, and white clover .82.

relationship from Table 2. Although this is only an approximation, it highlights certain principles. The relationship clearly indicates the importance of CP content, but, more importantly, it highlights the importance of energy supply to the rumen if the protein is going to reach the intestines. The dis- crepancy between temperate and tropical forages is a function of both ME and GP content.

The message from this section is that legumes are introduced into pastures because they are thought to increase protein supply to the animal. Yet data clearly show that although legumes increase protein intake, they generally do not increase intestinal protein supply per unit of DM1 to the same extent, if at all. Legumes are known to promote higher intakes, and this is the most likely cause of improved animal performance. Legumes have the potential t o increase intestinal protein supply, but increases are limited by protein loss in the rumen. This is clearly a fruitful area for manipulation of the plant or for strategic use of energy supplements to improve ruminal N utilization.

Live Weight Gain Response to Added Protein

The preceding studies have illustrated the extent of protein loss from the rumen, but is this an important constraint to live weight gain from pasture in the wet season? Pasture in the wet season is not thought to be deficient in CP because values are generally in excess of 7 to 8% CP, below which intake is usually decreased. However, responses to supplemental protein have occurred with such diets. This section will outline examples of responses to protein and essential amino acids and evaluate some of the strategies that might be employed to increase protein supply.

With animals grazing temperate pastures, significant losses of ingested protein may occur from the rumen. For instance, in spring those animals grazing white clover can lose approximately 30% and those on alfalfa can lose approximately 40% of ingested protein (Cruickshank et al., 1992). When lambs grazing white clover (approximately 30%’ CP) or fescue (approximately 20% GP) were supplemented with escape protein equal to that estimated to be lost from the rumen, live weight gain was significantly in-

creased (Table 6). The efficiency of utilization of absorbed protein from pastures and their silage has been calculated to be less than .5 (Beever et al., 1988; Poppi, 1990), which is much lower than the .6 to .8 used in nutrient requirement tables (ARC, 1984; AFRC, 1992) but similar to NRC (1985) values, suggesting that some essential amino acids were limiting growth. A series of experiments using protected amino acids or infusions of amino acids demonstrated that an infusion of six amino acids ( 6 M , Met, Lys, His, Arg, Thr, Cys) was needed to give a live weight gain response equal to that of whey protein infusion (Table 6 ) . When other combinations were tried (Met + Lys and Met + Lys + His + Arg) no significant increase in live weight gain or N balance was obtained. These experiments demonstrated that 6AA were the limiting nutrients for growth from these high-protein, high-digestibility pastures. The eff- ciency of utilization of extra intestinal protein can be low. Webster ( 1992) has calculated the efficiency of

4 200 -

+l

.H U

E 150 - n M

g 100 - U rl

8 50 - m 0 c, .H U

, Temperate grass & legume 80% DMD P

I I legume

Tropical 50% DMD

Temperate legume Tropical legume

Tropical grass Temperate. grass

M O L ’ I I I I

0 50 100 150 200 250 300 350 g CPkg DM

Figure 2. The predicted relationship between intestinal CP supply (g of CP/kg of DM intake) and CP content (g of CP/kg of DM) for condensed tannin-free herbages consumed in the fresh state and assuming that there is incomplete net transfer of ingested protein when CP content exceeds 210 g of CP/kg of DOM. The range in values for tropical grasses and legumes and temperate grasses and legumes most likely to occur for growing plants is given.




282 POPPI AND McLENNAN

Table 6. The live weight gain of lambs grazing fescue [approximately 20% CP) or white clover [approximately 30% CP) and supplemented with fish meal (Exp. 1)

or protected methionine and lysine supplements (Exp. 1) or intra-abomasal infusions of whey protein or a mixture of six amino acids (Exp. 2)

Treatment

M+L+H+ Experiment Diet Control Proteina M + Lb A+C+TC

Weight gain, gid 1 Fescue 20 1 266

2 White clover 326 374 - 379 White clover

- 331 388 350

- -

aFish meal 3.25 g k g BW.d (Exp. 1, Poppi et al., 19881, whey 2.31 g of proteinkg BW-l.d-l (Exp. 2,

bProtected methionine and lysine (52 mg Met + 82 mg Lys.kg BW-l.d--'). CAbomasal infusion mixture of 50.6 mg Met, 227.7 mg Lys, 48.3 mg His, 66.7 mg k g , 50.6 mg Cys, and

Fraser et al., 1991).

