Efficient use of grazed herbage by dairy cows

Transcription

1 Efficient use of grazed herbage by dairy cows Contract Ref.: 96/R3/06 Report to the Milk Development Council R.J. Orr, Grazing Ecology and Behaviour Group, Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB

2 Efficient use of grazed herbage by dairy cows 2 Executive summary Grazed herbage is a major feed source for dairy cows, supplying nutrients at a lower cost than alternative feeds. It is therefore desirable to maximise the contribution of grazed herbage to the total nutrient supply of dairy cows whilst ensuring the efficient utilisation of pastures. Previous research at North Wyke has shown that the main meal of the day for grazing dairy cows on continuously stocked pastures occurs following afternoon milking, with up to 40 % of the daily intake occurring in the four to five hours before dusk. In addition, grass leaves have been shown to accumulate highly digestible sugars over the day, and the percentage of dry matter of the herbage increases as water is lost from the leaves. These findings suggested that the optimum time to give dairy cows their daily allocation in a strip grazing system would be following afternoon rather than morning milking. This hypothesis was tested in a strip grazing experiment in which spring-calving Holstein-Friesian cows were given a daily herbage allowance, calculated to be twice their estimated intake, either following morning or afternoon milking over a 10-week period. The milk yields of the two groups of cows gradually diverged over the course of the experiment, and in the last four weeks were approximately 5% higher (23.1 v 21.8 kg d -1 ) for the cows given their allocation in the afternoon compared with those receiving their allocation in the morning. Although the total times spent grazing (462 min d -1 ) and ruminating (444 min d -1 ) were similar for both groups, the rate of intake of herbage dry matter during the first hour of grazing was higher in the cows receiving their allocation in the afternoon (54.4 v 46.6 g DM min -1 ). In addition, cows receiving their allocation in the afternoon had a substantially longer evening meal (more than 4 hours duration) compared with those receiving their allocation in the morning (which had a 2-3 hour evening meal). This short-term study demonstrates how a relatively simple change to management practice in stripgrazing systems can give better pasture utilisation. There is now a need for further studies to see if giving cows their fresh pasture allocation following afternoon milking improves the quality (higher water soluble carbohydrate concentration) and quantity of ingested herbage and consequently brings a worthwhile improvement in milk yield in the longer term.

3 Efficient use of grazed herbage by dairy cows 3 Introduction Grazed herbage is the cheapest feed available for dairy cows but total intake of nutrients from grazed pasture alone is generally insufficient to allow high yielders to reach their production potential. Cows have their main daily meal following afternoon milking in the period up to sunset (e.g. Rook et al., 1994 ). In addition, both the dry matter (DM) concentration and water soluble carbohydrate (WSC) concentration of grass increase over the day (e.g. Orr et al., 1997) due to the loss of moisture and the accumulation of the products of photosynthesis. Based on this evidence, this study tested the hypothesis that dairy cows receiving strip-grazing allocations following the afternoon rather than the morning milking would have higher intake rates and consequently higher milk yields. Material and methods The grazing experiment ran for 10 weeks from 1 May until 9 July Sward The sward (2.65 ha) was sown on 6 September 1996 with a mixture of diploid (AberElan, AberSilo and Gilford) and tetraploid (Tivoli and Fetione) perennial ryegrass varieties at a total seed rate of 35 kg ha -1. A total of 160 kg N ha -1 was applied by the end of the experiment. Animals Twenty spring-calving Holstein-Friesian cows (median calving date 10 February 1997) were offered silage and concentrate diets indoors in early lactation. Following turnout to pasture in mid April, the cows were offered 4 kg head -1 day -1 of a proprietary concentrate (18 % crude protein) and stripgrazed a perennial ryegrass pasture in four groups of 5 cows from 25 April onwards. They were given a daily herbage allowance of 32 kg DM cow -1 (measured to ground level), which was calculated to be approximately twice their estimated daily herbage intake, and had access to the previous day s strip as a lie-back area. Treatments Daily herbage allocations were given either after morning milking (2 groups at 07:45 h approximately, AM 1 and AM 2 ) or afternoon milking (2 groups at 16:45 h approximately, PM 1 and PM 2 ). The cows allocated to the two treatments had been similar in live weight on days 4 and 5 after calving and for AM and PM treatments weighed v kg (± 3.78, P=0.73) respectively. In order to calculate the areas required each day to supply the herbage allowance, herbage mass was

