Applied Animal Behaviour Science, 16 (1986) 233--247 233 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE EFFECT OF FORAGE SUPPLEMENTATION ON THE BEHAVIOUR OF GRAZING DAIRY COWS

C.J.C. PHILLIPS ~ and J.D. LEAVER

The West of Scotland A&ricultural Colle#e, Crichton Royal Farm, Dumfries, DG1 4SZ (Gt. Britain)

(Accepted for publication 8 January 1986)

ABSTRACT

Phillips, C.J.C. and Leaver, J.D., 1986. The effect of forage supplementation on the behaviour of grazing dairy cows. Appl. Anim. Behav. Sci., 16: 233--247.

In a series of three experiments, the effect of offering conserved forage indoors to grazing dairy cows was examined. In Experiment 1, hay was offered ad libitum for 45 min after morning milking. In Experiments 2 (early season) and 3 (late season), silage was offered ad libitum either after morning milking or overnight. All experiments in- cluded a control grazing treatment that received no supplementary forage. Grazing time was reduced by offering forage, particularly at the high levels o f forage intake when cows were housed overnight. The bite size and rate of herbage DM intake were reduced for cows eating large quantities of silage, but the rate of biting was not affected. As the grazing season advanced there was a small increase in the rate of biting and grazing time in all treatments, but a large reduction in bite size and the rate of herbage intake. Heifers had a 25% smaller bite size, 5% faster rate of biting and similar grazing times to cows. Spring-calving cows had higher grazing times than autumn-calving cows, with little difference in biting rate. Forage was consumed at a rate between 2 and 4 times that of grazed herbage. Offering forage increased rumination times, particularly when it was eaten in large quantities. Rumination time was reduced for cows on spring grass and when total intakes were low.

INTRODUCTION

Recent work has shown that conserved forage is a useful supplement to pasture for grazing dairy cows, since it tends to be eaten only when fresh herbage is in short supply (Phillips and Leaver, 1985a,b). In contrast, supple- mentary concentrates are generally preferred to grazed herbage and hence cannot be used as a buffer against variations in herbage availability.

A major factor limiting the herbage DM intake of the grazing dairy cow is its ability or willingness to consume sufficient herbage within the time

1Present address: Department of Agriculture, University College of North Wales, Bangor, Gwynedd, Gt. Britain.

0168-1591/86/$03.50 © 1986 Elsevier Science Publishers B.V.

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available at pasture (Leaver, 1981), and it is not known to what extent offering supplementary forage affects the ingestive behaviour of the cow. This paper records the effects of offering supplementary forage on the grazing behaviour of dairy cows in a series of experiments, the effects on animal product ion having been previously reported (Phillips and Leaver, 1985a,b). Animal production, and in particular milk fat yield, was most increased when conserved forage was offered (1) to high-yielding cows, (2) with a restricted availability of grazed herbage, and (3) in autumn.

MATERIAL AND METHODS

Experiment 1

Forty-eight, spring-calving British Friesian cows were allocated to two treatments in a continuous design. Treatment B received supplementary forage (chopped hay) for 45 min after morning milking before being re- turned to pasture, and Treatment A received no supplementary forage and were held in a cubicle house whilst the cows in Treatment B were eating hay.

Cows were set-stocked on three fields of predominantly perennial rye- grass (Lolium perenne) pasture for the duration of the experiment (15 April--30 September 1981). Stocking rates averaged 6.3, 4.0 and 3.4 cows ha -1 in the first, second and third 8-week periods, respectively, with addi- tional cows being used in the first and second periods to maintain the higher stocking rates. Within treatments, cows were grazed at two stocking rates {mean for the season 4.9 and 4.3 cows ha-l), but as this had little effect on animal production or behaviour, the presented results are for the mean of the two stocking rates. The cows were milked twice daily at about 0.700 and 16.00 h, and both treatments were at pasture for about 20 h/day.

Groups of 12 cows, balanced for initial milk yield, live weight and stage of lactation, were used for the observation of grazing behaviour. Cows were identified by numbers painted on their sides in white gloss paint. At monthly intervals, records of time spent grazing, standing, lying, ruminating and walking were made by a team of observers over a 24-h period. Observation was aided during the day by a pair of binoculars and during the night by a 6-V torch. Cows were condit ioned to the presence of an observer both day and night prior to the first observation. Conditioning was also aided by the regular examination of grazing cows by stockmen both in daylight and in darkness with a torch. Records were made at 5-min intervals during the day and 15-min intervals at night.