126.5 mg Thr.kg BW-'.d-'.

utilization of extra absorbed protein to be .2 for lactating cows of high intake and milk production. In the above experiments this marginal response to extra intestinal protein (including that deposited in wool) was about .15, and the marginal response to the 6AA was about .66. Thus, although tissue energy supply, rather than protein supply, may be the overriding influence (Webster, 19921, the nature of the response curve, in particular in the area of diminishing returns, can be markedly influenced by the AA composition of the extra protein. This may be of practical significance in many situations.

The situation with tropical pastures is less clear. Numerous experiments have examined supplements of various types to animals grazing dry season pasture or low-quality roughage. By contrast, there are few data to assess whether protein is a limiting nutrient for cattle grazing wet-season pasture. Most experiments have used only one or two levels of protein supplementation, so no assessment of response curves has been obtained. A collation of experiments from a variety of basal forage types is seen in Figure 3, where three categories have been defined: low-quality forage ( c .55 digestibility), medium- to high-quality forage (> .6), and silage. Clearly, protein supplements to silage and low-quality forage result in a marked increase in live weight gain. There is less effect with high-quality forage, but, when the protein supplement has a high CP content, a live weight gain response of 200 to 300 gid can be obtained.

Recently, Mbongo et al. ( 1994) supplemented steers (272 kg) grazing setaria (Setaria anceps) pasture (3.6 t of green leaf DM/ha; 12% CP in extrusa) with increasing levels of formaldehyde- treated casein (0, 150, 300, and 500 g of CP.steer-l.d-l). Live weight gain was 780, 1,020, 900, and 790 gid, respectively. Live weight gain increased by 240 gid to a supplement of 150 g of casein protein but decreased back to the control at high levels of protein supplement, possibly as a consequence of

catabolizing excess protein. McLennan et al. (unpublished data) in two separate experiments examined the response of steers t o increasing amounts of cottonseed meal ( CSM). In the first experiment with Rhodes grass hay ( Chloris gayana), live weight gain increased linearly from . l to .95 kg/d (Table 7) in response to increasing CSM supplementation. In the second experiment, animals grazed Rhodes grass

c 0 0 0

0 0

0 OO

00

0

*0

0

0

- 100 t , I

0 1 2 3 4 Supplemental protein intake, g.kg BW-' .d-'

Figure 3. The live weight gain response of supplemented animals above control animals (gld above control) when supplemented with increasing amounts of protein (g of CP. kg BW-l. d-l). Basal diets were low- quality roughage ( 0 ) , silage ( * ) , and high-quality roughage (0) . The equation for the low-quality roughage was y = 183 + 189x; similar equations could not be obtained for silage and high-quality roughages. The mean response for silage was 292 g/d and for high- quality roughage 160 gld. Data collated from Hennessy et al., 1983; England and Gill, 1985; Smith and Warren, 1986a,b; Gill et al., 1987; Irlbeck et al., 1989; Perdok and Leng, 1990; Karges et al., 1992; Moss and Murray, 1992; Sanderson et al., 1992; Mbongo et al., 1994; McLennan et al., unpublished data.





Table 7. Live weight gain of steers supplemented with cottonseed meal (CSM)

Experiment 1 (Initial live weight U 137 kg, S 155 kg)a Intake of CSM, g/d 0 250 500 750 1,000 1,500 Live weight gain, kgid U . l 0 .46 .56 .70 .73 .94 S .l3 .49 .58 .64 .80 .95

Experiment 2 (Initial live weight 233 kg)b Period 1: Intake of CSM, g/d 21 290 570 1,160 1,700

Period 2: Intake of CSM, g/d 0 290 610 1,170 1,860 Live weight gain, kg/d .85 .64 .85 .95 1.03