4 Efficient use of grazed herbage by dairy cows 4 estimated from prediction equations which related mass to compressed sward height (CSH) as measured by a rising plate meter (Holmes, 1974). At approximately weekly intervals, measurements of CSH and mass were made in 6 quadrat areas (30.5 x 30.5 cm), deliberately chosen to encompass the range of sward height observed in the sward at that time, in order to develop a calibration equation. One hundred measurements of CSH were made across the area ahead of where the 4 groups of cows were grazing and herbage mass was calculated from the mean sward height and the current calibration set for that week. Measurements Milk yield was recorded daily from calving onwards. Milk samples were taken at four consecutive milkings (Monday pm through to Wednesday am inclusive) in each week of the trial and subsequently analysed for fat, protein and lactose concentrations. Temporal patterns of grazing behaviour were recorded automatically over 24 h on 9 occasions for 2 cows per group using automatic behaviour recorders (Rutter et al., 1997). Briefly, this system recorded jaw movements and these data were subsequently analysed to give periods of eating, ruminating and idling, and counted the numbers of bites and mastications during eating. In addition, herbage intake rate and bite mass were estimated each week on 3 cows per group by weighing before and after a period of approximately 1 h grazing (with the actual eating time and number of bites being recorded using the automatic behaviour recorders), making an allowance for insensible weight loss (Huckle et al., 1994). In weeks 1 to 3, intake rate and bite mass were measured during the first hour after herbage allocations for both treatments. In weeks 4 to 10, concurrent measurements were made in the morning and the afternoon for both treatments. Thus in weeks 1 to 3 the treatments were compared after herbage allocation while in weeks 4 to 10 they were compared at the same time of day. Whenever intake rate was measured, pluck samples of herbage from the grazed horizon, representative of material selected by the cows, were collected. These were divided into halves, one of which was freeze-dried and analysed for WSC, digestibility (DOMD), nitrogen and neutral detergent fibre (NDF). The other half was oven-dried to determine DM concentration. Also, 3 quadrats per replicate were cut to ground level using scalpels whenever intake rate was measured. During weeks 1 to 3, 3 quadrats per replicate were also cut at the end of each grazing period (OUT cuts) whenever

5 Efficient use of grazed herbage by dairy cows 5 intake rate was measured. The quadrat samples were separated into leaf, pseudostem and dead fractions. Results and discussion Milk yield and composition Because individuals in a group are not statistically independent, due to competition for herbage and social facilitation effects (Rook and Huckle, 1995), the mean measurements from the 5 cows in each group were treated as the experimental unit for statistical analysis (i.e. the analyses were based on n=4, with only 2 residual df). Milk yields in the second week after calving (when the cows were on a common diet) were used as a covariate to adjust yields during the subsequent grazing period. These were similar for the AM and PM groups and were v kg day -1 (± 0.377, P=0.83) respectively. Milk yields (Figure 1) declined more rapidly for cows given their new allocation after morning milking and over the last 4 weeks of the experiment this led to milk yields being approximately 5 % higher for cows moved following afternoon milking (21.8 v 23.1 kg day -1 ± 0.36, P= 0.076) AM PM 32 Milk yield (kg per d Week

6 Efficient use of grazed herbage by dairy cows 6 Figure 1. Mean daily milk yield (kg cow -1 ) in each week over the 10-week experiment, adjusted by covariance for yields two weeks post partum, for treatments AM and PM (± s.e.d). Milk fat concentration and fat yield (Table 2) tended to be higher for PM cows than AM cows, whereas the yields and concentrations of protein and lactose were generally similar. Table 2. Mean milk fat, protein and lactose composition and yield over the 10-week trial. Time of allocation AM PM ± s.e.d Probability Milk yield # (kg day -1 ) Fat % Protein % Lactose % Fat yield (kg day -1 ) Protein yield (kg day -1 ) Lactose yield (kg day -1 ) # adjusted for yields in the second week after calving, when the cows were on a common diet Intake rate, bite mass and jaw movement rates Intake rate (Table 3) tended to be higher during the first hour after allocation when the new allocation of grass was given to cows in the afternoon compared with the morning. During weeks 4 to 10 of the experiment, cows offered their new allocation in the afternoon had a significantly lower intake rate in the morning compared to those offered the new allocation in the morning. This effect was reversed when measurements were made in the afternoon. Table 3. Intake rate (g DM min -1 ) measured on 3 cows per group for AM and PM treatments in the morning (am) or afternoon (pm). Weeks 1-10 Intake rate (gdm/min) Time of allocation Morning Afternoon AM_am AM_pm PM_pm PM_am ± s.e.d Probability

