The effect of the spatial scale of heterogeneity of two herbage species on the grazing behaviour of lactating sheep

Applied Animal Behaviour Science 88 (2004) 61–76

The effect of the spatial scale of heterogeneity of two herbage species on the grazing behaviour of lactating sheep R.A. Champion∗ , R.J. Orr, P.D. Penning, S.M. Rutter Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, UK Received 12 May 2003; received in revised form 3 February 2004; accepted 6 February 2004

Abstract The benefits of including clover in sheep pasture are well known, this study investigates a method to enhance these benefits by facilitating diet selection. Lactating ewes and their twin lambs grazed ryegrass (G; Lolium perenne cultivar ‘Parcour’) and white clover (C; Trifolium repens cultivar ‘Kent Wild White’) as either separate or conterminal (G:C) monocultures, or as mixtures (M), at a mean sward surface height (SSH) of 6.6 cm. There were two replicates of each treatment and there were four ewes and their lambs on each replicate. The grazing behaviour of the ewes was measured during an 8-day experimental period. For treatments M and G:C, clover content of the diet was estimated using an n-alkane technique. Ewes on treatments M and G:C selected higher proportions of clover (0.35 and 0.62) than was offered in the paddocks (0.09 and 0.43). The ewes on G:C made frequent transitions between the two herbage species and there was a significant linear increase in this frequency over the day (P < 0.001). The times spent eating were; G, 579 min per day; C, 495 min per day; M, 664 min per day and G:C, 592 min per day; P = 0.072. Grazing times (eating times+intra-bout intervals ≤ 6 min) were; G, 673; C, 573; M, 706 and G:C, 640 min per day; P = 0.062. Although not significant, the apparent longer eating time for ewes on M was reflected in the proportion of the day that the ewes spent standing (G, 0.50; C, 0.49; M, 0.58 and G:C, 0.52 of the day). The number of lying bouts per day was significantly higher on G than on the other treatments and significantly lower on M than on the other treatments (G, 32; C, 26; M, 18 and G:C, 24; P = 0.001). The mean duration of these lying bouts was significantly less on G than it was on the other treatments (G, 23 min; C, 29 min; M, 34 min and G:C, 29 min; P = 0.026). During eating the ewes on C and M appeared to walk more slowly than the other ewes. The ewes on G:C appeared to walk more quickly than the other ewes during eating and during intra-bout intervals, although none of these differences in speed were significant. The intake rates of the ewes were; G, 5.4 g DM min−1 ; C, 6.5 g DM min−1 and M, 4.2 g DM min−1 ; P = 0.086. Daily intakes of the ewes were calculated (intake rate×eating time) as; G, 3126 g DM per day; C, 3233 g DM per day; M, 2797 g DM per day and G:C, 3569 g DM per day; P = 0.119. During the experimental ∗

Corresponding author. Tel.: +44-1837-883516; fax: +44-1837-82139. E-mail address: [email protected] (R.A. Champion). 0168-1591/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.02.011

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period the daily live weight gain of the lambs was significantly different between treatments (G, 17.2 g kg−1 ; C, 26.4 g kg−1 ; M, 14.3 g kg−1 and G:C, 19.8 g kg−1 live weight; P = 0.002). So it appears there were benefits from offering grass and clover as separate monocultures and this may have been due to less time spent searching for clover. © 2004 Elsevier B.V. All rights reserved. Keywords: Sheep; Grazing; Heterogeneity; Energy expenditure; Diet selection