Rate of biting was measured on 20 additional days on the same cows during the first grazing period after morning milking (09.00--11.00). The number of grazing bites (assessed by movement of the cow's head) taken in 90 s was recorded, but if there was an interuption in the biting action of more than 15 s, a new recording was initiated. The sequence for recording cows for each day was determined by allocating the 24 cows to 6 blocks at

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random, followed by random allocation of the treatments within blocks. Hay feeding times were recorded twice weekly for cows in Treatment

B, with observations at 3-rain intervals. The number of daily grazing bites was estimated from the grazing time

× biting rate. Herbage DM intake for each cow was estimated from the recorded animal production data (Phillips and Leaver, 1985a) using the standards for metabolisable energy (ME) requirements of MAFF et al. (1975). Herbage intake divided by the number of daily bites estimated bite size, and herbage intake divided by the grazing time estimated the rate of herbage DM intake. Rate of hay DM intake was calculated from the mea- sured hay DM intake divided by the hay feeding time.

The height of the herbage grazed by the cattle was recorded weekly using a rising-place meter (Baker, 1980).

Experiment 2

Thirty-two British Friesian cows were allocated to four treatment groups in a Latin-square design. The behaviour results are presented for three treatments: A, grazing only; B, grazing + ad libitum silage for 45 min after morning milking; C, grazing during the day + ad libitum silage overnight in a feeding passage. The experiment ran from 12 April to 14 July 1982 (four 3-week periods).

The animals employed were 12 autumn-calving cows, 4 autumn-calving heifers, 4 spring-calving cows and 12 spring-calving heifers, which were allocated to the eight blocks of the Latin Square design so that the animals within each block were as similar as possible in milk yield and live weight. Within blocks, animals were allocated to the four treatments at random.

Cows were set-stocked in treatment groups with the same available herbage height for all treatments. This was achieved by rotating the groups dally around four areas of predominantly perennial ryegrass (Lolium perenne) pasture of 1.2 ha each. The cows on treatments A and B were at pasture for 21.5 and 20.0 h/day, respectively, and those on Treatment C for 7.3 h/day. The latter treatment had access to silage for 14.5 h/day.

Records of the time spent grazing, standing, lying, ruminating, walking and eating silage were made in two 24-h observations in the last week of each period. Records were taken at 5-min intervals between morning and afternoon milking, and 10-min intervals between afternoon and morning milking.

Rate of biting at pasture was measured in the last 6 days of each period using the techniques of Experiment 1, but the measurements were taken four times in the day (06.00, 09.00, 14.00 and 19.00 h) for cows in Treat- ments A and B, and twice (09.00 and 14.00 h) for cows in Treatment C. Two measurements at each time were taken for Treatments A and B and four measurements at each time for Treatment C, giving 8 measurements/ cow/period.

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The total number of daily bites, bite size, herbage DM intake rate and silage DM intake rates were derived as in Experiment 1.

Experiment 3

Using similar t reatments and design to Experiment 2, but with 8 spring- calving heifers and 12 spring-calving cows allocated to Treatments A, B and C in a 3 X 3 Latin-square design, this experiment investigated silage as a grazing supplement in the later par t of the season (4 August--6 Oc- tober 1982). As in Experiment 2, all cows were set-stocked and had the same available herbage daily, using three of the four grazing areas em- ployed in Experiment 2. Treatment B was slightly modified to allow in- creased forage intakes by returning these cows to pasture only when two- thirds of them had finished eating. This extended the mean feeding time by about 15 min. Otherwise the times available for grazing and eating silage were as for Experiment 2.

Behaviour measurements were taken as in Experiment 2, except that the biting rate of cows in Treatments A and B was only measured on three occasions each day (09.00, 13.00 and 18.00 h) as very little grazing was done before morning milking. For these cows, three measurements were taken at each time, giving 9 measurements/cow/period.

Statistical analysis

In Experiment 1 the statistical significance of t rea tment differences was examined by the analysis of variance for a continuous design experiment described by Steel and Torrie (1960). In Experiments 2 and 3, differences were examined by the changeover analysis of Patterson and Lucas (1962). The significance of animal type (cow or heifer, autumn- or spring-calving) in Experiment 2 was examined by the unequal replicate analysis of the statistical package Genstat 5 Mark 4.03 (Lawes Agricultural Trust, 1980).