Live weight gain, kg/d .02 . l9 .40 .59 .74

aExperiment 1: Brahman-crossbred steers initially supplemented with .5 kgld CSM over the dry season (S), or unsupplemented ( U ) so as

bExperiment 2: Santa Gertrudis steers, Period 1, 5 wk with green leaf on offer, Period 2, 6 wk when little green leaf available. Both periods to reach two different live weights and then supplemented with increasing CSM up to 1.5 kg/d.

with increasing CSM supplementation (McLennan et al., unpublished data).

pasture in the wet season. In the first period (summer) there was a high quantity of green leaf available and only a small response to CSM. In the second period (summer/autumn), there was little green leaf available (but pasture DM was not limiting) and live weight gain of the animals responded to CSM in a manner similar to that in the first experiment in which hay was fed (Table 7). These two experiments demonstrate that animals grazing wet-season pastures will respond to increased intestinal protein supply. However, if a protein supplement with a significant amount of non-protein available energy, such as CSM, is used, a response may not always occur, possibly because of a substitution effect when the intake and quality of the basal diet is high. Recently, Sanderson et al. (1992) showed that the use of increasing levels of fish meal with silage results in increasing levels of live weight gain irrespective of the intake of silage. Fish meal would be expected to have less substitution effect than CSM because most of the energy is available as protein that disappears in the intestines. The role of limiting amino acids for growth in these pasture-based systems has not been evaluated.

Targeting a Live Weight Gain Response

For a meaningful increase in annual animal growth rate (about 50 kgianimal) from tropical pastures, live weight gain in the wet season must increase by about 300 g/d. This might be achieved by supplying the limiting nutrient to an existing pasture base or by increasing the net energy value of the pasture base. The latter proposal is seen at its extreme in the difference in growth rate achieved from temperate and tropical pastures. This is no doubt the most successful strategy for increasing live weight gain, but improving the net energy value of tropical pastures has not always proved successful despite many years of plant genetic selection. An alternative that is frequently used is to graze pasture at an earlier stage of growth. In any case, native pasture (indigenous rangeland

pasture) forms the major pasture base and large scale replacement of this is unlikely because of ecological and economic considerations. Supplying the limiting nutrient becomes the next option.

An increase of 300 g/d for weanling cattle of approximately 200 kg live weight will require delivery of about 150 g of extra protein t o the intestines (Mbongo et al., 1994; Figure 3) . This might come from a legume, a protein supplement, or an energy supplement to the rumen. Each will be considered. In some cases N fertilizer may also be effective, but this will not be considered here.

A legume is most effective when it increases both energy and protein supply. This is best seen in Table 5, which shows a theoretical calculation for a tropical grass and white clover. However, most tropical legumes are similar in digestibility to tropical grasses and will not therefore markedly increase ME intake through increased dietary nutrient density (Table 5). The effect must therefore result from an increase in intake. Those legumes most likely to increase intestinal protein supply per unit of DM1 are the tannin- containing species, such as leucaena (Table 5). Some caution must be used with respect to condensed tannins, because high levels can interfere with intestinal absorption, and probably with intake. The target must be a legume of higher digestibility and CP content than the base forage. Of all the legumes used in tropical areas, leucaena has been the most successful in increasing live weight gain. Legumes of the StyZosanthes spp. have also been very successful in improving animal production in northern Australia (Cameron et al., 19931, but the effect is probably achieved more through an increase in intake than by a more effective protein delivery system. Diet selection studies indicate that the protein content and digestibility of StyZosanthes spp. is such that large increases in intestinal protein supply would not be expected (Table 8). Genetic engineering or plant selection programs to increase the protein content of an existing legume are unlikely to be successful because of soil




284 POPPI AND

Table 8. Crude protein content and digestibility of Stylosanthes spp. selected by steers (extrusa)

in the wet and dry season (R.E. Hendricksen, unpublished data)

g of CPkg OM g of CPkg of Season of OM digestibility digestible OM

Wet 140-150 .64-.66 217-225a Dry 41-92b .49-51 82-180

asmall losses in net transfer of protein will most likely occur. bLittle green leaf available.

