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Effect of time of feeding carbohydrate supplements and pasture on production of dairy cows
Livestock Production Science 85 (2004) 275 – 285 www.elsevier.com/locate/livprodsci
Effect of time of feeding carbohydrate supplements and pasture on production of dairy cows L.M. Trevaskis, W.J. Fulkerson *, K.S. Nandra M.C. Franklin Laboratory, University of Sydney, PMB 3, Camden, NSW 2570, Australia Received 13 June 2002; received in revised form 26 February 2003; accepted 24 March 2003
Abstract The productivity and rumen status of Friesian cows grazing short rotation Italian ryegrass (Lolium multiflorum) pastures was examined in relation to the time of grazing and of feeding cereal-based concentrates. In experiment 1, 42 cows grazed as seven separate groups of six cows on ryegrass pasture over a 2-day period, and were fed 2 kg crushed barley grain/cow at morning and evening milking on the first day. On the second day, no concentrates were fed at morning milking and a different group of cows was removed at random from grazing and a rumen sample collected by stomach tube at 0, 1, 2, 3, 4, 5, 7 and 9 h after commencement of grazing at 07:00 h. Sampling concluded before afternoon milking at 17:00 h. Rumen ammonia (NH3) concentration peaked and pH fell to its lowest value between 7 and 9 h after cows commenced grazing. In experiment 2, 42 cows were stratified into three groups of 14 cows on the basis of milk and component yield, age, liveweight and calving date and randomly allocated within blocks to one of three treatments. Over a 10-day adjustment period and a 21-day experimental period, cows were given their daily pasture allocation of 13.5 kg DM/cow (above 5 cm stubble height) either after morning milking, and fed crushed barley grain at morning and evening milking in the ratio of 3:1 kg/cow (Synch) or 1:3 kg (Asynch) or given their daily allocation of pasture after afternoon milking with barley fed at a ratio of 3:1 kg/cow (PM) in the morning and evening milking. Based on the results of experiment 1, the feeding schedule for the Synch group was predicted to more closely synchronise the availability of N from pasture and readily fermentable carbohydrates (RFC) from concentrates, in the rumen, compared to the Asynch cows. The comparison between Synch and PM groups was to determine the relative importance of ‘synchrony’ compared to feeding pasture (afternoon) high in water soluble carbohydrates (WSC) content. The WSC content of ryegrass pasture sampled after morning milking (when the Asynch and Synch groups received their new pasture allocation) was 70 and 74 g/kg DM, respectively, compared to 124 g/kg DM for samples taken after afternoon milking (when the PM group received a new pasture allocation). Pasture dry matter intake (DMI) and in vivo digestibility of individual cows was determined by use of alkanes as inert markers, whilst group DMI was determined from pre- and post- grazing pasture mass estimated by rising plate meter. Rumen samples were taken by stomach tube at 0, 3, 7 and 11 h after grazing commenced in the morning on the last day of the experiment. On this same day the % of cows grazing was recorded at 10-min intervals. Nitrogen intake (733 g N/cow/day) was 280% above requirements and this resulted in rumen NH3 concentrations being three times higher than the concentration at which microbial protein synthesis (MPS) would be expected to become limited. Milk (25.1, 24.3 and 26.8 l/ cow/day; ( P = 0.05)) and protein (809, 794 and 850 g/cow/day; P < 0.05) yield for the Asynch, Synch and PM groups, respectively, differed significantly. Pasture DMI was not significantly different ( P > 0.05) but DM digestibility was ( P < 0.001) at 84% for the mean of the Asynch and Synch cows compared to 88% for the PM cows, respectively. The results of the present study indicate a substantial benefit of feeding cows pasture with a high WSC content. This can be achieved by giving the cows their daily allocation of pasture after afternoon milking in the knowledge that WSC accumulates during the day and that over * Corresponding author. Tel.: +61-2-9351-1635; fax: +61-2-4655-2374. E-mail address: [email protected] (W.J. Fulkerson). 0301-6226/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0301-6226(03)00122-2
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70% of the intake of pasture by cows will be consumed in the first 3 – 4.5 h of grazing a fresh pasture allocation. Under the conditions of this study there was no production benefit from attempting to synchronise the availability of N and RFC in the rumen by timing of WSC supplementation. This presumably indicates that MPS was not limiting the daily milk yield of 25 – 26 l/cow when these cows consumed high N ryegrass pastures. D 2003 Elsevier B.V. All rights reserved. Keywords: Water soluble carbohydrates; Dairy cows; Short-rotation Italian ryegrass; Nutrient synchrony
1. Introduction Nitrogen (N) in grazed herbage is used by dairy cows to synthesise milk protein with an apparent efficiency as low as 20% (Van Vuuren, 1993), and this is often because the non-structural carbohydrate (NSC):rumen degradable protein (RDP) ratio is below microbial requirements. Whereas, Hoover and Stokes (1991) suggest a NSC:RDP ratio of 2.0 as being optimal for efficient microbial protein synthesis (MPS), the ratio can be ten times lower than this in N-fertilised pastures (Trevaskis and Fulkerson, 1999). The theoretical energy cost of detoxifying excess ammonia (NH3) to urea and excreting it in urine is between 0.035 and 0.052 MJ metabolisable energy (ME)/g N excreted (Lobley and Malino, 1993; NRC, 1989). Reducing the quantity of N fertiliser applied to pasture is a financially viable approach to improving the efficiency of N utilisation by dairy cows in some European countries where producers have monetary penalties imposed for excessive N loss to the environment (Mayne and Peyraud, 1996). However, the resultant lower pasture yield makes this an unacceptable option for Australian dairy farmers. The alternative approach is to improve the utilisation of pasture N in the cow by increasing the availability of readily fermentable carbohydrates (RFC) in the rumen. Previous studies by Trevaskis et al. (2003) demonstrated that the effectiveness of feeding ‘carbohydrates’ in terms of a milk production response depended on whether they were degraded in synchrony with N from pasture, in the rumen. However, the results of these studies, and those of Sinclair et al. (1993, 1995) and Witt et al. (1999, 2000), who also showed apparent benefits of synchronising N released from the basal diet with supplemental NSC, were confounded by the need to feed different sources of carbohydrate in the synchronised and asynchronised diets. Likewise, studies claiming no benefits associated with synchrony
were also confounded by different carbohydrate types (Cecava et al., 1991; Casper et al., 1999). Studies by Trevaskis et al. (2001) demonstrated that MPS could be significantly improved in sheep fed a diet in which the rumen availability of RFC and N were synchronised. In this case, a single source of carbohydrate (sucrose) was used and fed at different times in relation to NH3 concentration in the rumen. Consequently, there is reason to believe that the utilisation of pasture N could be improved in dairy cows by feeding NSC at a time coinciding with peak rumen NH3 concentration in the rumen. Studies by Van Vuuren et al. (1986) have shown rumen NH3 concentration to peak approximately 7 h after dairy cows commenced grazing ryegrass pastures. If, as is common practice, cows are given a new break of pasture immediately after morning milking, the NH3 concentration in the rumen should peak during the afternoon milking when cows can be fed NSC-rich supplements to effect synchrony. The objective of the present study was to schedule feeding of highly fermentable carbohydrate supplements and high-N pasture in such a way that the availability of N and NSC would be more synchronised in the rumen. This response could be compared to the benefits of simply feeding pasture in the afternoon which has a higher WSC:RDP ratio. Water soluble carbohydrate concentrate in ryegrass peaks at 2 – 4 h before sunset (see Fulkerson and Donaghy, 2001). Trevaskis et al. (2001) have demonstrated that MPS is higher in sheep when fed pasture harvested at 17:30 h compared to 07:30 h.