RESULTS

In Experiment 1, grazing time was only slightly less when supplementary forage was offered (Table I). This difference occured mainly when grazing times were high (Fig. 1) and was on average 14 min/kg forage DM eaten. In Experiments 2 and 3, reductions in grazing time were greater, averaging 37 and 30 min/kg forage DM eaten, respectively.

Grazing times for cows in Treatment A were low in April and May in both years and tended to be inversely related to herbage height, which declined through the grazing season (Fig. 1). For cows in Treatment C, grazing times tended to be lower later in the grazing season.

Grazing time was related to milk yield in Experiment 1 by the degree

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TABLE I

The effect o f offering supplementary forage on the behaviour of grazing cows in Ex- periments 1, 2 and 3

Time (h/day) Experiment 1 Experiment 2 Experiment 3 spent:

A B SED A B C SED A B C SED

Grazing 8.9 8.5 0.27 8.1 7.4 4.4 0.13 9.4 7°6 3.8 0.18 Ruminating 6.8 7.0 0.22 7.5 8,1 9.0 0.16 6.4 7.7 9.8 0.27 Standing and

ruminating 1.3 1.5 0.11 0.3 0.7 2.9 0.13 0,8 1.2 3.4 0.41 Lying and

ruminating 5.5 5.6 0.25 7.2 7.4 6.1 0.29 5,6 6.4 6.3 0.43 Standing 3.1 2.8 0.14 2.5 2.9 3.7 0.13 2,5 3.1 3.8 0.21 Lying 4.0 3.8 0.29 5.4 4.5 4.5 0.21 5.1 4.3 3.0 0.23 Walking 1.3 1.3 0.05 0.6 0.6 0.3 0.01 0.6 0.6 0.3 0.02 Eating forage -- 0.6 -- -- 0.4 2.1 0.11 -- 0.8 3.3 0.13

Experiment 1 Exper,ments 2 and 3 Experiments 2 and 3

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represented in the fo l lowing equations:

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y - mi lk yie ld (kg /day)

Ruminat ing t ime was increased by offering supplementary forage, par-

238

ticularly in Experiments 2 and 3 when forage DM intakes were high (Table I). For cows in Treatment A, ruminating time increased through the grazing season, except for low ruminating times in late season as a result of low herbage DM intakes. In Experiment 1, ruminating times/kg herbage DM for cows in Treatment G (assuming a negligible contribution from concen- trate intake) were 28,35 and 43 min in early, mid- and late season, respec- tively, compared with 36 and 38 min/kg herbage DM in Experiments 2 and 3, respectively. In this treatment, most of the ruminating occurred whilst the cows were lying down, while standing and ruminating mainly occurred when the cows were removed from the pasture for milking. The cows spent longer away from the pasture for milking in Experiment 1 than in Experi- ment 2 or 3, and as a result the standing and ruminating times were higher. Cows housed overnight (Treatment C) spent more time standing/standing and ruminating and less time lying/lying and ruminating (Table II). This was mainly a difference in their behaviour pattern overnight rather than during the day at pasture.

TABLE II

The effect of housing overnight on the time spent lying/lying and ruminating or standing/ standing and ruminating

Time (h) Experiment 2 Experiment 3

Cows at Cows Cows at Cows pasture housed pasture housed (Treatments overnight (Treatments overnight A and B) (Treatment A a n d B) (Treatment

c) c)

Lying/lying and 07--16.00' ruminating 16--07.00

Standing/standing 07--16.00 and ruminating 16--07.00

3.8 2.8 1.7 3.2 8.4 7.7 8.9 6,1

1.5 1.1 2.1 1.8 1.7 5.6 1.6 5.5

~Milking occurred at approximately 07.00 and 16.00 h.

In Experiments 1, 2 and 3, respectively, 90, 93 and 95% of walking activity was to or from the milking parlour. Cows stocked at the higher rate in Experiment 1 spent significantly more time walking at pasture {0.21 h) than those stocked at the lower rate (0.14 h).