fertility constraints and the homeostatic nature of the plant. As identified earlier, the problem is the ruminal loss of protein that occurs with most temperate, and some tropical, legumes, and this can only be addressed by modifying the degradability of the plant protein (e.g., Broderick and Buxton, 1991). Condensed tannins have been most effective in reducing ruminal degradability, but, because of their potential deleteri- ous effects in terms of overprotection, consideration of other avenues to reduce degradability should be made. In view of the previously discussed live weight gain response to a small group of essential amino acids, increasing their supply has potential (Table 6 ) . McNabb et al. ( 1993) have outlined the possibility of including SA8 genes into legume leaves to express sunflower albumin 8, a low degradability protein fraction, that has the complement of 6AA potentially limiting for weight gain (Kortt et al., 1991). Theoreti- cally, if the legume did not increase intake then SA8 protein would need to be about 49% of the protein in the plant to supply the necessary 6AA for a 200-kg animal (Table 91, an unlikely scenario. An increase in intake of 20% due t o the legume would require an SA8 expression of about 23%, which might

McLENNAN

also be too high to achieve through genetic engineering. The most feasible strategy is to obtain a higher intake due to the legume. A 30% increase in intake would supply most of the required protein. Alterna- tively, reducing the degradability of existing protein, perhaps by tannins, so that an extra 25% of the protein escaped, would be equally effective. These calculations address the issue of the best strategy for a plant modification program. In summary, increasing intake seems most effective, followed by a reduction in degradability of existing protein, and finally by incorporation of low degradability protein fractions via genetic engineering.

One other method of increasing protein supply is to manipulate ruminal microbes to supply additional protein or specific amino acids. Calculations similar to those outlined in Table 9 can be done. The amount of 6AA required to be equivalent to an extra 150 g of protein at the intestines can be calculated as approximately 36 g of 6AA. The grass-only diet (Table 9) will supply approximately 360 g of CP/d to the intestines (approximately 99 g of 6AA) if it is assumed that all protein arriving at the intestines is microbial. This means expression of the 6AA in this protein fraction will need to increase by approximately 42% to supply the extra 36 g of 6AA needed. Another way to examine this is to look at the data of Fraser et al. (199 1) and Cruickshank et al. ( 1992) for the 6AA needed and microbial protein supply to lambs grazing white clover. Fraser et al. (199 1) needed to supply .57 g of 6AA.kg BW-I.d-l to increase live weight gain of lambs grazing white clover by approximately 20%. Cruick- shank et al. ( 1992) measured microbial protein supply from white clover as 5.2 g CP-kg BW-l.d-I, which approximates to 1.3 g of 6AA.kg BW-l.d-l. Thus, expression of these 6AA will need to increase in these microbes by approximately 43%. These values

Table 9. The calculated effect of increasing legume intake on intestinal protein supply and the subsequent requirement for escape protein to supply an extra 150 g/d intestinal protein, or for level of expression of

low degradability protein fractions in these plants to supply an equivalent amount of 6AAa

Item

Grass + 30% Grass + 30% Grass t 30%' legume, legume, legume,

no change in 20% increase 30% increase Grass intake in intake in intake

Intake kgld 4 4 4.8 5.2 Digestibility .58 .6 .6 .6 Intestinal CP, gkg of OM intake 90 96 96 96 Total intestinal protein, gld 360 384 460 500 Deficit from required extra 150 g of CPid, g of CPid - b 127 50 10 Escape legume protein t o supply the deficit, g k g of DM of legume - b 106 35 6 Proportion of legume protein needed to escape - .76 .25 .04 SA8 protein to supply the deficit (in terms of 6AA), gid - b 68 23 3.9 SA8 expression, % of CP in plant - b 49 16 2.8

b

aThe legume was assumed to have a digestibility of .65 and a CP content of 140 gkg DM (similar to Stylosanthes spp. from Table 8). The protein fraction was calculated assuming a requirement of 6AA (Table 6 ) and a level of proportional composition of ,374 of sunflower albumin 8 (SA8) in this form.

bNot applicable.





seem too high to achieve and take no account of the extra energy or NH3 requirements that would be needed, but they provide a target by which genetic manipulation of microbes might be assessed.