2. Material and methods Two experiments were conducted in September 2001. In the warm temperate climate of coastal New South Wales, Australia (latitude 35j 30VS, longitude
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150j 0VE). In each study, Friesian cows in early lactation ( < 100 days as at the commencement of the study) were randomly stratified into treatment groups based on milk and component yield, liveweight, age and days since calving in the week prior to the experimental period in experiment 1, or the adjustment period in experiment 2. In both experiments, cows were offered 13.5 kg DM/cow/day of ryegrass (Lolium multiflorum cv. Concord) pasture above a 5-cm stubble height which was always in a vegetative state of growth. 2.1. Experiment 1: Grazing behaviour and changes in rumen NH3 and pH when cows grazed ryegrass pasture Forty two cows were allocated to one of seven groups of six animals and grazed as separate herds for 2 days. On day 1, the cows received 2 kg crushed barley/cow/milking but on day 2 no concentrates were fed at the morning milking. On day 2, a different group of cows was sequentially removed from grazing and a rumen fluid sample was immediately collected by stomach tube at 0, 1, 2, 3, 4, 5, 7 and 9 h after cows commenced grazing in the morning. Any samples visibly contaminated by saliva were discarded and a new sample taken. 2.2. Experiment 2: Grazing behaviour and change in rumen NH3 and pH in relation to feeding pasture and supplements Forty-two cows were allocated to one of three groups of 14 cows each and grazed as three separate herds and treated as shown in Table 1. The provision of the daily allocation of pasture in the morning and then a higher proportion of cerealbased concentrate in the afternoon (Synch group) was predicted to lead to greater nutrient synchrony in the
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rumen compared to feeding a higher proportion of concentrate in the morning (asynch group). The PM group was a direct comparison with the synch group of providing the daily allocation of pasture in the morning or the afternoon. Cows had access to their daily allocation of pasture for only 1 day with the interval between grazing based on pasture regrowth rate and related to the time taken for three new leaves/tiller to regrow (see Fulkerson and Donaghy, 2001) with the actual grazing interval varying from 12 to 35 days. Each group was grazed separately over a 10-day adjustment period followed by a 21-day experimental period. 2.3. Animal measurements 2.3.1. Milk and liveweight In experiment 2, milk yield and milk fat and milk protein content (composite morning and afternoon samples) were recorded at 4 day intervals using inline milk meters. Milk fat and milk protein content were measured by infra-red spectroscopy (Dairy Express, Armidale, New South Wales, Australia). Liveweight was recorded after morning milking prior to the adjustment period and on a weekly basis during the experimental period. 2.3.2. Rumen fluid The pH of rumen fluid was determined immediately after collection with a pH probe and a subsample strained through eight layers of standard muslin cloth and acidified with four drops of concentrated H2SO4 before it was frozen at 16 jC for subsequent NH3 analysis by the colorimetric method of McCullough (1967). 2.3.3. Grazing behaviour The percentage of cows grazing was recorded at 10-min intervals for 7 h from 07:00 to 14:00 h on day
Table 1 Barley (kg/cow/day) fed at morning and evening milking to treatment groups and given access to a new daily allocation of pasture after morning (07:00 h) or evening (16:50 h) milking in experiment 2 Treatment
Time of access to new daily allocation of pasture
Time of barley (kg/cow) feeding Morning milking
Afternoon milking
Asynch Synch PM
Morning Morning Evening
3 1 1
1 3 3
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2 of experiment 1 and for 17 h from 07:00 to 24:00 h on the last day of experiment 2.
Table 2 Mean nutrient content (g/kg DM) of pluck samples of ryegrass pasture from experiment 2
2.3.4. Estimation on pasture on offer and pasture intake The pasture allocated to each group on a daily basis was estimated from pre-grazing pasture mass on offer above a 5-cm stubble height (actual height), determined by a rising-plate meter (RPM) (Earle and McGowan, 1979) based on a pooled calibration curve for that type of pasture (Fulkerson and Slack, 1994). Basing pasture availability on a post-grazing residue above 5 cm related to previous studies (see Fulkerson and Donaghy, 2001) which indicated that cows could be grazed to a 5-cm stubble height to maximise pasture utilisation by stock without prejudicing pasture regrowth or milk yield/cow. In experiment 2, intake and in vivo digestibility of pasture consumed by six cows in each group representing the total group in terms of age, liveweight, days in lactation and milk component yield, was estimated using plant cuticular alkanes as inert markers (Mayes et al., 1986; Reeves, 1998). Thus, the cows were selected at random and dosed with controlled-release capsules containing C32 and C36 alkanes (Captec (NZ)) 7 days prior to the commencement of the experimental period to allow the alkanes to equilibrate within the rumen contents. Faecal samples were then taken immediately prior to afternoon milking for a period of 8 days and immediately frozen for 24 h, before freezedrying. Previous studies Reeves (1998) showed no significant difference in alkane content of faeces taken in the morning or afternoon. The faecal samples, as well as pre-grazing pasture samples (see Section 2.3.5), were extracted for alkane analysis according to the method of Mayes et al. (1986). The faecal samples were collected in the paddock after being excreted by the cows in over 90% of samples with the remainder collected per rectum. The nutrient content of pasture offered to cows in experiment 2 is shown in Table 2.
