CSIRO PUBLISHING

www.publish.csiro.au/journals/ajea Australian Journal of Experimental Agriculture, 2006, 46, 975–979

Rate of feed degradation in strip-grazing dairy cows

Y. J. WilliamsA,C,E, P. T. DoyleB and A. R. EganA,D

AInstitute of Land Food Resources, The University of Melbourne, Parkville, Vic. 3010, Australia.BDepartment of Primary Industries, Primary Industries Research Victoria, Kyabram, 120 Cooma Road,

Kyabram, Vic. 3620, Australia.CCurrent address: CSIRO Livestock Industries, Centre for Environment and Life Sciences, Private Bag 5,

Wembley, WA 6913, Australia.DCurrent address: 145 Sterling Street, Bunbury, WA 6230, Australia.

ECorresponding author. Email: yvette.williams@csiro.au

Abstract. Nylon bags containing Persian clover, perennial ryegrass or perennial pasture hay were incubated for12-h periods in lactating dairy cows grazing Persian clover pasture. The hypothesis was that the rate of dry matterdisappearance over the first 12 h of incubation in the rumen would be lower when bags were inserted at a timewhen the rumen pH was at its lowest point for the day compared with a time when it was at its highest. It was alsohypothesised that the reduction in rate of DM loss over 12 h at the initially low and then fluctuating rumen pH wouldbe lower for Persian clover than for perennial ryegrass and the hay. Rumen fistulated cows grazing at 4 differentpasture allowances (9, 16, 32 and 53 kg DM/day; 4 cows per treatment) were used in a completely randomised,split-plot design. The nylon bag incubation periods were 0700 to 1900 hours (period 1) and 1900 to 0700 hours(period 2) and rumen fluid pH was measured every 3 h during these periods. The ruminal fluid pH at the time ofinsertion of the nylon bags was higher (6.3 v. 5.7; P<0.001) in incubation period 1 than in period 2, and there wasan interaction between the effects of incubation period and pasture allowance on the average rumen fluid pH in eachperiod. Rate of DM loss was higher (3.47 v. 3.28%/h; P = 0.019) in incubation period 1 than in incubation period 2,highest (P<0.001) for clover, followed by ryegrass and then hay (5.05 v. 3.15 v. 1.93%/h) and higher (P<0.001) incows grazing at the low (9 and 16 kg DM/cow) compared with high (32 and 53 kg DM/cow) allowances. There wasa significant linear relationship (P<0.05) between DM loss rate calculated over 12 h and rumen fluid pH for grassand hay, but for clover this relationship only occurred in incubation period 2. These results suggest that more than1 factor is important for determining feed degradation rates in the rumen at any particular time and that only usingrates of forage DM loss that are obtained from nylon bags inserted prior to the morning feed, may overestimate theextent of rumen degradation of the feed eaten over the whole day in cows strip-grazing highly digestible pastures.

Additional keywords: in sacco degradation, milk production, nutrition models, rumen digestion.

IntroductionMathematical models to predict the performance of dairycows are often based on steady-state kinetics and derived fortotal mixed ration systems, and although they can providerealistic predictions of nutrient supply to cows and milkproduction in pasture-based dairy management systems, theyfail to consider the varying feed intake and rumen conditionsthroughout the day observed in these systems (Kolver et al.1998). The pattern of pasture intake by grazing cows resultsin fluctuations in substrate inputs into and fermentationconditions within the rumen over a day, particularly theextent to which pH is depressed (Wales and Doyle 2003;Williams et al. 2005b). Studies with continuous fermentershave demonstrated that digestibility of perennial ryegrass

(Lolium perenne L.) is reduced when the pH of the culture isreduced from 6.3 to 5.4 for only 4 h in the day (De Veth andKolver 2001). Diurnal variation in pH, around an average of5.6, but ranging from 5.3 to 5.9, in continuous fermentersalso reduces grass digestibility compared with maintaining aconstant pH of 5.6 (Wales et al. 2004). The impact of diurnalfluctuations in rumen fluid pH on the degradation rate offorages in the rumen of cows grazing highly digestible pastureis not clear, but could be a critical factor in reconciling dailysubstrate removal rate and rumen retention times for grazedherbages.

The degradation parameters of forages obtained fromnylon bag studies are used to predict the effectivedegradability of dietary components in the rumen. However,

© CSIRO 2006 10.1071/EA06008 0816-1089/07/060975

976 Australian Journal of Experimental Agriculture Y. J. Williams et al.

nylon bag studies are often conducted under standardisedfeeding conditions to avoid large fluctuations in the rumenenvironment (AFRC 1992). It is likely that the disappearancerates of forages from nylon bags during the first 12 h ofincubation in the rumen would differ if the bags wereinserted in the rumen of grazing cows at different timesduring the day, and that this difference could significantlyinfluence the ability to predict nutrient yields to the animal.In addition, the extent to which rate of degradation of forageis affected by rumen conditions may vary between speciesdue to differences in the availability of neutral detergentfibre (NDF) to microbes for digestion (Akin 1989; Wilson1993). This is particularly important on high digestibilityforages, in particular clovers, where retention times in therumen may be short (Stockdale 1993).