1. Introduction Recent work has shown that when sheep were offered a free choice between eating grass and eating clover, they chose to eat a diet that was 60–70% clover (Parsons et al., 1994), i.e. a partial preference for clover, rather than a diet that was 100% clover. The preference for clover was higher in the morning than it was in the evening. The existence of this partial preference is likely to lead to selective grazing behaviour by sheep, when they are grazing mixtures of grass and clover. Mixtures of grass and clover are now commonly used for sheep grazing because these pastures permit less nitrogen fertiliser to be applied than pure grass swards. Orr et al. (1990) found that the herbage-mass production of a grass/clover sward, receiving no nitrogen fertiliser, was 80% of that of a grass sward receiving 420 kg N ha−1 . This nitrogen economy was achieved because there was effective recycling of the nitrogen fixed by the clover (Orr et al., 1995). Another benefit of using grass/clover mixtures is the potential for improving individual animal performance. Penning et al. (1995) found that when lambs were reared on clover monocultures they grew faster than those reared on grass. Although improvements in individual animal performance may rely on the grazing of clover monoculture, there are certainly benefits associated with the use of grass/clover mixtures. In the present experiment, we tested the effects of different spatial scales of heterogeneity of grass and clover; firstly, on the ability of the animals to select a preferred diet and secondly on their grazing behaviour. We chose four treatments; grass only, half clover (i.e. a block of clover next to a block of grass), clover only and finally one where clover was available as part of a mixture with grass. In terms of grazing behaviour we examined the rate with which the animals ingested herbage on the different treatments and how long the animals spent grazing per day. We knew that the sheep on the mixture would have to search to include clover in their diet so we also decided to measure energy expenditure. This cost-benefit approach was also used by Thornley et al. (1994) in their model of grazing intake and diet selection in grass/clover swards. They suggested that selection costs would be limited by the animals as soon as the costs became greater than the benefits. They also suggested that increasing time spent grazing increases the vulnerability of the animals to predators. This is a cost of diet selection that we did not consider in our experiment. The overall objective of the experiment was to enable us to have a sound basis upon which to make recommendations on how to provide sheep with their preferred diet and thus obtain the likely benefits in animal performance. Using a grazing system that includes clover means that little nitrogen needs to be applied to achieve high outputs, giving environmental and economic benefits. This paper may indicate ways in which these benefits can be maximised.

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2. Materials and methods 2.1. Sward establishment Perennial ryegrass (Lolium perenne L. cv. Parcour) and white clover (Trifolium repens L. cv. Kent Wild White) were sown in 1990 as monocultures or as mixtures. They were grazed by sheep and goats each year from 1991 to 1993 and by sheep in 1994. The experiment was conducted in 1995 and the whole experimental area was fertilized with 40 kg P2 O5 and 40 kg K2 O ha−1 on 15 March. 2.2. Treatments The experiment was conducted between 12 and 20 June 1995 (i.e. 8 days in early summer). There were four sward treatments; (i) monocultures of grass (G); (ii) monocultures of clover (C); (iii) mixtures of grass and clover (M) and (iv) adjacent monocultures of grass and clover, where half the area in each paddock was grass and half was clover (G:C). Each treatment was replicated twice and all eight paddocks (each of 0.3 ha) had a target sward surface height (SSH) of 6 cm and were managed using continuous variable stocking with sheep. The clover and mixed swards received no nitrogen fertiliser, whereas grass swards were fertilised with 30 kg N ha−1 on each of 9 May and 5 June. All swards were irrigated to maintain a soil moisture deficit of less than 40 mm. 2.3. Management At the start of the grazing season, the swards were stocked with non-lactating ewes to establish the target SSH on each treatment. Thirty-two lactating Scottish Halfbred (Border Leicester × Cheviot) ewes between 3 and 5 years old with twin lambs were then selected from a larger group that had undergone synchronised parturition on 24 April (Penning and Gibb, 1977). On 27 April, these ewes and lambs were turned out to mixed grass/white clover swards, on a separate area of the experimental field and grazed these until 5 June, when they were moved to four swards which were similar to the treatment areas. This allowed them to become acclimatised to their designated diets for 1 week. Then core groups of four ewes and their twin lambs per replicate, moved to the treatment areas on 12 June and this group size (Penning et al., 1993) was not changed during the subsequent 8-day measurement period. 2.4. Measurements 2.4.1. Sward height, composition and leaf area SSH was measured, using a sward stick (Hill Farming Research Organisation, 1986), twice each week, up to and during the 12–20 June measurement period, with 50 contacts being made per paddock (grass and clover on G:C were measured separately). Between 12 and 20 June, SSH was measured on 13 and 16 June. On 15 June, herbage from within three quadrats (each 0.25 m × 0.50 m) per replicate (three quadrats on grass and three quadrats