Nutrient (g/kg DM)
Treatment group Asynch
Synch
PM
Water soluble carbohydrate Crude protein Neutral detergent fibre Acid detergent fibre
70 308 410 190
74 328 390 170
124 268 390 180
2.3.5. Chemical analysis Pasture samples were plucked pre-grazing to the post-grazing height of previously grazed paddocks three times/week. The pasture samples, together with concentrates fed, were pooled on a weekly basis and then dried in a forced-draught oven at 80 jC for 48 h,
3.1.1. Grazing behaviour Most cows grazed pasture for approximately 2 h after they were given their new daily allocation of pasture in the morning (see Fig. 1). This was followed by a rapid decline in grazing activity and within a 0.5 h all cows had ceased to graze.
then ground through a 1-mm sieve and analysed. Ammonia was determined colorimetrically by the phenol –hypochlorite reaction adapted from the method of McCullough (1967) using sodium nitro-prusside as a catalyst; N by combustion using a Leco FP-428 analyser (Leco Corp., St. Joseph, MO, USA); for neutral detergent fibre (NDF) by the method of Van Soest and Wine (1967); for acid detergent fibre (ADF) using the method of Van Soest (1963) and for WSC by a modification of Technical Industrial method number 302-73A as described by Fulkerson et al. (1998). 2.3.6. Statistical analysis In experiment 1, the pH and NH3 concentration in rumen fluid over time was analysed by analysis of variance with differences between times being indicated by least significant differences (L.S.D.). In experiment 2, the data (milk and milk component yield data) was subject to ANOVA based on the general linear model with data obtained in the week prior to the adjustment day, used as covariate factors. Differences between individual means were tested by L.S.D. A linear regression equation, describing the relationship between urinary N excretion and N intake, was fitted.
3. Results 3.1. Experiment 1: Grazing behaviour and changes in rumen pH and NH3 when cows grazed ryegrass pasture
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daily pasture allocation in the afternoon (PM group) compared to the two groups given their allocation in the morning (Asynch and Synch). 3.2.2. Grazing behaviour Grazing activity, expressed as the summation of % cows grazing each hour, was similar for all groups (see Table 3) but the diurnal pattern of grazing differed. Reference to Fig. 3 indicates that grazing activity was 39% higher after afternoon milking in the PM group than for the other groups at that time of day.
Fig. 1. Rumen ammonia concentration (mg/l) (a), rumen pH (b), and the percentage of cows grazing (c) at specified times after being given their new daily allocation of ryegrass pasture in experiment 1. L.S.D. ( P = 0.05) for time of sampling: NH3 = 28; pH = 0.11.
3.1.2. Rumen status The NH3 concentration in the rumen of dairy cows rose rapidly, almost doubling ( P < 0.001) within 2 h from the time they commenced grazing. The highest rumen ( P < 0.001) NH3 concentration was in samples taken at 5 and 7 h after grazing had commenced. Rumen pH fell to reach its lowest value between 5 and 9 h from the time of commencement of grazing (see Fig. 1). 3.2. Experiment 2: Grazing behaviour and changes in rumen pH and NH3 in relation to time of feeding pasture and supplements 3.2.1. WSC content The WSC content of afternoon pasture was 52 g/kg DM higher than morning pasture and this led to an in vivo digestibility 8% units higher for cows given their
3.2.3. Rumen status There was a significant ( P < 0.001) effect of treatment on rumen NH3 concentration (See Fig. 2). The NH3 concentration in the PM group was 46 and 37% lower ( P < 0.001) at 3 (174 F 20 and 323 F 24 (mean F S.E.) mg/l) and 7 (212 F 24 and 337 F 33 (mean F S.E.) mg/l) h, respectively, after commencement of grazing than the Synch group. In contrast, the NH3 concentration of the Synch group was 39% lower ( P < 0.001) than the PM group 3 h after evening grazing commenced (341 F 42 and 208 F 36 (mean F S.E.) mg/l, respectively). The mean rumen NH3 concentration of PM cows over the 17h monitoring period was 17% lower ( P < 0.10) than the mean of the other two groups (234 F 23 and 277 F 24 (mean F S.E.) mg/l for the PM, and the other two groups, respectively). There was no significant treatment effect on rumen pH (7.04 F 0.04 (mean F S.E.)), however, there was a significant effect ( P < 0.001) of time on rumen pH with the lowest values at 7 h after commencement of grazing in the morning (6.89 F 0.06 (mean F S.E.)) and 3 h after evening milking (6.7 F 0.06 (mean F S.E.)). Rumen pH was highest at 0 h (7.5 F 0.06 (mean F S.E.)) after grazing commenced. Table 3 Grazing activity (summation of % cows grazing at each hour) recorded at 10-min intervals in experiment 2 Time of day (h)
Treatment groups (summation of % cows grazing at each hour) Asynch
Synch
PM
07:00 – 15:00 16:00 – 24:00 Total
564 379 944
532 394 926
373 539 912
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Fig. 2. Rumen ammonia concentration (mg/l) (a) and rumen pH (b) at specified times after Asynch (.) Synch (n) and PM (D) cows were given their daily allocation of ryegrass pasture in experiment 2. L.S.D. ( P = 0.05) for the time of sampling were: NH3 = 89 mg/l; pH 0.17.