Supplying energy to the rumen can be an effective way to deliver extra protein to the animal, either by enabling ammonia normally lost to be captured or by the use of non-protein N and energy supplements to produce more microbial protein. This approach has the advantage that it supplies energy as well as protein, but substitution often occurs (Doyle, 1987; Hunter, 1988). In the absence of substitution, about 1.2 kg of grain would be needed to provide an additional 150 g of microbial protein (AFRC, 1992). On an energy basis alone this extra ME intake would be expected to increase live weight gain by about 250 gld, so it is difficult to determine whether the response is due to additional protein or to additional ME (Thomas et al., 1988).

The type of energy supplement seems to be important. Energy supply to the rumen would be most effective when there is rapid NH3 production and loss of protein. This most certainly occurs with temperate pastures, especially in spring, with some of the tropical legumes, and most probably with tropical grasses immediately following rain (Table 3). To be effective there must be synchrony between energy and NH3 release (Broderick et al., 1991; Galyean and Owens, 1991; Obara et al., 1991).

The types of energy supplement for forages fall into three categories: starch (e.g., sorghum, barley), sugars (e.g., molasses), and fiber (e.g., sugar beet pulp and pineapple pulp). All these have potential substitution effects. Present nutrient requirement tables do not distinguish between these sources and assume a constant microbial production per unit of OM fermented in the rumen, yet evidence suggests that the energy sources differ in this attribute (Obara et al., 1991). Classic experiments on microbial synthesis/ kilogram of fermentable OM focusing on the dietary situation of excess NH3 do not seem to have been done. Beever and Siddons 1986) have reported that sugar beet pulp was very effective in capturing the ammonia, and Toharmat et al. ( 1992) reported beneficial effects of a pineapple pulp supplement. These fiber sources are high in digestibility and low in protein. The slower degradation of the fiber probably enables better synchrony between energy and NH3 release. However, the high fiber can lead to substitution effects, and use of these feeds as supplements is obviously better with basal diets of lower fiber content than with tropical forages. Molasses, however, is rapidly fermented and it would be difficult for good synchrony to occur, but rate of intake is lower than with grain (McLennan, 19921, so asynchrony may not be as extreme as suggested. Also, molasses would not be expected to contribute significantly to rumen distension and so may fit better with high-fiber diets.

Grain or starch supplementation is where most work has been concentrated (Galyean and Owens, 19911, and substitution effects are well documented. Of interest, though, is the distinction between rapidly fermented grains (e.g., wheat and barley) and those slowly fermented (e.g., sorghum and corn) from which significant amounts of starch escape fermentation in the rumen. It might be deduced that grains would differ markedly in quantity of OM fermented in the rumen, as observed in starch digestion (Axe et al., 19871, and thus in the capture of NH3 and microbial synthesis in the rumen and consequently protein supply to the intestines. Comparative experiments on the effect of energy supply on NH3 capture do not seem to have been done but are essential if meaningful supplementation strategies are to be devised to enhance the transfer of plant protein to animal protein. A recent comparative experiment with supplements of sugars and starch indicated that microbial protein production clearly varied with the carbohy- drate source (Chamberlain et al., 1993). Aspects of protein supplementation and requirement have been reviewed recently (Wilkerson et al., 1993).

Compensatory Growth

The previous discussion has focused on the ineffi- ciencies of transfer of plant protein to animal protein and how this might be improved. It concluded that in many situations animals would respond to additional protein and that wet season tropical pastures would most probably be one such situation. Animals coming into the wet season often display compensatory growth, and this can negate the responses to previous dry-season supplementation (Winks, 1984). The ques- tion is whether these compensating animals need more protein than continually growing animals and whether the compensatory effect can be enhanced. This may place additional emphasis on the protein delivery capacity of legumes.