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on clover, on each G:C replicate) was cut to ground level using scalpels. The herbage from each replicate was sub-sampled, separated into leaves, vegetative and reproductive stems, and dead material, and the number of vegetative and reproductive tillers were counted. Reproductive tillers were identified on the basis of internode elongation. After oven drying at 80 ◦ C for 18 h, the weight of leaf, stem and dead dry matter (DM) ha−1 , was calculated for all the swards. From the total DM mass of grass and clover in the mixed and G:C swards, the percentage of clover (on a live and dead basis) in these two sward treatments was calculated. 2.4.2. Herbage digestibility and nitrogen concentration Hand-plucked samples of herbage (plucks) weighing approximately 100 g fresh, to simulate herbage grazed by the animals, were taken from all swards on 13 June. These pluck samples were freeze-dried and analysed for DM, ash, N and digestibility in vitro (Jones and Hayward, 1975). The plucks from treatments G:C and M were also analysed for n-alkanes (see Section 2.4.3). 2.4.3. Diet selection All four ewes on each of the G:C replicates were observed during daylight (04:00–22:00 h, approximately) on 12 and 14 June. Recordings were made every 4 min as to whether they were positioned on grass, clover, or on the boundary between the two herbage species; also, whether they were eating, idling or ruminating and whether they were standing or lying. The observations were performed from a tower equipped with a hide, from which a single observer could see and identify all the G:C ewes. The ewes were identified with different coloured marker sprays and each observer watched for 3 h so that six observers were used during the 18 h period. From the 4 min intervals when the ewes were eating we calculated eating time on each species. We also used the observations to identify when the ewes moved from one species of herbage to the other and to calculate the intervals of time that they spent on each species during the day. Faecal samples were collected from the ewes on the M and G:C treatments on 13 June, by observing the ewes and picking up the faeces from the pasture immediately after defecation. The samples were bulked separately for each replicate and freeze-dried. Herbage pluck samples (see Section 2.4.2) and the faecal samples from treatments M and G:C, were analysed for their n-alkane content. The concentrations of the n-alkanes (C28 , C29 , C30 , C31 , C32 and C33 ) in the herbage and faeces samples were used to calculate the composition of the diet that was selected by the ewes on treatments M and G:C. This technique depends on the fact that the relative amounts of different n-alkanes in grass and clover are different. Gas–liquid chromatography was used to characterise this ratio pattern in the herbage on offer. The ratios found in the faeces were used to calculate diet composition by least-squares optimisation (Dove and Moore, 1996). Since some of the n-alkane ingested is not recovered, the amounts detected in the faeces must be corrected for recovery rates. In our experiment, we did not measure faecal recovery rates, so we used the rates given by Dove and Mayes (1991) for sheep that were fed fresh perennial ryegrass. 2.4.4. Lying/standing and walking behaviour Measurements of lying/standing and walking behaviour were made on two occasions, using an automatic recording system (Champion et al., 1997) fitted to one ewe per replicate.

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The measurements were made between 00:00 h on 12 June and 00:00 h on 13 June, on one ewe per replicate and then on a different ewe in each replicate, over the same time period (24 h) on 14 June. The automatic system used a mercury tilt switch attached to one of the animal’s forelegs to measure walking and lying/standing. The signal from the tilt switch was logged by the IGER Behaviour Recorder (Rutter et al., 1997) and the recorder also logged jaw movements over the same 24 h period. The lying/standing and walking data were analysed later using the ‘Graze’ program (Rutter, 2000; Champion and Rutter, 2000) to give the number of steps taken, the length of time spent lying/standing and the number of lying bouts (where a lying bout was a period starting with the animal laying down and ending with it standing up). We calculated which steps occurred during eating and which occurred during intra-bout intervals with The Observer® software; a program for analysing observations of behaviour (Noldus Information Technology, b.v., Wageningen, The Netherlands). As a result of measuring walking and standing/lying behaviour, it was possible to calculate several variables. These were; distance moved per day and walking speed during various activities, proportion of the day spent standing, number of lying bouts and mean lying bout length and estimated energy expended. Rook et al. (2004) investigated the movement patterns of grazing cattle and sheep on perennial ryegrass paddocks and used a scaled map to calculate the distance moved per step. They found that the distance moved per step for ewes (97 ± 3.9 kg) was 0.23 m while they were grazing and 0.26 m while they were moving between grazing bouts. We did not measure distance moved per step directly, but our ewes (70.2 ± 1.5 kg) were a similar size and the same breed as those used by Rook et al. (2004) and so we assumed that our ewes moved 0.25 m per step (the mean of the two figures found by Rook et al.). We calculated walking speed during eating, grazing and intra-bout intervals, from the distances moved and the durations of each activity. The durations were derived from the ‘Graze’ analyses of ingestive behaviour. 2.4.5. Ingestive behaviour The jaw movements of the measured ewe in each replicate (see Section 2.4.4) were recorded with the IGER Behaviour Recorder over 24 h on 12 June and 14 June. These recordings were subsequently analysed with the Graze program to give eating, grazing (Gibb, 1998), ruminating and idling time. Eating time was made up of periods of eating jaw movements including pauses of up to 3 s. Grazing time comprised grazing bouts, which were defined as periods of eating separated by pauses of between 3 s and 6 min (these pauses were described as intra-bout intervals as per Penning et al. (1993)). As well as recording ingestive behaviour, intake rate (IR) was measured on the G, C and M treatments. It could not be measured on the G:C treatment since time spent on the G or C component was variable. IR was calculated from the changes in weight of four ewes per replicate measured before and after a period of grazing of approximately 1 h (Penning and Hooper, 1985), with adjustment for insensible weight loss. These measurements were made in the morning (10:00–11:00 h, approximately) and the evening (18:00–19:00 h, approximately). The number of minutes the ewes spent eating during the grazing period was obtained from recordings of ingestive behaviour (made with the IGER Behaviour Recorder), allowing the intake per minute of eating to be calculated. Measurements were made on one replicate per treatment on 15 June (evening) and 16 June (morning); and on the other replicate on 16 June (evening) and 19 June (morning).