3.2.4. Production, intake and N excretion The milk and component yield is shown in Table 4.
Milk yield was 2.1 l/cow/day higher ( P < 0.001) in the PM cows than the mean of the other two groups. The protein yield of the PM group was also
Table 4 Mean milk (l/cow/day) and component (g/cow/day) yield and pasture nutrient intake for cows grazing ryegrass in experiment 2 Parameters
Milk yield (l/cow/day) Milk fat (%) Milk fat yield (g/cow/day) Milk protein (%) Milk protein yield (g/cow/day) Liveweight change (kg/cow/day) Pasture dry matter intake (DMI) (kg/cow/day)a Pasture (kg/cow/day)b Pasture DMI (kg/cow/day)c Digestible dry matter (DDM) DDM intake (kg/cow/day) N intake (g/cow/day)d
Treatment group Asynch
Synch
PM
25.1 3.84 936 3.28 809 0.25 14.7 13.0 13.7 0.74 11.0 791
24.3 3.54 874 3.25 794 0.24 14.7 13.0 13.8 0.74 11.0 836
26.8 3.39 904 3.19 850 0.6 16.4 13.2 13.7 0.82 13.6 835
** Level of significance P = 0.06. a Estimated by faecal and pasture alkane analysis (Reeves, 1998). b Estimated by rising plate meter (RPM) for the period during which intake was estimated by alkane analysis as per above. c Estimated by RPM over the whole experimental period. d Including concentrate N.
L.S.D. ( P = 0.05) 0.8 0.14 NS 0.06 36 0.14 NS NS NS 0.03 2.9** NS
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significantly higher ( P < 0.01) despite a small, but significantly ( P < 0.02) lower milk protein content. There were no differences in milk fat yield between groups (913 F 14 (mean F S.E.) g/cow/day), although the milk fat content of the PM group was significantly lower ( P < 0.001). Cows in the PM group also gained significantly more ( P < 0.001) body weight. The pasture DMI of individual cows, estimated by alkane technology, was not significantly different between groups (15.3 F 2.1 (mean F S.E.) kg DM/ cow/day), however, the DDMI of pasture consumed by cows in the PM group was significantly higher ( P < 0.06) than that observed in the other groups (13.6 F 1.0 and 11.0 F 0.5 (mean F S.E.) % DM, respectively) and this reflected the substantially higher in vivo dry matter digestibility (DDM) of 82% compared to 74%, respectively. Nitrogen intake (733 F 23 (mean F S.E.) g/cow/day) was not different between groups but was 280% above recommendations (178 g N/cow/day; SCA, 1990), assuming an N use efficiency of 70% (SCA, 1990).
4. Discussion Friesian milking cows given access to their daily pasture allocation after afternoon milking produced 2.5 l milk/cow/day, 56 g milk protein/cow/day and gained 0.36 kg liveweight/cow/day more than the Synch ‘control’ cows given access to their daily pasture allocation immediately after morning milking. This was despite the fact that the Synch group was fed concentrate and pasture at times predicted to synchronise more closely the availability of N and RFC in the rumen. Milk fat, and to a lesser extent milk protein content, was lower in the PM cows, however, this appears to have been a dilution effect given their higher milk protein yield. The increased productivity of the PM group was associated with an increase of 8% units in pasture DDM and 2.6 kg/cow/day higher DDMI by these cows. These findings are in line with the previous observations (Van Vuuren et al., 1986; Fulkerson and Donaghy, 2001) that afternoon pasture, containing more WSC than morning pasture, should provide forage of higher ME and should therefore be more appropriate in terms of ruminal availability of N and
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RFC (Hoover and Stokes, 1991). In this study, afternoon pasture contained 52 g/kg DM units more WSC than morning pasture. The relatively long hours of unimpeded solar radiation (maximising photosynthesis), accompanied by relatively low temperatures (minimising respiration and growth) in the present study, were conditions conducive to a high rate of WSC accumulation in ryegrass (Fulkerson and Donaghy, 2001). Two recent studies confirm the beneficial effect of high WSC content of pasture on milk production in dairy cows. In a similar study to our own, Orr et al. (2001) reported a 1.3 l milk/cow/day (23.1 vs. 21.8 l milk/day cow) increase in milk production for cows, given their daily allocation of pasture in the afternoon when WSC content of perennial ryegrass pasture was 20.4% DM, compared to morning when it was only 17.5% DM. In vivo digestibility was not measured in this study. In the second study by Miller et al. (2001), milk yield was significantly increased from 12.6 to 15.3 kg/ cow/day when Holstein –Friesian cows were fed a perennial ryegrass variety with a high WSC content (165 g/kg DM) compared to low WSC content (126 g/ kg DM). In this study the DM digestibility of the high WSC variety was 0.71 g/g DM compared to 0.64 g/g DM for the low WSC variety and this is similar to the results of the present study. Interestingly, in this study the N efficiency was significantly increased in the cows fed a high WSC variety of ryegrass. Both the studies of Orr et al. (2001) and Miller et al. (2001) used small numbers of animals. Previous studies (Fulkerson and Dobos,unpublished data), have shown that 60 –90% of the daily pasture allocation was consumed in the first 4 h of grazing. In experiment 2 of the present studies, the PM group’s afternoon grazing activity lasted 39% longer than that observed in the remaining groups at that time, given their daily allocation in the morning (Table 3) and this presumably equated to a greater proportion of pasture DMI consumed at a higher WSC content. The lowest rumen pH in PM cows in experiment 2 was recorded 3 h after afternoon milking, presumably reflecting the higher intake of WSC from pasture. This is supported by the observations of Van Vuuren et al. (1986) who also observed that the rumen pH of dairy cows grazing ryegrass pasture was lowest after evening milking.