The diurnal variation in the time spent grazing, ruminating or eating forage is shown for Treatment A in Experiment 1 in Fig. 2, and for all treatments in Experiments 2 and 3 in Figs. 3 and 4. In Experiment 1, the reduction in daylight hours in late season resulted in a concentration of grazing time between 08.00 and 20.00 h. Grazing was not, however, ex- clusively confined to daylight hours in any part of the season. On average

239

E a r l y s e a s o n

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over the whole grazing season, 54% of grazing was between the afternoon and morning milking and 46% between morning and afternoon milking. Ruminating tended to be concentrated into the hours of darkness, but was also interspersed between the major grazing bouts in the daylight. In Experiments 2 and 3, the longest period of continuous grazing occurred after the afternoon milking. In early season (Fig. 3) there were three peaks

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of grazing activity between morning and afternoon milking, but in late season (Fig. 4) there were only two. In both Experiments 2 and 3, offering silage after morning milking replaced the first grazing bout. The grazing bout before morning milking was much more pronounced in Experiment 2 than in Experiment 3, but in the latter there was a further grazing bout around midnight.

Cows offered forage overnight (C) ate most of their forage in the early evening in Experiment 2, but in Experiment 3 there was a second peak of forage-eating activity before going to pasture in the morning.

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Fig. 4. Proportion of each hour of the day spent grazing, ruminating or eating silage, Experiment 3 (late season).

Biting rate at pasture was not significantly affected by offering forage in any experiment, although it tended to be reduced by offering silage in Experiment 3 (Table III). In Experiment 2 the biting rate increased as the season advanced, and it remained high throughout Experiment 3 (Fig. 2). Biting rate tended to increase through the day in all treatments in Ex- periments 2 and 3 (Fig. 5).

There was no significant effect of offering hay in Experiment I on herbage bite size or DM intake rate (Table III). In Experiment 2 herbage bite size was significantly increased for cows offered forage overnight. In Experi-

242

T A B L E I n

Ingest ive behav iouz o f grazing dairy c o w s

E x p e t i m e n t 1

A B S E D

Herbage Biting zates (b i tes m i n - I ) 6 6 . 6 6 4 . 0 1 .87 Bite s ize (g D M ) 0 . 3 4 0 . 3 8 0 . 0 2 4 In take rate (g D M m l n -~) 22 .6 2 4 . 6 1 , 4 3 N o o f b i tes day -~ 3 5 5 4 5 3 2 7 4 4 1 4 7 3

Fol~tge In take rate (g DM m in -1) - - 4 9 . 5

65

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Time of day (h )

Fig. 5. Diurnal variation in biting r a t e . . , Treatment A; A, Treatment B; m, Treatment C.

ment 3, however, cows offered forage overnight had a significantly lower herbage bite size. These differences are reflected in the herbage DM intake rates, which were increased and reduced in Experiments 2 and 3, respec- tively, compared to cows in Treatment A. The total number of daily grazing bites was reduced by offering supplementary forage, reflecting the reduced grazing times, in particular when silage was offered overnight.

The rate of forage DM intake was highest where forage was offered

2 4 3

Exper iment 2 Exper iment 3

A B C SED A B C S E D

59 .7 59 .2 60 .3 0 .77 66 .1 64 .7 63 .0 1 .25 0 .45 0 . 4 0 0 .48 0 . 0 3 5 0 .29 0 .26 0 . 2 0 0 .026

26 .4 23 .7 28 .6 1 ,80 18 .3 16 .2 12 .6 1 .83 29 273 27 076 15 9 5 3 771 37 252 29 338 14 0 1 0 958

63 .0 48 .5 - - - - 69 .7 55 .6

after morning milking, and was higher for silage than hay, and higher in Experiment 3 and 2. Forage DM was eaten at between 2 (Treatment B, Experiment 1) and 4 (Treatment C, Experiment 3) times the rate of her- bage DM.

The effect of animal type (spring- or autumn-calving, cow or heifer) in Experiment 2 is shown in Table IV. There were no significant type:treat- ment interactions and so results are presented as means for the four treat- ments. There were no significant differences between cows and heifers in grazing or silage eating times, but spring-calvers spent longer in each of these activities than autumn-calvers. Spring-calvers also spent longer stan- ding/standing and ruminating, and less t ime lying/lying and ruminating than autumn-calvers. Heifers did more of their ruminating whilst lying down compared to cows and less whilst standing. Animal-type effects on total ruminating time were small but significant. Cows ruminated for longer

TABLE IV

The effect of animal-type on behaviour (Experiment 2)