Many reviews on compensatory growth have been written (Allden, 1970; O’Donovan, 1984; Ryan, 1990). A most interesting aspect was advanced by Ryan (19901, who noted that compensation occurs when undernutrition has markedly reduced the size of the metabolically active tissue pool of the gut, and of the liver in particular. Most studies have restricted growth by reducing level of intake, but in the tropics dry-season growth is largely restricted because of a protein deficiency and its effect on intake. This might be expected to have a major depressive effect on growth of the metabolically active tissues (Drouillard et al., 1991). It is rare for any supplementation strategy in the dry season to result in significant growth, so it might be argued that all animals coming into the wet season show some level of compensatory growth irrespective of dry season supplementation strategy. Some animals may show more compensatory growth than others depending on the previous degree of undernutrition.




286 POPPI AND

Orskov et al. (1976) and Hovel1 et al. (1987) demonstrated that compensating animals responded more to increased protein supply than non-compensating animals. Both these experiments were conducted with sheep given either concentrate-based diets or intragastric infusion. No experiments specifically directed to the effects of one nutrient, as opposed to a complete change to a higher-quality diet, seem to have been completed, but Drouillard et al. ( 199 1) made reference to unpublished data showing a response to protein supplementation in compensating animals. Recently, McLennan et al. (unpublished data) examined this aspect with two groups of weanling calves. The supplemental group gained 10 kg over a 60-d period on a protein-deficient roughage supplemented with .5 kg of CSM, and the unsupplemented group lost 8 kg. Animals were then given Rhodes grass hay in pens and supplemented with increasing amounts of CSM up to 1.5 kgcalf-l.d-l (Table 7). The response to extra protein was substan- tial, but there was no difference in the response curve between groups. This implies that both groups were using the extra protein with equal efficiency. But, because no plateau was reached, the experiment did not test the concept that the two groups might reach a plateau in live weight gain at different levels of protein inclusion. Both groups might also have been compensating at a similar rate. A similar conclusion can be drawn from findings of Gibb and Baker ( 199 l), who noted that supplementation of control and compensating animals with fish meal/maize/molassine meal and monensin resulted in a similar increase in live weight gain for both groups. Animals exposed to stress or various preconditioning treatments also seem to respond better to extra protein (Cole and Hutche- son, 1990).

Both the gut and the liver are known to actively metabolize essential amino acids (Fitch et al., 1989; McBride and Kelly, 19901, and it is interesting to speculate whether this might also be a mechanism for the differential response of both types of animals (Compensating and normal) in some studies. The protein requirement of compensating animals is an important issue to resolve because it sets a target for protein supply during the wet season that must be met from introduced tropical legumes. Overgrazing has resulted, at times, from some supplementation strategies in the dry season. It might be a better strategy to increase the intake and utilization of the higher-quality material grown in the wet season, when soil moisture allows pasture regrowth to occur, by trying to exploit the compensatory growth effect in the wet season through increased protein supply.

Heat

As animals move into the wet season, they experience higher temperatures and humidity than during the dry season. For example, Table 10 outlines

McLENNAN

Table 10. Seasonal values for mean maxima and minima temperatures and relative humidity

at Kununurra, Western Australia, a summer rainfall area

Summer Winter Environment (wet) Autumn (dry) Spring

Mean maxima, "C 36.3 34.4 31.3 37.7 Mean minima, "C 24.5 20.7 15.0 22.1 Relative humidity, % 59.8 47.0 33.4 40.3