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Although PM cows received the same quantity of concentrates as the Synch cows at morning milking, rumen NH3 levels of the PM group were 37 and 39% lower than their Synch counterparts at 3 and 7 h following morning milking, respectively, and this presumably reflects their lower pasture N intake over this period. Of further interest was the lack of difference between the PM and Asynch rumen NH3 levels 3 h after afternoon milking despite the latters’ increased grazing activity and presumably pasture intake. Hence, we can speculate that the more stable NH3 levels in the PM group resulted from a higher intake of WSC. This same phenomenon has previously been demonstrated in sheep (Trevaskis et al., 2001), when urinary allantoin excretion was increased by as much as 50% in sheep fed kikuyu grass harvested in the afternoon compared to morning. However, the increased milk production (2.5 l/cow/day) and liveweight gain (0.36 kg/cow/day) observed in the PM group in experiment 2 could be accounted for entirely by their 2.6 kg/cow/day increase in DDMI. This would provide an additional 31 MJ ME, more than the extra 26 MJ ME predicted to be required (assuming 4.63 MJ ME required to produce 1 l of milk and 34 MJ ME for 1 kg of liveweight gain; SCA, 1990) for the extra milk production. The first objective of the present study was to test the hypothesis that feeding RFC at a time when rumen NH3 levels peaked would improve N-utilisation, MPS and consequently animal productivity. The benefit of synchronising N and RFC has previously been shown in sheep (Trevaskis et al., 2001) when timing of an intra-ruminal infusion of sucrose was controlled relative to a once-daily meal of pasture. In the present studies, rumen NH3 peaked at 7 and 9 h after the initial grazing bout and this confirms previous observations by Van Vuuren et al., 1986), and those by Khalili and Sairanen (2000) and Soriano et al. (2000) which were published subsequent to the present study. Based on the pattern of change in rumen NH3 concentration, we hypothesised that the degree of synchrony between the availability of pasture N and supplemental carbohydrates in the rumen would be greater in cows fed a high proportion (3:1) of their 4 kg/cow/day crushed barley at afternoon milking ( c 8 h after grazing commenced), than morning milking (when NH3 was lowest). In turn, MPS (Trevaskis et al., 2001), and possibly milk production, could be
expected to be increased. The results of experiment 2 (see Fig. 3.) indicate that rumen NH3 was lowest after afternoon milking in the Synch group indicating success in improving the synchrony between N and RFC. However, despite substantial reduction in rumen NH3 in the Synch group, there was no response in production associated with improved diet synchrony. There are several reasons why an improvement in diet synchrony may have failed to improve productivity under the conditions of the present experiment. Firstly, milk production averaged 24.7 l/cow/day for the Asynch and Synch groups and was probably not sufficient to require additional metabolisable protein (MP). The pasture N and ME content of the diet in experiment 2 would be predicted to be sufficient for cows producing up to 25 milk l/cow/day (Beever and Siddons, 1986). In confirmation of this, Kolver et al. (1998) detected no change in milk production or liveweight gain when MPS was increased by synchronising the release of carbohydrate-rich supplements with the availability of pasture N in the rumen. They also attributed this lack of response, in part, to an already sufficient supply of MP from the asynchronous diet for cows producing in excess of 30 l/cow/ day. Recent studies by Hongerholt and Muller (1998) and McCormick et al. (1999) also showed that provision of additional protein post-ruminally as supplemental undegraded protein, did not improve milk production in cows producing c 30 l/cow/day and supports the conclusions of Kolver et al. (1998). Consequently, any additional MPS arising from an improvement in diet synchrony would be expected to be catabolised and excreted. In fact, Peyraud and Delaby (2001) have shown that the MP supplied from good quality ryegrass pasture can support milk yields as high as 28 l/cow/day. Secondly, the CP intake of cows in the present study (732 g N/cow/day) amounted to 280% of their daily recommended requirements (178 g N/cow/day; SCA, 1990). Assuming ryegrass protein is 85% digestible in the rumen (Corbett, 1987), the 622 g N degraded there would require 7.7 kg of fermentable carbohydrates to provide what is considered as the optimal 1:12.5 ratio between N and RFC (Hoover and Stokes, 1991). As the diets of the Asynch and Synch cows provided only 4.9 kg of RFC, it is conceivable that the benefits (of diet synchrony)
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Fig. 3. Percentage of cows grazing in the PM (a), Synch (b) and Asynch (c) groups in experiment 2.
were negated by an extreme imbalance in the ratio of N to RFC. This was evidenced by the extremely high urinary N excretion of these cows. In experiment 2, there was nearly a unit of N excreted in the urine for each unit of N ingested. With such an excessive N intake, the timing of input of RFC to the rumen microbial population would have been irrelevant but the additional RFC could have benefited whenever it was given. The lowest levels of rumen NH3 recorded in experiment 2 (see Fig. 3) were at least three times higher than what is commonly considered necessary for efficient MPS ( c 50 NH3 –N mg/l; Slyter et al., 1979). It seems reasonable to speculate that the benefits of diet synchrony will become more pronounced as rumen NH 3 becomes marginal to rumen microbial requirements and this is substantiated in our results in sheep
(Trevaskis, unpublished data). This situation is unlikely to be experienced in cows grazing ryegrass pastures in a vegetative stage of growth and fed moderate amounts of cereal-based concentrate. The benefits of a simple adjustment in grazing management to provide the herd’s daily pasture allocation after afternoon milking rather than the morning seems very worthwhile. Whether there are benefits lost by going to a 24-h rather than 12-h allocation of pasture may need to be tested.