Cows Heifers SED

Autumn- Spring- Autumn- Spring- calving calving calving calving

Time (h day - l ) spent Grazing 6.0 Eating silage 0.9 Ruminating 8.5 Standing and ruminating 1.6 Lying and ruminating 6.9 Standing 3.1 Lying 5.1

Ingestive behaviour Biting rate (bites min -1 ) 58.1 Bite size (g DM) 0.52

6.6 6.2 6.5 0.17 0.9 0.8 1.0 0.16 8.3 8.4 8.1 0.21 2.3 1.1 1.8 0.33 5.9 7.3 6.3 0.39 3.3 3.1 3.3 0.18 4.4 5.0 4.6 0.29

58.9 57.4 62.3 1.03 0.55 0.50 0.36 0.05

244

than heifers, and autumn-calvers for longer than spring-calvers. Spring- calving heifers had a significantly smaller bite size but faster biting rate than the other three groups.

DISCUSSION

The effect of offering supplementary forage on grazing time largely depends on whether it substitutes for, or supplements, the grazed herbage. In this series of experiments, forage offered in Experiments 1 and 3 largely supplemented herbage, whereas in Experiment 2 it substituted for herbage.

The absence of a significant relationship between milk yield and grazing time suggests that high yielders were not prepared to graze for much longer than low yielders. This is in accordance with work by Brumby (1959), who showed that for an increase of 1 kg/day in milk yield cows would graze on average for an extra 6.2 min. At the rate of intake measured by this author, this would provide an additional 0.19 kg DM intake, or approxi- mately one-half of the energy requirements for 1 kg milk. In Experiment 1, a greater response in milk output was obtained from providing supple- mentary forage to high yielders than low yielders (Phillips and Leaver, 1985a).

A major seasonal effect was the decline in herbage bite size as the season progressed. This appeared to be related to a decline in herbage height, although declining dry matter content and digestibility and increasing faecal contamination of the sward may also have been influential. Hodgson and Jamieson (1981) also found decreased bite size at reduced herbage heights. The concomittant increase in biting rate and grazing time in Ex- periments 2 and 3, while representing an attempt by the cow to maintain herbage DM intake, seemed to be limited by a ceiling of approximately 66 bites min -1 and 10 h/day, respectively (Fig. 1). Individual cows, however, recorded values as high as 91 bites min -1 and 12.8 h/day during periods of low herbage availability. The ceiling of 66 bites rain -1 and 10 h/day, i.e. approximately 40 000 bites/day, is in accordance with most reports of the grazing behaviour of lactating dairy cows (e.g. Brumby, 1959; Meijs, 1981), although under-feeding in hotter climates has resulted in grazing times of between 11 and 13 h/day (Smith, 1955; Stobbs, 1974) and up to 62 000 bites/day (Stobbs, 1974).

One factor probably limiting the number of bites/day at low herbage heights in Experiment 1 was an increase in walking time at pasture for cows stocked at a higher rate. This probably represents an increase in the time spent searching for suitable areas to graze. A second factor reducing the number of bites/day was housing the cows overnight. Vik (1956) found that cows grazed between morning and afternoon milking spent up to 98% of their time grazing at pasture, but still had total grazing times lower than cows which spent day and night at pasture. Housing overnight and offering silage had a greater depressing effect on grazing time than in the

245

experiment of Vik (1956). In Experiment 3, where there was evidence that silage was preferred to pasture, the herbage bite-size was also reduced.

The small bite-size of the spring-calving heifers and their higher biting rate means that they are more likely to suffer from herbage shortages. Previous work at this centre with silage-fed animals has shown that within parities, the rate of silage DM intake is directly proportional to live weight, but the rate of intake of heifers in early/mid-lactation is 23% less than cows of the same live weight and at the same stage of lactation (Phillips, 1983). This may not be important with animals offered ad libitum silage, as feeding times may be increased by the heifers to compensate. However, where rates of intake are inherently low and eating times high, as in the case of the grazing animal, the intakes of spring-calved heifers will fall more in a period of reduced herbage availability than cows. The higher bite-size and lower biting rate of the autumn-calving heifers in Experiment 2 may have resulted from their greater weight (mean 551 kg at the start of the experiment compared with 481 kg for the spring-calving heifers) and therefore larger muzzle size (Waite et al., 1951).