7 Efficient use of grazed herbage by dairy cows 7 Weeks 4-10 Intake rate (gdm/min) Intake rate (gdm/min) Intake rate is the product of bite mass and biting rate. Bite masses (Table 4) were similar for AM and PM cows during the first hour on the new area. The significantly lower intake rate in the morning for cows offered their allocation in the afternoon was due to the lower (P<0.05) bite mass. Table 4. Bite mass (mg DM bite -1 ) measured on 3 cows per group for AM and PM treatments in the morning (am) or afternoon (pm). Time of allocation Morning Afternoon AM_am AM_pm PM_pm PM_am ± s.e.d Probability Weeks 1-10 (mg DM bite -1 ) Weeks 4-10 (mg DM bite -1 ) (mg DM bite -1 ) The cows on the AM treatment tended to have lower bite masses in the afternoon compared with PM cows. These differences are probably an effect of the reduction in sward height as each strip was depleted. The larger effect on morning measurements is consistent with the greater intake rate immediately after allocation by cows receiving their new allocation in the afternoon. PM cows had higher intake rates than AM cows in the first hour after receiving their new allocation because they significantly increased their total jaw movement rate (P<0.05) and biting rate tended to be

8 Efficient use of grazed herbage by dairy cows 8 higher (Table 5). As bite mass is largely constrained by sward characteristics this represents the best strategy for the cows to increase their intake rate at a potentially advantageous time of day. Table 5. Biting, masticating and total jaw movement (TJM) rates (min -1 ) measured on 3 cows per group for AM and PM treatments in the morning (am) or afternoon (pm). Time of allocation Morning Afternoon AM_am AM_pm PM_pm PM_am ± s.e.d Probability Weeks 1-10 Bites min Mast. min TJM min Weeks 4-10 Bites min Mast. min TJM min Bites min Mast. min TJM min Chemical analysis of herbage pluck samples DM and WSC concentrations in herbage pluck samples (Table 6), taken during the first hour after herbage allocation, were higher in the afternoon than the morning whereas the digestibilities and NDF concentrations were similar. This suggests that the higher intake rates observed in the afternoon may be an optimal foraging strategy designed to maximise intake per unit of energy expended in foraging. In

9 Efficient use of grazed herbage by dairy cows 9 weeks 4 to 10, differences in DOMD %, NDF % and N % for swards compared at the same time of day reflected changes in leaf/pseudostem ratio as the cows grazed-down the daily allowance of herbage. Table 6. Herbage dry matter (DM %), water soluble carbohydrate (WSC), digestibility (digestible organic matter in the dry matter; DOMD), neutral detergent fibre (NDF) and nitrogen (N) measured for AM and PM treatments in the morning (am) or afternoon (pm). Time of allocation Morning Afternoon AM_am AM_pm PM_pm PM_am ± s.e.d Probability Weeks 1-10 DM % WSC (% of DM) DOMD (% of DM) NDF (% of DM) N (% of DM) Weeks 4-10 DM % WSC (% of DM) DOMD (% of DM) NDF (% of DM) N (% of DM) DM % WSC (% of DM) DOMD (% of DM) NDF (% of DM) N (% of DM)

10 Efficient use of grazed herbage by dairy cows 10 Diurnal meal patterns Figure 2 shows the patterns of meals over 24 h recorded on 9 occasions throughout the experiment. Recordings started at 08:00 h for both treatments (i.e. they were measured over the same time-frame). Removal of the cows from the pasture twice-daily for milking and their subsequent return (06:30 to 07:45 h and 15:30 to 16:45 h approximately), along with times of

11 Efficient use of grazed herbage by dairy cows 11 12/5 21/5 30/5 AM 1 5/6 19/6 26/6 9/7 8/5 12/5 21/5 AM 2 30/5 5/6 11/6 26/6 9/7 Date 8/5 12/5 21/5 5/6 11/6 PM 1 19/6 26/6 9/7 8/5 12/5 21/5 30/5 PM 2 11/6 19/6 26/6 9/ Hour Figure 2. Patterns of meals, indicated by dark shading, for individual cows during the day.