Acknowledgements The authors would like to thank Kim McKean and Madelaine Hall for their careful and diligent assistance. The financial support provided by the Dairy
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Research and Development Corporation is gratefully acknowledged. References Beever, D.E., Siddons, R.C., 1986. Digestion and metabolism in the grazing ruminant. In: Milligan, L.P., Grovum, W.L., Dobson, A. (Eds.), Control of Digestion and Metabolism in Ruminants. Prentice-Hall, Englewood Cliffs, NJ, pp. 479 – 497. Casper, D.P., Maiga, H.A., Brouk, M.J., Schingoethe, D.J., 1999. Synchronization of carbohydrate and protein sources on fermentation and passage rates in dairy cows. J. Dairy Sci. 82, 1779 – 1790. Cecava, M.J., Merchen, N.R., Berger, L.L., Mackie, R.I., Fahey Jr, G.C., 1991. Effects of dietary energy level and protein source on nutrient digestion and ruminal nitrogen metabolism in sheep. J. Anim. Sci. 69, 2230 – 2243. Corbett, J.L., 1987. Energy and protein utilization by grazing animals. In: Wheeler, J.L., Peasron, C.J., Robards, G.E. (Eds.), Temperate Pastures: Their Production, Use and Management. Australian Wool Corporation/CSIRO, Canberra. Earle, D.F., McGowan, A.A., 1979. Evaluation and calibration of an automated rising plate meter for estimating dry matter yield of pasture. Aust. J. Exp. Agric. Anim. Husb. 19, 337 – 343. Fulkerson, W.J., Donaghy, D.J., 2001. Plant-soluble carbohydrate reserves and senescence—key criteria for developing an effective grazing management system for ryegrass-based pastures: a review. Aust. J. Exp. Agric. 41, 261 – 275. Fulkerson, W.J., Slack, K., 1994. Estimating mass of temperate and tropical pastures in the subtropics. Aust. J. Exp. Agric. 33, 865 – 869. Fulkerson, W.J., Slack, K., Henessy, D.W., Hough, G.M., 1998. Nutrients in ryegrass (Lolium multiflorum), White clover (Trifoolium repens) and Kikuyu (Pennisetum calndestinum) pastures in relation to season and stage of regrowth in a subtropical environment. Aust. J. Exp. Agric. 38, 227 – 240. Hongerholt, D.D., Muller, L.D., 1998. Supplementation of rumenundegradable protein to the diets of early lactation Holstein cows on grass pasture. J. Dairy Sci. 81, 2204 – 2214. Hoover, W.H., Stokes, S.R., 1991. Balancing carbohydrates and protein for optimum rumen microbial yield. J. Dairy Sci. 74, 3630 – 3644. Khalili, H., Sairanen, A., 2000. Effect of concentrate type on rumen fermentation and milk production of cows at pasture. Anim. Feed Sci. Technol. 84, 199 – 212. Kolver, E., Muller, L.D., Varga, G.A., Cassidy, T.J., 1998. Synchronization of ruminal degradation of supplemental carbohydrate with pasture nitrogen in lactating dairy cows. J. Dairy Sci. 81, 2017 – 2028. Lobley, G.E., Malino, G.D., 1993. Regulation of hepatic nitrogen metabolism in ruminants. Proc. Nutr. Soc. 57, 547 – 563. Mayes, R.W., Lamb, C.S., Colgrove, P.M., 1986. The use of dosed and herbage n-alkanes as markers for the determination of herbage intake. J. Agric. Sci. Camb. 107, 161 – 170. Mayne, C.S., Peyraud, J.L., 1996. Recent advances in grassland
utilization under grazing and conservation. In: Grass. Land Use Systems, pp. 347 – 360. McCormick, M.E., French, D.D., Brown, T.F., Cuomo, G.J., Chapa, A.M., Fernez, J.M., Beatty, J.F., Blouin, D.C., 1999. Crude protein and rumen undegradable effects on reproduction and lactation performance of Holstein cows. J. Dairy Sci. 82, 2697 – 2708. McCullough, H., 1967. The determination of nitrogen in whole blood by a direct colorimetric method. Clin. Chem. Acta 17, 297 – 304. Miller, L.A., Moorby, J.M., Davies, D.R., Humphreys, M.O., Scollan, N.D., MacRae, J.C., Theodorou, M.K., 2001. Increased concentration of water-soluble carbohydrates in perennial ryegrass (Lolium perenne L): Milk production from late-lactation dairy cows. Grass Forage Sci. 56, 383 – 394. NRC, 1989. National Research Council. Nutrient Requirements of Dairy Cattle, 6th Revise Edition. National Academy of Science, Washington, DC. Orr, R.J., Rutter, S.M., Penning, P.D., Rook, A.J., 2001. Matching grass supply to grazing pattern for dairy cows. Grass Forage Sci. 56, 352 – 361. Peyraud, J.L., Delaby, L., 2001. Ideal concentrate feeds for grazing dairy cows—responses to supplementation in interaction with grazing management and grass quality. In: Recent Advances in Animal Nutrition. Nottingham University Press, Nottingham, UK, pp. 203 – 220. Reeves, M., 1998. Milk Production from Kikuyu (Pennisetum clandestinum) grass pastures. PhD thesis, University of Sydney, Camden, NSW, Australia. SCA, 1990. Standing Committee on Agriculture. Feeding Standards for Australian Livestock, 2nd Edition. PWS – Kent, Boston. Sinclair, L.A., Garnsworthy, P.C., Newbold, J.R., Buttery, P.J., 1993. Effect of synchronizing the rate of dietary energy and nitrogen release on the rumen fermentation and microbial protein synthesis in sheep. J. Agric. Sci. 120, 251 – 263. Sinclair, L.A., Garnsworthy, P.C., Newbold, J.R., Buttery, P.J., 1995. Effects of synchronizing the rate of dietary energy and nitrogen release in diets with a similar carbohydrate composition on rumen fermentation and microbial protein synthesis in sheep. J. Agric. Sci. 124, 463 – 472. Slyter, L.L., Satter, L.D., Dinius, D.A., 1979. Effect of ruminal ammonia concentration on nitrogen utilization by steers. J. Anim. Sci. 48, 906 – 912. Soriano, F.D., Polan, C.E., Miller, C.N., 2000. Milk production and composition, rumen fermentation parameters, and grazing behaviour of dairy cows supplemented with different forms and amounts of corn grain. J. Dairy Sci. 83, 1520 – 1529. Trevaskis, L.M., Fulkerson, W.J., 1999. The relationship between various animal and management factors and milk urea, and its association with reproductive performances of dairy cows grazing pasture. Livest. Prod. Sci. 57, 255 – 265. Trevaskis, L.M., Fulkerson, W.J., Gooden, J.M., 2001. Provision of certain carbohydrate based supplements to pasture-fed sheep, as well as time of harvesting of pasture, influences pH, ammonia concentration and microbial protein synthesis in the rumen. Aust. J. Exp. Agric. 41, 21 – 27. Trevaskis, L.M., Fulkerson, W.J., Nandra, K.S., 2003. Slowly ru-
L.M. Trevaskis et al. / Livestock Production Science 85 (2004) 275–285 men degradable carbohydrate supplements increase productivity and nitrogen utilisation in dairy cows grazing kikuyu (Penniseetum clandestinum), but not ryegrass (Lolium perenne) pastures. Aust. J. Exp. Agric. (in press). Van Soest, P.J., 1963. Use of detergents in the analysis of fibrous feeds. II. A rapid method for the determination of fibre and liquid. Assoc. Agric. Chem. J. 46, 829 – 835. Van Soest, P.J., Wine, R.H., 1967. Use of detergents in the analyses of fibrous feeds. IV. Determination of plant cell wall constituents. J. Assoc. Off. Anal. Chem. 50, 50 – 55. Van Vuuren, A.M., 1993. Digestion and nitrogen metabolism of grass fed dairy cows. PhD thesis, Wageningen Agricultural University, The Netherlands.