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2.4.6. Live weight The ewes and lambs were weighed 24 h after lambing and then approximately every 2 weeks until 15 June. The weights of the lambs on 5 and 15 June were used to calculate their daily live weight gain (DLWG) while they were on their designated diets. We also calculated the weight loss by the ewes during the same period. 2.5. Statistical analysis The four sward treatments were replicated twice giving eight paddocks altogether. The grazing and movement behaviour (eating time, grazing time, distances moved, walking speeds and energy expenditure) of the ewes were measured on two occasions. To estimate the energy cost due to the various activities, the energy values for activities (ARC, 1980) were used. Different ewes were used at each occasion and one-way analyses of variance were performed on the results with a split-plot design (d.f. = 3). When measurements were made on more than one animal in each group (i.e. for intake rate) analyses were made on group means as the individuals cannot be regarded as independent (Rook and Penning, 1991). Sward surface height was also measured on two occasions, but there were 10 measurements of SSH (not eight) on each occasion because the G:C replicates had separate measurements for their grass and clover components. An analysis of variance was performed on these 20 values of SSH with a split-plot design to test whether there were differences between the five herbage categories (G, C, G:C grass, G:C clover and M; d.f. = 4) and whether there was an interaction between herbage category and the time between the occasions (d.f. = 4). Sward composition, herbage digestibility and nitrogen concentration were all measured on one occasion only, so for these, there was no split-plot design for the analyses of variance. We used the results of when transitions occurred between the two species of herbage in G:C (see Section 2.4.3), to calculate the mean of the number of transitions in each hour across the four animals in each replicate and then the means across the replicates and the two occasions. The overall mean hourly pattern of transitions, during daylight hours, was subjected to a linear regression analysis against hour of the day. Daily live weight change was calculated for the lambs and the ewes while they were on the treatments, from the weight measurements of all the animals on 5 and 15 June (Section 2.4.6). To remove the differences related to the different size of the lambs, daily live weight gain in g kg−1 of live weight was calculated (using live weight on 5 June). The daily live weight gain of the lambs and the daily live weight loss of the ewes were subjected to one-way analyses of variance with no split-plot design (d.f. = 3).

3. Results 3.1. Sward characterisation Sward heights were measured twice during the experimental period. The observations and recordings of behaviour were made on 12 and 14 June, so one set of sward heights

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Table 1 Herbage mass, tiller and stolon growing point numbers, and stolon lengths for ryegrass and white clover in monoculture or with two scales of heterogeneity (G:C, large scale; mixture, small scale) Monoculture

G:C

Grass

Grass

Clover

Mixture

F prob

S.E.D.

Clover

Sward surface height (cm) 6.98 6.89 6.49 6.18 6.21 0.586 0.597 Herbage mass (kg DM ha−1 ) 3626b 3017a,b 3452b 2569a 3341b (0.09)a 0.049 257.4 1004 1064 1046 850 769 (0.15)a 0.365 157.9 Green leaf mass (kg DM ha−1 ) Dead herbage mass (kg DM ha−1 ) 1631b 445a 1424b 489a 1818b (0.02)a <0.001 161.9 990 – 982 – 598 – – Grass stem mass (kg DM ha−1 ) Stolon, petiole and flower – 1509 – 1229 156 – – mass (kg DM ha−1 ) Grass tillers (×103 m−2 ) Vegetative Reproductive Clover growing points (×103 m−2 ) Stolon lengths (m m−2 )

15.6 1.9

– –

13.2 5.0

– –

9.2 0.3

– –

– –

–

6.6

–

8.7

1.2

–

–

–

195.4

–

147.9

26.7

–

–

The measurements were made on 15 June 1995 and there were 2 replicates of each treatment. Letters that include the same letter indicate that their values are not significantly different. a Proportion of clover in total.