seasonal values for temperature and humidity for Kununurra in northwestern Australia. Humidity in Australia varies with distance from the coast. Ambient temperature is uniformly high across northern Austra- lia in the wet season. Heat dissipation poses a major limitation to production in these environments and the widescale adoption of Bos indicus breeds is associated with their ability to cope with heat stress (Vercoe and Frisch, 1982). What has not been resolved is whether high growth rates of 1.2 to 1.4 kg/ d from pasture-based systems can be sustained given the level of heat production by the animal that is positively linked to growth rate (Blaxter, 1962; MacRae and Lobley, 1986; Lobley, 1990). Animals may not suffer obvious heat stress symptoms but merely restrid intake and thus growth rate to a level at which heat can be comfortably dissipated. Evapora- tive heat loss becomes the most important pathway for heat loss (Blaxter, 19621, and in those regions where humidity is high, heat dissipation should be examined as a possible constraint to high levels of production. This might explain or contribute to the lower than expected live weight gains from high-quality tropical pastures in areas of the wet tropics (e.g., approximately .5 kganimal-l.d-l on signal grass; Teitzel et al., 1991) or the poor responses to supplementation. In the case of leucaenaipangola (Digitaria decumbens) pastures in the Kununurra region of Western Australia, S. Petty (personal communication) has recorded higher humidity (ca. 10% units higher) and less wind speed in the inter-row spaces between leucaena tree rows than outside this alley system. This may partially account for the differences in growth rates of steers of 750 g/d on leucaena but 1.2 kg/d in a nearby feedlot experiment (Petty et al., unpublished data). This effect can be seen in data of Cropper and Poppi ( 199 1) where lambs fed a concentrated diet and exposed to constant temperatures of 27 to 29.8"C and high humidity grew at 169 g/d whereas those exposed to a daily variation of 12 to 20.3"C grew at 430 g/d. It has also been well documented in the various reports from the Missouri Agricultural Experi- ment Station between the 1940s and 1960s.

Heat production accounts for a large part of the ME intake, varying from 100% at maintenance to 70 to 90% for animals at twice maintenance, a level needed





to produce a live weight gain of approximately 1 kgld by cattle. MacRae and Lobley ( 1986) have demonstrated the relationship between heat production and live weight gain with a linear relationship between heat production and protein synthesis. Trying to improve growth rate by increasing protein supply might not show the expected response because upper levels of heat dissipation are being approached. However, data of MacRae and Lobley (1986) were derived by increasing food intake rather than protein supply independent of food intake, and, as a result, the supply of non-protein energy-yielding substrates was also increased linearly. Where protein has been increased independently of intake, it can be calculated that heat productionlunit of food intake was decreased (MacRae et al., 1985; Leng, 1990; Fraser et al., 1991). The theoretical estimates of heat production/peptide bond synthesis would suggest that the extra heat produced by protein synthesis would be small, but this depends on the extent of protein synthesis and degradation associated with net deposition under the dietary conditions and physiological state of the animal. This can be quite significant when protein turnover is high (Lobley, 1990). At present, therefore, the literature suggests either no increase or a large increase in heat production if growth rate is increased by protein supplementation. High growth rates in excess of 1.0 kgid have been achieved in the wet season (McLennan et al., 1988; Table l), so presumably body heat can be dissipated in the absence of high humidity. The point is that nutritional strategies to increase growth rate may not work in certain environments because the upper levels of heat dissipation may limit live weight gain irrespective of nutrition or the use of tropically adapted breeds. Altering the proteinienergy ratio of absorbed products through use of appropriate legumes or supplementation strategies might be one way of bypassing this limit either through improving the efficiency of use of ME for energy retention (kf) and so less heat production, or by stimulating protein synthesis without a major change in heat production. This principle was highlighted by Leng (1990). Ames et al. (1980) and White et al. (1992) have demonstrated that extra protein supply to thermally stressed animals increases growth rate, providing further support for this idea. Another aspect is stage of maturity of the animal. Although younger animals have a greater surface area per unit weight, they also have a higher proteinifat ratio in deposition products, leading to a much lower kf (more heat production) than for animals approaching maturity (Geay, 1984). Fat deposition, and hence kf, is sensitive to the supply of NADPH from glucogenic precursors (e.g., protein). Thus, fattening animals might be more sensitive to proteinlenergy manipulations than weanlings in the environmental situation of high temperature and humidity because kf is more easily increased and heat production decreased by dietary manipulation.

Nutrient Balance

The above discussion has, as its underlying principle, the importance of nutrient balance to metabolism and the production of heat, protein, and fat. The effects of nutrient imbalances (deficiency and excess) are well documented for protein and various minerals, but the implication of nutrient balance within the extremes of deficiency and excess is a matter for conjecture. This arises largely because the experimental approach usually does not measure nutrient absorption. It is well known that a manipulation of a nutrient in the diet may not change the quantity of that nutrient that is absorbed. Poppi et al. (1990) highlighted the scarcity of such data when modeling metabolic regulation of intake. Our understanding of the importance of nutrient balance comes from metabolism models (Gill et al., 1984; France et al., 1987; Danfaer, 199 1). The balance of proteinlenergy is always of importance in these models. We are interested in nutrient balance because the new genetic engineering techniques give opportunities to markedly alter nutrient balance arising from digestion of plants. Yet, with the exception of some aspects of protein/ energy ratios, we do not have good experimental evidence of animal response. For example, the level of expression of SA8 needed to increase only S-amino acids for wool growth is much less than that needed to increase six essential amino acids required for live weight gain.