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Van Vuuren, A.M., Van der Koelen, C.J., Vroons-de Bruin, J., 1986. Influence of level and composition of concentrate supplements on rumen fermentation patterns of grazing dairy cows. Neth. J. Agric. Sci. 34, 457 – 467. Witt, M.W., Sinclair, L.A., Wilkinson, R.G., Buttery, P.J., 1999. The effects of synchronizing the rate of dietary energy and nitrogen supply to the rumen on the metabolism and growth of ram lambs given food at a restricted level. Anim. Sci. 69, 627 – 636. Witt, M.W., Sinclair, L.A., Wilkinson, R.G., Buttery, P.J., 2000. The effects of synchronizing the rate of dietary energy and nitrogen supply to the rumen on milk production and metabolism of ewes offered grass silage based diets. Anim. Sci. 71, 187 – 195.
Effect of time of feeding carbohydrate supplements and pasture on production of dairy cows L.M. Trevaskis, W.J. Fulkerson *, K.S. Nandra M.C. Franklin Laboratory, University of Sydney, PMB 3, Camden, NSW 2570, Australia Received 13 June 2002; received in revised form 26 February 2003; accepted 24 March 2003
Abstract The productivity and rumen status of Friesian cows grazing short rotation Italian ryegrass (Lolium multiflorum) pastures was examined in relation to the time of grazing and of feeding cereal-based concentrates. In experiment 1, 42 cows grazed as seven separate groups of six cows on ryegrass pasture over a 2-day period, and were fed 2 kg crushed barley grain/cow at morning and evening milking on the first day. On the second day, no concentrates were fed at morning milking and a different group of cows was removed at random from grazing and a rumen sample collected by stomach tube at 0, 1, 2, 3, 4, 5, 7 and 9 h after commencement of grazing at 07:00 h. Sampling concluded before afternoon milking at 17:00 h. Rumen ammonia (NH3) concentration peaked and pH fell to its lowest value between 7 and 9 h after cows commenced grazing. In experiment 2, 42 cows were stratified into three groups of 14 cows on the basis of milk and component yield, age, liveweight and calving date and randomly allocated within blocks to one of three treatments. Over a 10-day adjustment period and a 21-day experimental period, cows were given their daily pasture allocation of 13.5 kg DM/cow (above 5 cm stubble height) either after morning milking, and fed crushed barley grain at morning and evening milking in the ratio of 3:1 kg/cow (Synch) or 1:3 kg (Asynch) or given their daily allocation of pasture after afternoon milking with barley fed at a ratio of 3:1 kg/cow (PM) in the morning and evening milking. Based on the results of experiment 1, the feeding schedule for the Synch group was predicted to more closely synchronise the availability of N from pasture and readily fermentable carbohydrates (RFC) from concentrates, in the rumen, compared to the Asynch cows. The comparison between Synch and PM groups was to determine the relative importance of ‘synchrony’ compared to feeding pasture (afternoon) high in water soluble carbohydrates (WSC) content. The WSC content of ryegrass pasture sampled after morning milking (when the Asynch and Synch groups received their new pasture allocation) was 70 and 74 g/kg DM, respectively, compared to 124 g/kg DM for samples taken after afternoon milking (when the PM group received a new pasture allocation). Pasture dry matter intake (DMI) and in vivo digestibility of individual cows was determined by use of alkanes as inert markers, whilst group DMI was determined from pre- and post- grazing pasture mass estimated by rising plate meter. Rumen samples were taken by stomach tube at 0, 3, 7 and 11 h after grazing commenced in the morning on the last day of the experiment. On this same day the % of cows grazing was recorded at 10-min intervals. Nitrogen intake (733 g N/cow/day) was 280% above requirements and this resulted in rumen NH3 concentrations being three times higher than the concentration at which microbial protein synthesis (MPS) would be expected to become limited. Milk (25.1, 24.3 and 26.8 l/ cow/day; ( P = 0.05)) and protein (809, 794 and 850 g/cow/day; P < 0.05) yield for the Asynch, Synch and PM groups, respectively, differed significantly. Pasture DMI was not significantly different ( P > 0.05) but DM digestibility was ( P < 0.001) at 84% for the mean of the Asynch and Synch cows compared to 88% for the PM cows, respectively. The results of the present study indicate a substantial benefit of feeding cows pasture with a high WSC content. This can be achieved by giving the cows their daily allocation of pasture after afternoon milking in the knowledge that WSC accumulates during the day and that over * Corresponding author. Tel.: +61-2-9351-1635; fax: +61-2-4655-2374. E-mail address: [email protected] (W.J. Fulkerson). 0301-6226/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0301-6226(03)00122-2
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70% of the intake of pasture by cows will be consumed in the first 3 – 4.5 h of grazing a fresh pasture allocation. Under the conditions of this study there was no production benefit from attempting to synchronise the availability of N and RFC in the rumen by timing of WSC supplementation. This presumably indicates that MPS was not limiting the daily milk yield of 25 – 26 l/cow when these cows consumed high N ryegrass pastures. D 2003 Elsevier B.V. All rights reserved. Keywords: Water soluble carbohydrates; Dairy cows; Short-rotation Italian ryegrass; Nutrient synchrony