was taken between these days, on 13 June. The other set of sward heights was taken on 16 June, during the IR measurements. Between these two measurements, the SSH on C went up slightly and the SSH on M went down slightly. However, there were no significant differences in SSH between treatments (P = 0.586) and there was no interaction between treatment and change over time (P = 0.727). The means of the two measurements of SSH are shown in Table 1. The sward composition measurements made on 15 June are also shown in Table 1. Of the total dry matter on the M plots (including live and dead material), 9.1% was live or dead clover. The herbage mass for the clover component of G:C was lower than for the grass component of G:C and lower than for treatments G and M (P = 0.049). There was no difference in green leaf mass between the treatments. The dead herbage mass was lower on the areas of clover monoculture than on grass monoculture or treatment M (P < 0.001). The nitrogen contents and digestibilities were similar in all the grass samples and similar in all the clover samples (Table 2). The nitrogen content was higher for clover than it was for grass (P < 0.001). The digestibility tended to be higher for clover than for grass (P = 0.11). 3.2. Preference for grass or clover Analysis of herbage and faeces for n-alkanes showed that diet selection on treatment M gave rise to a diet with a mean clover content of 34.8% (where the mixture on offer

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Table 2 Herbage nitrogen concentration and digestibility for ryegrass and white clover in monoculture or with two scales of heterogeneity (G:C, large scale; mixture, small scale)

Nitrogen (%) Digestible organic matter in dry matter (%)

Monoculture

G:C

Mixture

Grass

Clover

Grass

Clover

Grass

Clover

3.2a 75.8

5.4b 80.0

2.8a 60.0

5.1b 77.9

3.5a 71.0

4.9b 77.8

F prob

S.E.D.

<0.001 0.112

0.305 6.11

The measurements were made on 13 June 1995 and there were 2 replicates of each treatment. Different letters indicate significant differences between their values.

contained 9% clover). On treatment G:C, the amount of clover in the diet was also measured by n-alkane contents and was found to be 62%. The diet on offer to the G:C ewes was 43% clover by mass. Preference on G:C was also measured by observation during daylight hours. The ewes spent 57% of their eating time on clover. This result was corrected for the different IR of grass and clover to give a percentage clover in the diet of 61% by mass—similar to the value found using the alkane technique. The observations were also used to calculate the number and frequency of transitions between grass and clover and vice versa by the ewes on G:C. The mean intervals of time that the ewes spent on clover and grass were 53 and 24 min, respectively (preference on a time basis was for 57% of eating time to be spent on clover). From the interval lengths described above it can be seen that there were frequent transitions between the two species of herbage in G:C during daylight hours (ca. 04:00–22:00 h). The frequency of these transitions varied between 0.25 h−1 at 04:00 and 2.5 h−1 at 21:00 (Fig. 1). There was a significant linear increase in the frequency of transitions over the course of the day (P < 0.001), which accounted for 66.9% of the variation. 3.3. Lying, standing and walking measurements Table 3 shows the activity by the ewes and the estimated energy costs that were incurred per ewe per day, on the four treatments. The only measurements that were significantly affected by treatment were the number of lying bouts per day (P = 0.001) and the mean length of the lying bouts (P = 0.026). There were more lying bouts on G than there were on the other treatments and less on M than there were on the other treatments. The mean duration of the lying bouts was less on G than on the other treatments. Although the differences for the other measurements were not significant, the ewes on G:C tended to walk further and faster than the other ewes (Table 3). The ewes on M tended to stand for longer and walk more slowly than the other ewes, except where the C ewes appeared to walk slowly while eating. This apparent effect for the C ewes did not occur when considering grazing. The overall estimated energy cost of grazing was greatest for G mainly because these ewes rose from a lying position more times than the others. In second place was G:C because these animals used more energy than the others for walking. The lowest energy cost was

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Number of transitions per hour

3

2.5

2

1.5

1

0.5

0 4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

Time of day (h) Fig. 1. The number of transitions between the two herbage species on treatment G:C, in each hour during the manual observations of behaviour; the fitted line is y = 0.1079x + 0.364 where y is the number of transitions and x is the time of day (h) (r = 0.67).

found in M because the ewes had fewer lying bouts and therefore had to rise a smaller number of times. The average amount of energy expended by all the ewes in the experiment, as a result of activity, was 1040 kJ per ewe per day. Assuming a daily maintenance requirement of 7411 kJ Table 3 Activity and the resulting energy costs per ewe per day for ewes grazing ryegrass and white clover where the herbage was in monoculture or had two scales of heterogeneity (G:C, large scale; mixture, small scale)

Eating time (min 24 h−1 ) Grazing time (min 24 h−1 ) Total distance moved (km 24 h−1 ) Walking speed during eating (m h−1 ) During intra-bout intervals During grazing Time spent standing (min 24 h−1 ) Number of lying bouts Mean length of lying bout (min) Total energy cost (kJ 24 h−1 )

Grass (G)

Clover (C)

Mixture (M)

Grass:clover F prob (G:C)

S.E.D.