An experiment was conducted to investigate thehypothesis that rates of dry matter (DM) disappearance offorages in sacco during the first 12 h of incubation in therumen would be lower when bags were inserted at a timewhen the rumen pH was at its lowest point for the daycompared with a time when it was at its highest. It was alsohypothesised that the reduction in the rate of DM loss over12 h at initially low and then fluctuating rumen pH would belower for Persian clover (Trifolium resupinatum L.) than forperennial ryegrass and hay.

Materials and methodsExperimental detailsThe experiment was conducted in winter (May–June) 2001 using16 rumen fistulated cows, in late lactation, in a completely randomised,split-plot design. There were 4 pasture allowance treatments (9, 16,32 and 53 kg DM/cow.day of Persian clover) to create a range inintakes and in patterns of rumen fermentation. Each of the 4 cowsallocated to a treatment grazed a separate plot. The details of cows,their allocation to treatments, their management and methods of pasturemeasurement and laboratory analyses have been reported elsewehere(Williams et al. 2005a).

Pasture offered to the cows on the day of nylon bag measurementshad a pregrazing mass of 2.7 t DM/ha and comprised, on a DM basis,Persian clover (77%), weeds (17%), of which the majority was capeweed[Arctotheca calendula (L.) Levyns], and dead matter (6%). It had anOM concentration of 87%, an in vitro DM digestibility of 73%, a crudeprotein (CP) concentration of 24% and NDF concentration of 26%.

Cows were offered half their daily pasture allowance after themorning milking (0530–0700 hours) and the remaining amount afterthe afternoon milking (1400–1530 hours), with the opportunity tobackgraze the morning allocation. All cows were dosed directly intothe rumen twice daily with 20 mL of Teric bloat preventative liquidimmediately after milking.

The amount of pasture eaten by each cow in its individual plot wasassessed every day using a sward sampling technique similar to thatdescribed by Stockdale and King (1983).

In sacco rate of dry matter lossChopped samples of Persian clover, perennial ryegrass and perennialpasture hay were incubated in nylon bags in the rumen of cows, 19 daysafter they commenced their pasture allowance treatments, to determinethe rates of DM loss in 2 consecutive 12 h periods (0700–1900 and1900–0700 hours). Persian clover [26.3% CP, 19.4% NDF, 16.4% aciddetergent fibre (ADF), 1.5% lignin] and perennial ryegrass (24.7% CP,

46.9% NDF, 25.9% ADF, 1.8% lignin) herbages were harvested to 5 cmusing a sickle bar mower. Harvested pasture was sorted so that an almostpure sample of Persian clover or perennial ryegrass was created. Pasturewas chopped into 1-cm lengths using scissors and frozen at −20◦Cbefore putting into nylon bags. The irrigated perennial pasture hay(11.6% CP, 58.7% NDF, 33.3% ADF, 3.0% lignin), comprising perennialryegrass, paspalum (Paspalum dilatatum Poir.), white clover (Trifoliumrepens L.), and other weed species, was also chopped into 1-cm lengthsand frozen before in sacco incubation.

Two sets of 36 bags (3 feed types × 4 incubation times × 3 replicatesof each bag) of dimensions 10 by 24 cm, made from a nitrogen-free, woven polyester cloth with a mean pore size of 50 µm (AnkomTechnologies, Macedon, NY, USA) were prepared for each cow. Theequivalent of about 5 g DM of herbage was weighed into each bag and thebag was then heat-sealed. Extra bags of each feed were also prepared toestimate immediate solubility. The set of bags for the first 12 h incubationperiod were inserted into the rumen at about 0700 hours, just before thecows went onto the pasture after the morning milking, when the ruminalfluid pH is known to be at its highest in strip-grazing cows. The secondset of bags was inserted into the rumen at about 1900 hours, when theruminal fluid pH was expected to be lowest (Williams et al. 2005b). Bagswere removed from cows 3, 6, 9 and 12 h after insertion. Portable raceswere set up in the paddock to enable rumen sampling and insertion andremoval of nylon bags without having cows out of their grazing plots forlong periods. After incubation, all bags, including those not incubated,were washed in a washing machine for 45 min followed by a slow spin(500 rpm) for 5 min. All bags were then freeze dried and weighed for DMloss determination with the bag replicates averaged for each incubationtime and feed type.