In circumstances where the ability to harvest sufficient feed in the time available is limited, the rate of DM intake of the feed is of paramount importance. The rate of silage DM intake in Experiments 2 and 3 declined as silage DM intakes increased, and the rates observed were considerably higher than where silage was the sole forage (Campling and Morgan, 1981). The rate of hay DM intake in Experiment 1 was substantially less than that of silage in Experiments 2 and 3, although several other authors have found that hay is eaten at a faster rate of DM intake than silage (see review by Campling and Morgan, 1981).

In Experiments 2 and 3, biting rate increased as the day progressed, as did the proportion of each hour spent grazing. Jamieson and Hodgson (1979) found that the biting rate of strip-grazed calves was highest in the afternoon, but this was confounded by the introduction of a new pasture at 16.00 h. The rate of DM intake of conserved forage has been found to be higher in the afternoon than in the morning (Butt, 1957).

The increase in ruminating time which was observed as the season pro° gressed and as the herbage increased in maturity, and also when silage replaced grazed herbage, confirms the observation of Hancock (1954) that ruminating time is positively related to the stage of maturity of the herbage. Animal-type also affected ruminating time, since heifers spent longer ru- minating/kg herbage DM than cows, possibly because of reduced rumen capacity, and autumn-calvers tended to spend longer ruminating/kg DM intake than spring-calvers.

In both Experiments 2 and 3, the time spent lying/lying and ruminating was reduced by 0.7--1.7 h/day and the time spent standing/standing and ruminating increased by 3.5--3.6 h/day by housing the cows overnight and offering them silage instead of pasture. Castle et al. (1950) found that cows spent less time lying/lying and ruminating in a cow stall with

246

yokes than at pasture, but Burt (1957) found that the pattern of lying/ lying and ruminating in a straw yard was similar to that at pasture. Clearly the total time spent lying down is reduced for cows in a cubicle house or cow stall compared with cows at pasture. The longer time spent lying/ lying and ruminating and reduced time standing/standing and ruminating for the autumn-calvers compared with the spring-calvers may arise from their advanced stage of pregnancy at this time of the year.

The behavioural observations in these experiments confirm that bite size is the critical factor limiting intake of grazed herbage, as emphasised by Hodgson (1981). The decline in bite size as the season progresses leads to reduced intakes because grazing time and rate of biting cannot be in- creased sufficiently to compensate. Therefore, unless a forage is offered as a buffer feed, total intakes and milk production levels decline (Phillips, 1983). The degree to which the forage supplement will compensate for reduced herbage intakes will depend on the degree of restriction of herbage, the quality of the forage and the duration of its availability.

ACKNOWLEDGEMENTS

We thank staff of the West of Scotland Agricultural College (Crichton Royal Farm) for their assistance in behaviour observation. C.J.C.P. is grate- ful to the West of Scotland Agricultural College for the provision of facilities and the Dr. William Stewart postgraduate research scholarship.

REFERENCES

Baker, A.-M.C., 1980. A study of herbage production and its utilization by dairy cattle from continuously grazed swards. M.Sc. Thesis, Faculty of Science, University of Glasgow.

Brumby, P.J., 1959. The grazing behaviour of dairy cattle in relation to milk produc- tion, live weight and pasture intake. N.Z.J. Agric. Res., 2: 797--807.

Butt, A.W.A., 1957. The effect of variations in nutrient intake upon the yield and com- position of milk. II. Factors affecting rate of eating roughage and responses to an increase in the amount of concentrates fed. J. Dairy Res,, 24: 296--315.

Campling, R.C. and Morgan, C.A., 1981. Eating behaviour of housed dairy cows -- a reviews. Dairy Sci. Abstr., 43: 57--63.

Castle, M.E., Foot, A.S. and Halley, R.J., 1950. Some observations on the behaviour of dairy cattle with particular reference to grazing. J. Dairy Res., 17: 215--230.

Hancock, J., 1954. Studies of grazing behaviour in relation to grassland management. I. Variations in grazing habits of dairy cattle. J. Agric. Sci., 44 : 420--432.

Hodgson, J., 1981. Influence of sward characteristics on diet selection and herbage intake by the grazing animal. In: J.B. Hacker (Editor), Nutritional Limits to Animal Produc- tion from Pastures. Commonwealth Agricultural Bureaux, Farnham Royal, pp. 153-- 166.

Hodgson, J. and Jamieson, W.S., 1981. Variation in herbage mass and digestibility, and the grazing behaviour and herbage intake of adult cattle and weaned calves. Grass Forage Sci., 36: 39--48.

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