579 673 1.14 88.1 117 86.1 725 32c 23a 1151

495 573 1.12 69.0 200 87.7 701 26b 29b 1010

664 706 1.19 80.9 129 81.6 829 18a 34b 946

592 640 1.40 102.7 224 111.6 751 24b 29b 1054

43.0 33.5 0.195 11.7 40.4 9.9 74.3 1.16 2.10 53.5

0.072 0.062 >0.1 >0.1 >0.1 >0.1 >0.1 0.001 0.026 0.073

Measurements were made on 12 and 14 June 1995 on one ewe on each replicate (two replicates per treatment). Assumes that sheep move 0.25 m for each step, the energy cost for walking is 2.6 J kg−1 m−1 , the energy cost for standing over lying is 10 kJ kg−1 24 h−1 and the energy cost for rising is 0.26 kJ kg−1 . Also note that grazing time is the sum of eating time and intra-bout interval time (Gibb, 1998). Different letters indicate significant differences between their values.

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Table 4 Ingestive and ruminative behaviour for ewes grazing ryegrass and white clover where the herbage was in monoculture or had two scales of heterogeneity (G:C, large scale; mixture, small scale) Grass (G)

Clover (C)

Grass:clover (G:C)

Mixture (M)

F prob

S.E.D

Intake rate (g DM min−1 eating) (h) 10.00–11.00 18.00–19.00

4.95 5.86

6.63 6.42

– –

4.00 4.43

0.080 0.092

0.735 0.600

Mean

5.40

6.53

–

4.21

0.086

0.658

0.232 0.482 0.119

0.1054 77.7 234.0

(min−1

eating)a

Total jaw movement rate Time spent ruminating (min 24 h−1 ) Daily intake (g DM 24 h−1 )

135 342 3126

122 276 3233

122 402 3569

133 303 2797

Measurements of jaw movement rate and ruminating time were made on 12 and 14 June 1995 on one ewe on each replicate (two replicates per treatment). Intake rate was measured on 4 ewes per replicate at 10:00 h and at 18:00 h between 15 June and 19 June 1995. Daily intake was calculated as the product of eating time (min 24 h−1 ) and intake rate (g DM min−1 eating). a Bites + eating chews.

for these ewes without activity (based on ARC, 1980 and Ministry of Agriculture Fisheries and Food, 1984), this represents an increase, over and above maintenance requirement of 14%. 3.4. Ingestive behaviour IR was highest for the ewes grazing clover monoculture compared with those grazing grass monoculture or the mixture (Table 4). Eating time tended to be shorter on clover monoculture and longer on the mixture (Table 3). Grazing time was apparently shortest on C and longest on M (Table 3). We determined total daily intake (Table 4) as the product of IR (mean of a.m. and p.m. measurements) and eating time and the greatest intake was found on G:C, although these differences (P = 0.119) were not significant. Jaw movement rate was not significantly different between treatments (Table 4) although there is a suggestion that the jaw movement rate on C and G:C was low compared with G and M. Fig. 2 shows the results for eating time and total intake. Ruminating times were not significantly different between treatments (Table 4). 3.5. Live weight changes During the period between the two weighings when the lambs were on the treatment diets, their daily live weight gain was significantly different (G, 17.2 g kg−1 ; C, 26.4 g kg−1 ; M, 14.3 g kg−1 and G:C, 19.8 g kg−1 live weight; P = 0.002). The DLWG for the lambs on G:C was significantly higher than for lambs on M. The DLWG for the lambs on C was significantly higher (P < 0.05) than for lambs on all the other treatments. The treatment means for the ewe weights were all lower at the end of the measurement period than they were at the start. For treatments G, C, M and G:C, respectively, the individual ewe liveweight losses were: 0.2, 1.6, 2.0 and 0.3 kg. The differences between treatments were not significant (P = 0.237).

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Fig. 2. Eating time per 24 h for each treatment (measured with the IGER Behaviour Recorder) and daily intake for each treatment (calculated as the product of eating time in minutes and intake rate).