12 Efficient use of grazed herbage by dairy cows 12 sunrise and sunset (05:45 and 20:45 h at the start; 04:45 and 21:45 h at the end of the experiment) can be seen to have influences on synchrony in the initiation and cessation of grazing bouts. Similar patterns were seen for each replicate and the cows given their allocation in the morning (AM 1 and AM 2 ) had an initial meal of approximately 2 h duration. Their evening meal, from returning to the pasture until sunset, was more intermittent (ca. 3 h duration) than was seen for PM 1 and PM 2 cows, which had a substantial meal of >4 h duration. AM cows did very little grazing in the period from sunrise until morning milking compared with PM cows, which grazed less than AM cows in the period between morning and afternoon milkings. In both cases the cows were on depleted swards and would therefore have had their intake rates constrained, making it less advantageous to graze at this time. These effects are summarised in Table 7. Table 7. Time spent eating, ruminating or idling (mins 24 h -1 ) measured on 2 cows per replicate for AM and PM treatments in weeks 1 to 10. Eating time in the periods between milkings is shown (excluding time spent eating concentrates in the parlour), along with % of total eating time. Time of allocation AM PM ± s.e.d Probability Eating (mins 24 h -1 ) Ruminating (mins 24 h -1 ) Idling (mins 24 h -1 ) Time spent: (mins) Eating (07:45 to 16:45 h) 246 (53%) 137 (30%) Eating (16:45 to 07:45 h) 215 (47%) 325 (70%) Total times spent eating were similar (462 min day -1 ) but the temporal distribution of grazing was significantly different (P = 0.003) between the two times of allocation. Cows given the new pasture in the morning had a relatively even distribution of grazing time across the day whereas the cows given the new pasture following afternoon milking concentrated much of their day s grazing time in the period up to sunset. Ruminating time was significantly higher for AM cows than PM cows. It may be speculated

13 Efficient use of grazed herbage by dairy cows 13 that this could be related to more-rapid rate of digestion for the PM cows as a result of their greater intake of WSC rich herbage Daily herbage dry matter intakes Herbage intakes (kg DM cow -1 ) are shown in Table 8. These were calculated from the products of intake rates (Table 3) and eating times (Table 7). Table 8. Herbage intakes (kg DM cow -1 ) in the periods from am to pm milking, from pm to am milking and total intake per day for AM and PM treatments measured in weeks 4 to 10. Mean daily intakes (kg DM cow -1 ) Time of allocation AM PM ± s.e.d Probability (07:45 to 16:45 h) (16:45 to 07:45 h) Total Whilst the intakes per day were very similar for AM and PM cows, the diurnal distribution of their pattern of intake differed markedly. Cows given their new allocation in the afternoon obtained significantly more of their daily DM intake in the evening, when the WSC concentration (% of DM) was higher (Table 6). The mean daily intake across both treatments (17.9 kg cow -1 ) represented 56 % of the daily herbage allowance (32 kg cow -1 ). Herbage mass and compressed sward height Herbage mass and CSH when intake rate measurements were made, are shown in Table 9. Measurements made immediately before the cows were given the allocation (AM_am and PM_pm) showed that on average the cows moved onto swards with 16 cm CSH (IN height) and total herbage mass to ground level of 5.8 t DM ha -1 (IN cuts). This was composed of 34 % leaf, 44 % pseudostem and 22 % dead material on average. The daily herbage allowance (32 kg DM cow -1 ) was twice the estimated herbage intake, but the actual herbage intake was approximately 18 kg DM cow -1, implying that both leaf and pseudostem were consumed. When the measurements were made later in the 24 h