Rumen fluid samplingRumen fluid was collected from all cows via the fistula for immediatemeasurement of pH every 3 h which coincided with nylon bag insertionor removal times. The methods of collection and measurement have beendescribed by Wales et al. (2001). The average rumen fluid pH duringeach 12 h incubation period was determined as the mean of the pHmeasurements taken while the bags were incubating and also includedthe measurement taken immediately before insertion of bags.

Statistical analysisRates of disappearance of DM over each 12 h incubation period for eachforage in each cow were determined individually by linear regressionof DM loss against incubation time. The effects of incubation period,pasture allowance and their interaction on rumen pH variables wereanalysed using analysis of variance (ANOVA) according to a split-plotdesign with cow as the whole plot and period as the subplot. Theeffects of incubation period, forage sample type, pasture allowanceand their interactions on DM loss rate were analysed using ANOVAaccording to a split-split plot design with cow as the whole plot,period as the subplot and feed as the sub-subplot. Significant treatmenteffects were established at P = 0.05. Relationships between rate ofDM loss and average rumen fluid pH during the incubation periodacross all cows were tested separately for each forage sample type withlinear regression.

ResultsIntakes of cows on the day of nylon bag measurements(mean ± s.e.) were estimated to be 5.4 ± 0.29, 9.4 ± 0.27,15.8 ± 0.42 and 12.9 ± 0.81 kg DM, respectively, forthe 9, 16, 32 and 53 kg DM/cow.day pasture allowancetreatments.

The straight line regressions fitted to the in sacco DM lossdata generally had a good fit with the average MSPE and100R2 of these equations being 22.5 ± 2.77 and 94.3 ± 1.09,

Feed degradation rates in dairy cows Australian Journal of Experimental Agriculture 977

respectively, for clover, 6.2 ± 0.59 and 96.4 ± 0.43 for grassand 6.0 ± 0.55 and 91.1 ± 0.94 for hay.

There was no interaction (P>0.05) between the effectsof incubation period, feed type or pasture allowance onthe rate of DM loss. Rate of DM loss was higher(P = 0.019) in incubation period 1 than in incubationperiod 2, highest (P<0.001) in clover, followed by ryegrass,followed by hay and higher (P = 0.046) in cows grazing at9 and 16 kg DM/ha allowances compared with cows grazingat 32 and 53 kg DM/ha (Table 1).

Rumen fluid pH fluctuated over the day and patterns offluctuation varied with pasture allowance treatment (Fig. 1).The ruminal fluid pH at the time of insertion of the nylon bagswas higher (P<0.001) in incubation period 1 than in period 2and there was no effect (P = 0.635) of pasture allowanceor any interaction between the effects of insertion time andpasture allowance on the pH at each insertion time (Table 1).There was an interaction (P = 0.003) between the effectsof incubation period and pasture allowance on the averageruminal fluid pH in each incubation period (Table 1). Averagerumen fluid pH was lower in incubation period 1 than period 2for cows grazing at the 9 and 15 kg DM allowances, but wasnot different between periods for cows grazing at the higherallowances. Pasture allowance had no effect on the averagerumen fluid pH in period 1, but in period 2, cows grazing at9 kg DM allowance had a higher pH than cows grazing and30 and 50 kg DM allowances.

There was a significant (P<0.05) linear relationshipbetween DM loss rate and average rumen fluid pH withinthe 12 h period incubation period for each feed type (Fig. 2).Incubation period had a significant effect on the relationshipsfor ryegrass (P<0.05) and hay (P<0.001) and there wasan interaction (P<0.01) between the effects of rumenfluid pH and incubation period on the relationship forclover. Rate constants for the relationships in period 2 were0.72 ± 0.215 for hay, 1.24 ± 0.328 for grass and 1.16 ± 0.391for clover.

Table 1. Dry matter (DM) loss rates over 12 h of clover, ryegrass and hay incubated in the rumen at 2 different times of the day, rumenfluid pH at time of insertion of nylon bags and average rumen fluid pH during the 2 incubation periods in cows grazing Persian clover

at 4 pasture allowances in winter

Values in rows within main effects followed by the same lower case letters are not significantly different at P = 0.05. Values in columnsfollowed by the same upper case letter are not significantly different at P = 0.05

Pasture allowance (kg DM/cow.day) Incubation period Feed type9 16 32 53 s.e.d. 1 2 s.e.d. Clover Grass Hay s.e.d.

DM loss (%/h) 3.60 3.46 3.18 3.27 0.140 3.47b 3.28a 0.071 5.05c 3.15b 1.93a 0.061Insertion pH 6.0 6.0 6.0 5.8 0.19 6.3b 5.7a 0.10 — — — —Average pH — — — — — — — — — — — —Period 1 6.0aA 5.9aA 5.9aA 5.8aA 0.13A — — — — — — —Period 2 6.4bB 6.2abB 5.8aA 5.9aA 0.05B — — — — — — —

AFor comparison between pasture allowances.BFor comparison between periods within an allowance.