4. Discussion The measurement period in this experiment was short, however, the ewes and lambs involved had experience of grazing grass and clover from soon after lambing, for over a month, before they went onto the treatment diets on 5 June. Because the swards were prepared by grazing with other ewes to establish the correct sward heights, we were able to keep core groups of four ewes on each plot for the 8-day measurement period, with little change in sward conditions. The mean preference of the ewes on G:C for clover (62%) was in line with previous findings for sheep (Parsons et al., 1994). The motivation that the ewes had to achieve their preferred diet, led to very effective selective grazing by the ewes on M. It is striking that, although their sward contained only 9% clover (assessed to ground level), the ewes managed to obtain a diet that contained 35% clover. It is therefore likely that their grazing behaviour included a significant amount of time searching for clover. It has been shown that sheep can use cues to recognise a preferred feed (Edwards et al., 1997) but presumably this searching was predominantly visual, or possibly by smell. The intake of clover by the ewes on treatment G:C was not constrained by a low mass of clover and they were able to obtain their preferred diet (62% clover). Since the ewes on M had a lower clover content in their diet, it is suggested that they could not select sufficient clover to achieve their preferred diet. Similar results for preference were obtained from the observations and the n-alkane measurements, despite the fact that the observations were only carried out during daylight

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hours. Using only daylight hours should have given a reasonable estimate of preference, because grazing activity at night would have been limited at this time of year (Leclerc and Lecrivain, 1979). The behaviour observations of the G:C ewes also showed that there were frequent transitions between the two herbage species, and that the mean time interval spent on clover was longer than that spent on grass (this was not subjected to statistical analysis because the length of one interval is the complement of the other). If the idle time on each species was the same proportion of the total time on each species we would expect the intervals to be in the same ratio as the preferences for grass and clover. Given the interval lengths found here, the ewes on G:C spent 69% of their total time on clover and 57% of their eating time on clover. This suggests that the ewes were idling for a higher proportion of the time when on clover than when on grass. It may be that these ewes preferred to remain in the clover area (where the preferred herbage was located) until they had to eat some grass and consequently moved to grass, where they ate intensively, but for a short period. This possible explanation supports the idea that sheep exhibit partial preference for physiological reasons (Cooper et al., 1995), i.e. adding small amounts of a different dietary component is essential for nutritional/metabolic well-being. Recent work using an in vitro simulated rumen culture system (Merry et al., 2002) investigated the effect of varying the proportion of clover in a grass/clover mixture fed to the system. Merry et al. (2002) found that the highest efficiency of microbial protein synthesis was achieved with a mixture that was 70% clover, this is the proportion of clover that we tend to see in the diet of sheep when their preference is not restricted. Another finding in this experiment was that transitions between the two species of herbage in the G:C treatment increased in frequency towards the end of the day. Previous work showed that feeding intensity increased towards the end of the day (Champion et al., 1994), as the sugar and the dry matter content of the leaves increased. It may be that our findings simply reflect this, or that the temporal pattern of mixing of the two components becomes more critical as ingestion becomes more sustained. This is another indicator that a mixed diet may be necessary for nutritional/metabolic well being. We can imagine that towards the end of the day, as IR would have increased (Orr et al., 1997) the time required for a given mass of clover or grass to be eaten would have reduced and thus the need for transitions might have increased. The frequency of transitions between the two herbage species on G:C may have had consequences for the energy consumption of the ewes on this treatment. The centres of the two monocultures on G:C were approximately 30 m apart, so it is likely that a transition would have involved walking several metres, especially if the herbage at the boundary became depleted over time. This may be why the G:C ewes tended to walk further than the ewes on the other treatments. This extra ‘walking’ cost for the ewes on G:C, might have reduced the net benefit of separating the grass and clover. This could possibly be overcome by growing the grass and clover in narrow strips, sufficiently wide to allow easy selection, but allowing the animals to switch between grass and clover without walking large distances. Further research is needed to determine the optimum strip width. The G ewes had more lying bouts than the others and this appears to have made their energy expenditure high, because of the cost of rising to a standing position. Rumination often takes place when sheep are lying down and although the ewes on G:C ruminated