Leng ( 1990) has provided an excellent, thought- provoking review in this area. One conclusion, however, that of the inappropriateness of the ME system, needs to be examined more carefully. His discussion relating to figure 8 of his paper suggests that responses to supplements in the tropics are much greater than predicted from the ME system. This is indeed the case, but is largely because the ME system cannot accurately predict intake, and not because the partial efficiency values (i.e., kf, etc.) change outside accepted limits. The danger is that the gross efficiency values used may be misinterpreted by others less familiar with energy metabolism and thermodynamic principles to imply that large responses can be obtained for little input. This is best illustrated in Figure 4 using data from experiments in Table 7. In Figure 4 (top panel) the response is as outlined in Leng ( 19901, but in the bottom panel of Figure 4 the response is plotted in a conventional manner. This clearly shows the protein response occurred within the normal limits of kf and ME intake, as outlined by Geay (1984) and thermodynamic principles of metabolism (Blaxter, 19621, but the gross efficiency, as outlined by Leng (1990), changed markedly. The slope kf in Figure 4 (bottom panel) is not strictly correct as diet supplements changed ME intake but it does show that energy retention is within accepted limits for kf (or kg). The ME system need not be discarded. It and the metabolizable protein system




288 POPPI AND McLENNAN

extensive protein degradation in the rumen and insufficient energy for the microbes to capture the released NH3. Effecting a more complete transfer of ingested protein to the intestines than is currently occurring, especially from legumes, would improve live weight gain. Energy supplements t o the rumen and supplements of high escape protein would be equally beneficial. Using protein fractions high in limiting essential amino acids but of low degradability is another strategy, but the level required is such that it may not be biologically feasible. Animals coming into the wet season often exhibit compensatory growth and are exposed to high temperatures and humidity. Increasing the protein supply seems to be beneficial under these circumstances. These conditions also place more pressure on legumes to supply protein, and it seems that with the possible exception of the condensed tannin-containing legumes, the tropical legumes cannot meet the animals’ requirements. Other strategies of energy and protein supplementation will need to be used until such time that the protein delivery capacity of legumes can be improved.

l 2 r

I

12 16 20 24 28 32 Energy value of diet, kcal M E k g DM

70 r

lo t 0 I

0 50 100 150 200 250 ME intake, kcal/(kg BW.75.d)

Figure 4. (Top panel) The relationship between efficiency of live weight gain (g/kcal of ME intake] and energy density (kcal of MElkg of DM] of the diet. The dashed line represents the value predicted from the ME of the diet (Leng, 1990). [Bottom panel) The same data expressed as energy retained (kcal . kg BW-.75. d-l) against ME intake (kcal. kg BW-.75. d-l) with kf = .50 (from Table 7 [a] previously supplemented, [o] unsupplemented). Energy retained is estimated from the live weight change of the animal (ARC, 1984) and ME intake is estimated from the composition of the diet.

provide a valuable means to examine strategies to increase growth in the wet season. We do emphasize, however, that the animal response must be the final arbiter of new ideas. We, like Leng, believe animals of the appropriate physiological state exposed to the appropriate environmental conditions and nutritional manipulations must be used.

Implications

Animals often respond to extra protein during the wet season, a period when pasture quality in terms of digestibility and protein content is high. Incorporation of legumes is one strategy to increase protein supply, but it is not as effective as it could be because of

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Protein and Energy Utilization by Ruminants at Pasture...PROTEIN AND ENERGY USE BY RUMINANTS 279 Table 1. Frequency of distribution (%] of live weight gain in the wet season in northern