DiscussionIn agreement with the hypothesis, the rate of DM loss offorages over 12 h was lower in incubation period 2 wheninitial pH was low, compared with incubation period 1 wheninitial pH was higher. At the beginning of incubation period 2,rumen pH was at the lowest point for the day, which for halfof the animals was <5.6, a level indicative of subclinicalacidosis (Owens et al. 1998) and, potentially, a change in themicrobial population. Although the reduction in rate of DMloss is consistent with the effects of low and fluctuating pHin continuous culture (De Veth and Kolver 2001; Wales et al.2004), the pH did not remain <5.6 for long (<3 h for thelowest allowance and 3–6 h for higher allowances) and it isnot possible from the measurements we made to establishthe reason for the lower digestion rates in period 2. It issuggested that the higher initial pH in period 1 favoureda different microbial population or more rapid colonisationof the forages and that this occurred despite the subsequentdecline in rumen pH to low levels by the end of this period.

It is unlikely that rumen pH was the only factor affectingthe rates of DM loss between the periods. At the beginningof period 2, rumen outflow rates would have been maximal,as cows had been offered their evening strip of pasture afew hours earlier (Moseley and Jones 1984). There may alsohave been a low ratio of fibre digesting microbes to residualfibre at the start of period 2 because of the short time thatplant fibres had remained in the rumen. This combinationof low pH, high outflow rates and low microbe to plant fibreratio may have reduced opportunities to colonise the forage inthe nylon bag samples. Conversely, at the start of incubationperiod 1 outflow rates would have been low and microbe toplant fibre ratios relatively high as cows had not had anysubstantial pasture intake for several hours — up to 10 h forcows grazing the low pasture allowances. This combination ofconditions, including high pH, is more suited to rapid activityof cellulolytic organisms and possibly more opportunity tocolonise the nylon bag samples.

978 Australian Journal of Experimental Agriculture Y. J. Williams et al.

0400 0800 1200 1600 2000 0000 0400 08005.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

Incubation period 2Incubation period 1

Rum

en fl

uid

pH

Time of day (hours)

Fig. 1. Diurnal pattern of ruminal fluid pH during nylon bagincubation periods in cows grazing Persian clover at allowances of 9 (•),16 (�), 32 (�) or 53 (N ) kg DM/day in winter. Values are the mean of4 cows and error bars indicate standard error. Arrows indicate time ofmorning and afternoon pasture allocations.

Rumen fluid pH

5.6 5.8 6.0 6.2 6.4 6.6

DM

loss

rat

e (%

/h)

1

2

3

4

5

6

Fig. 2. Relationship between rate of dry matter (DM) loss and averagerumen fluid pH during the 12 h incubation period for clover (circles),ryegrass (squares) and hay (triangles) incubated in the rumen from0700 hours (solid symbols and lines) or from 1900 hours (open symbolsand dotted lines) in cows grazing Persian clover in winter.

The extent of ruminal digestion of feed stuffs dependson the product of rate of digestion and retention time. Giventhat digesta mean retention times in the rumen of cowswith high intake levels of highly digestible Persian cloverpastures can be as little as 12 h (Stockdale 1993), the extent

of ruminal digestion of herbage entering the rumen laterin the day when the pH is low may be reduced comparedwith the same forage when entering at the start of the day.Therefore, using only rates of forage degradation that areobtained from nylon bags inserted prior to the morning feedmay overestimate the extent of rumen degradation of thefeed eaten over the whole day in cows strip-grazing highlydigestible pastures.

Unlike perennial ryegrass and hay, the rate of DM lossof clover was constant across the range of average ruminalfluid pH values observed in period 1. This is consistent witha concept that NDF in clover is more available to microbesfor digestion, possibly because lignin in clover does notprotect cellulose and hemicellulose from digestion to thesame extent as it does in grasses (Akin 1989; Wilson 1993).However, the differing responses of clover degradation ratesto the ruminal fluid pH between the 2 incubation periodssuggests that there are other factors that are important indetermining feed degradation rates at any particular timein the rumen.

AcknowledgmentsThe technical support provided by Chris Hazelman,Amanda Tiller, Ric Dabrowski, Marg Jenkin, Brett Petersonand David Price is gratefully acknowledged along with theassistance of the farm staff. The assistance of Sally Francis,Bill Wales, Janna Heard and Sharni Lyttle with rumenfluid sampling and nylon bag incubations is also gratefullyacknowledged. Jim Maden and Kevin Murray providedstatistical advice. The Victorian Department of PrimaryIndustries and Dairy Australia provided financial assistancefor this research.

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Received 25 November 2005, accepted 4 April 2006

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