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for longer (not significant) than the G ewes, it may be that ewes eating only grass had a requirement to ruminate more frequently. Also, the G ewes would have had more time for lying down as they did not have an opportunity to search for clover. Conversely, ewes on treatment M appeared to have low energy expenditure because they had few lying bouts and thus fewer times to rise than ewes on the other treatments. Presumably the ewes on M had few lying bouts because they had little time to lie down. Overall, the activity of the ewes in this experiment increased their energy requirements by 14%, above that required for maintenance only. This figure is similar to the findings of Di Marco and Aello (2001), who found that in cattle, grazing activity increased energy requirements by 8–30% above the maintenance requirement. In our experiment, the sheep did not need to move far to obtain herbage and the mass of herbage that was present meant that intake was not limited. Lachica et al. (1999) investigated energy expenditure by goats in range land and found that the increase above maintenance was 32–47%. Fig. 2 suggests that the ewes on treatment M spent more time eating than the others, perhaps because they searched more for clover, and as a result their total daily intake was low. The total daily intake of the G:C ewes tended to be higher than the others and it is possible that spending short periods eating grass enabled the ewes to eat more than ewes eating clover alone. Perhaps there is a maximum amount of clover that can be eaten. Indeed the question has been asked before: “why do sheep stop eating clover” (Penning et al., 1991)? In our experiment, the shortest eating time was on clover, so they probably did not stop because they were tired. Jaw movement rates appeared to be low on the ‘high clover’ treatments (G:C and C), although this was not significant. This effect would be consistent with findings by Penning et al. (1995), where mastication rate was lower for clover than for grass, because each bite of clover required fewer mastications than a bite of grass. Ewes on treatment C appeared to walk relatively slowly during eating. This is surprising, considering that the C ewes achieved the highest intake rate. However, the growth habit of clover allows all the leaf to be harvested in a given area and thus the requirement for the grazer to move is reduced. The animal would then need to move to a new patch during an intra-bout interval. This argument is born out by the walking speed for the C ewes during intra-bout intervals, where it appeared to be faster. This resulted in the walking speed of C ewes during grazing (includes intra-bout intervals) being similar to that for the other treatments; the apparent effect had disappeared. Even though the measurement period was short, the lambs on treatment C grew fastest. The lamb growth rate would have been affected by their own solid food intake and the C and G:C lambs had access to an abundant supply of clover. In previous work (Penning et al., 1995), lambs grew faster on clover than on grass prior to weaning. The G:C lambs did not grow as fast as the C lambs but they grew faster than the M lambs. The low growth rate for the M lambs may have been due to lower clover availability to them and presumably their mothers had lower milk yields than the other ewes (if their intakes were lower). In planning this experiment, we had some preconceptions about diet selection and the likely costs incurred by it, expecting that the main cost of diet selection would be increased energy expenditure. We found evidence that the ewes on G:C may have walked further than the ewes on the other treatments, but this did not lead to them having a higher total energy expenditure than the other ewes. The finding that we did not expect, was that on treatment M, where the scale of aggregation was fine, the main cost of diet selection appears

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to have been time. Although this extra time spent grazing did not lead to increased energy expenditure, it may have been a cost to the animals because they perceived an increased risk of predation during grazing (as suggested by Thornley et al., 1994). Our energy expenditure measurements only took account of movement and not energy that may have been expended as a result of stress induced by a perceived threat. A cost-benefit approach to investigating diet selection is difficult because not all the costs and benefits can be measured in the same units, for example, Joules of energy. The benefits of increasing the amount of clover in the diets of the animals may have included increasing the nitrogen content in the diet and the digestibility of the diet (neither of which can be measured in Joules). Another finding has resulted from facilitating diet selection by providing the two herbage species on a large scale of aggregation (G:C). In this case, we found evidence that there may have been a benefit to the animal in terms of increased intake, because of the ability to obtain a preferred diet easily. So in our experiment there was one case (M) where there was a cost to the animal, as a result of selection and another (G:C) where there was a benefit to the animal as a result of selection. Since we did this work, experiments have been carried out in New Zealand and Australia (Cosgrove et al., 2001) to test whether actual increases in intake can be achieved. In these experiments sheep grazing grass and clover planted in distinct strips, increased their intake by 25% (265 g per sheep per day, P < 0.05) compared with grazing conventional intermixtures and the milk production of dairy cows increased by 11% (2.4 kg per cow per day, P < 0.01).

5. Conclusions The results of this experiment suggest that the main cost of diet selection, to the ewes that grazed finely-mixed grass/clover swards, was the time that was required to search for clover. Where the ewes did not have to search for clover (i.e. on conterminal grass and clover monocultures or on pure clover) the growth rates of their lambs were higher than on the grass/clover mixture. Offering grass and clover as separate monocultures may have increased daily intake by the ewes.

Acknowledgements We thank C.F. Raine, M.J. Hill and C. Kelly for assistance with collection and processing of the data, Dr. A.J. Rook for help with statistical analysis and D.A. Jones for chemical analysis. We acknowledge financial support from the UK Ministry of Agriculture, Fisheries and Food and the Biotechnology and Biological Sciences Research Council during the execution of these studies.

References Agricultural Research Council, 1980. The Nutrient Requirements of Ruminant Livestock. Commonwealth Agricultural Bureaux, Slough.

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