14 Efficient use of grazed herbage by dairy cows 14 grazing period, there had been substantial reductions in leaf mass and, to a lesser extent in pseudostem mass (Table 9). From measurements made at the end of the 24 h grazing periods (OUT cuts) in weeks 1 to 3 it can be calculated, ignoring tissue turnover during the grazing period, that the cows removed approximately 70 % of the live leaf mass and 19 % of the live pseudostem mass that was present in the IN cuts. Table 9. Compressed sward height (CSH) and the partition of herbage mass (cut to ground level), measured for AM and PM treatments in the morning (am) or afternoon (pm). Time of allocation Morning Afternoon AM_am AM_pm PM_pm PM_am ± s.e.d Probability Weeks 1-10 CSH (cm) Leaf (kg DM ha -1 ) Pseudostem (kg DM ha -1 ) Dead (kg DM ha -1 ) Total (kg DM ha -1 ) Weeks 4-10 CSH (cm) Leaf (kg DM ha -1 ) Pseudostem (kg DM ha -1 ) Dead (kg DM ha -1 ) Total (kg DM ha -1 ) CSH (cm) <0.001 Leaf (kg DM ha -1 ) <0.001 Pseudostem (kg DM ha -1 ) Dead (kg DM ha -1 ) Total (kg DM ha -1 )

15 Efficient use of grazed herbage by dairy cows 15 In weeks 1 to 3, CSHs on the grazed area after 24 h (OUT heights) were 7.5 v 7.9 cm ± 0.20 for AM and PM respectively, compared with the CSH at the start (IN heights) which were 16.2 v 16.1 cm ± The OUT heights were only marginally lower than those measured part-way through the grazing period, which indicates that relatively little grazing occurred in the later stages of the 24 h grazing period on either treatment. This confirms the evidence from Figure 2 and Table 3 i.e. AM cows grazed very little towards the end of the period and waited for the next move; PM cows did graze to some extent towards the end of each allocation - but at a low intake rate. In summary, giving new daily allocations of pasture to cows in a strip-grazing system following afternoon milking resulted in a greater proportion of their grazing occurring when the WSC content of the herbage was higher. Milk yields gradually diverged over the 10-week period and tended to be higher for cows offered their new area of pasture following afternoon milking. Whilst this difference was not significant, the probability of it occurring by chance was approximately 1 in 13. Although these results are from just one, relatively short-term trial, they demonstrate that it should be possible to achieve higher milk yields on-farm in the longer term as a result of a relatively simple change in management practice. Conclusions Allocating new areas of pasture for grazing following afternoon milking enabled cows to: achieve high intakes of grass at the time of day when the concentrations of highly digestible sugars were high. produce up to 5 % more milk (worth 54 per day for 100 cows at June 1997 prices), compared with cows given their new allocation of herbage in the morning. Following this strategy should lead to: improved quality and quantity of herbage ingested. better pasture utilisation, because this pattern of allocation coincides with the cows main meal of the day.

16 Efficient use of grazed herbage by dairy cows 16 References Holmes C.W. (1974) The Massey grassmeter. Dairy Farming Annual, Massey Univ., pp Huckle C.A., Nuthall R. and Gibb M.J. (1994) The use of short-term weight changes to measure intake rates in grazing dairy cattle. 4 th Research Meeting of the British Grassland Society, Reading, September Orr R.J., Penning P.D., Harvey A. and Champion R.A. (1997) Diurnal patterns of intake rate by sheep grazing monocultures of ryegrass or white clover. Applied Animal Behaviour Science 52: Rook A.J., Huckle C.A. and Penning P.D. (1994) Effects of sward height and concentrate supplementation on the ingestive behaviour of spring-calving dairy cows grazing grass-clover swards. Applied Animal Behaviour Science 40: Rook A.J. and Huckle C.A. (1995) Synchronization of ingestive behaviour by grazing dairy cows. Animal Science 60: Rutter S.M., Champion R.A. and Penning P.D. (1997) An automatic system to record foraging behaviour in free-ranging ruminants. Applied Animal Behaviour Science 54:

17 Efficient use of grazed herbage by dairy cows 17 Telephone Fax Contacts: Dr A.J. Rook Dr S.M. Rutter Mr R.J. Orr