1. Introduction Nitrogen (N) in grazed herbage is used by dairy cows to synthesise milk protein with an apparent efficiency as low as 20% (Van Vuuren, 1993), and this is often because the non-structural carbohydrate (NSC):rumen degradable protein (RDP) ratio is below microbial requirements. Whereas, Hoover and Stokes (1991) suggest a NSC:RDP ratio of 2.0 as being optimal for efficient microbial protein synthesis (MPS), the ratio can be ten times lower than this in N-fertilised pastures (Trevaskis and Fulkerson, 1999). The theoretical energy cost of detoxifying excess ammonia (NH3) to urea and excreting it in urine is between 0.035 and 0.052 MJ metabolisable energy (ME)/g N excreted (Lobley and Malino, 1993; NRC, 1989). Reducing the quantity of N fertiliser applied to pasture is a financially viable approach to improving the efficiency of N utilisation by dairy cows in some European countries where producers have monetary penalties imposed for excessive N loss to the environment (Mayne and Peyraud, 1996). However, the resultant lower pasture yield makes this an unacceptable option for Australian dairy farmers. The alternative approach is to improve the utilisation of pasture N in the cow by increasing the availability of readily fermentable carbohydrates (RFC) in the rumen. Previous studies by Trevaskis et al. (2003) demonstrated that the effectiveness of feeding ‘carbohydrates’ in terms of a milk production response depended on whether they were degraded in synchrony with N from pasture, in the rumen. However, the results of these studies, and those of Sinclair et al. (1993, 1995) and Witt et al. (1999, 2000), who also showed apparent benefits of synchronising N released from the basal diet with supplemental NSC, were confounded by the need to feed different sources of carbohydrate in the synchronised and asynchronised diets. Likewise, studies claiming no benefits associated with synchrony
were also confounded by different carbohydrate types (Cecava et al., 1991; Casper et al., 1999). Studies by Trevaskis et al. (2001) demonstrated that MPS could be significantly improved in sheep fed a diet in which the rumen availability of RFC and N were synchronised. In this case, a single source of carbohydrate (sucrose) was used and fed at different times in relation to NH3 concentration in the rumen. Consequently, there is reason to believe that the utilisation of pasture N could be improved in dairy cows by feeding NSC at a time coinciding with peak rumen NH3 concentration in the rumen. Studies by Van Vuuren et al. (1986) have shown rumen NH3 concentration to peak approximately 7 h after dairy cows commenced grazing ryegrass pastures. If, as is common practice, cows are given a new break of pasture immediately after morning milking, the NH3 concentration in the rumen should peak during the afternoon milking when cows can be fed NSC-rich supplements to effect synchrony. The objective of the present study was to schedule feeding of highly fermentable carbohydrate supplements and high-N pasture in such a way that the availability of N and NSC would be more synchronised in the rumen. This response could be compared to the benefits of simply feeding pasture in the afternoon which has a higher WSC:RDP ratio. Water soluble carbohydrate concentrate in ryegrass peaks at 2 – 4 h before sunset (see Fulkerson and Donaghy, 2001). Trevaskis et al. (2001) have demonstrated that MPS is higher in sheep when fed pasture harvested at 17:30 h compared to 07:30 h.
2. Material and methods Two experiments were conducted in September 2001. In the warm temperate climate of coastal New South Wales, Australia (latitude 35j 30VS, longitude
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150j 0VE). In each study, Friesian cows in early lactation ( < 100 days as at the commencement of the study) were randomly stratified into treatment groups based on milk and component yield, liveweight, age and days since calving in the week prior to the experimental period in experiment 1, or the adjustment period in experiment 2. In both experiments, cows were offered 13.5 kg DM/cow/day of ryegrass (Lolium multiflorum cv. Concord) pasture above a 5-cm stubble height which was always in a vegetative state of growth. 2.1. Experiment 1: Grazing behaviour and changes in rumen NH3 and pH when cows grazed ryegrass pasture Forty two cows were allocated to one of seven groups of six animals and grazed as separate herds for 2 days. On day 1, the cows received 2 kg crushed barley/cow/milking but on day 2 no concentrates were fed at the morning milking. On day 2, a different group of cows was sequentially removed from grazing and a rumen fluid sample was immediately collected by stomach tube at 0, 1, 2, 3, 4, 5, 7 and 9 h after cows commenced grazing in the morning. Any samples visibly contaminated by saliva were discarded and a new sample taken. 2.2. Experiment 2: Grazing behaviour and change in rumen NH3 and pH in relation to feeding pasture and supplements Forty-two cows were allocated to one of three groups of 14 cows each and grazed as three separate herds and treated as shown in Table 1. The provision of the daily allocation of pasture in the morning and then a higher proportion of cerealbased concentrate in the afternoon (Synch group) was predicted to lead to greater nutrient synchrony in the
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rumen compared to feeding a higher proportion of concentrate in the morning (asynch group). The PM group was a direct comparison with the synch group of providing the daily allocation of pasture in the morning or the afternoon. Cows had access to their daily allocation of pasture for only 1 day with the interval between grazing based on pasture regrowth rate and related to the time taken for three new leaves/tiller to regrow (see Fulkerson and Donaghy, 2001) with the actual grazing interval varying from 12 to 35 days. Each group was grazed separately over a 10-day adjustment period followed by a 21-day experimental period. 2.3. Animal measurements 2.3.1. Milk and liveweight In experiment 2, milk yield and milk fat and milk protein content (composite morning and afternoon samples) were recorded at 4 day intervals using inline milk meters. Milk fat and milk protein content were measured by infra-red spectroscopy (Dairy Express, Armidale, New South Wales, Australia). Liveweight was recorded after morning milking prior to the adjustment period and on a weekly basis during the experimental period. 2.3.2. Rumen fluid The pH of rumen fluid was determined immediately after collection with a pH probe and a subsample strained through eight layers of standard muslin cloth and acidified with four drops of concentrated H2SO4 before it was frozen at 16 jC for subsequent NH3 analysis by the colorimetric method of McCullough (1967). 2.3.3. Grazing behaviour The percentage of cows grazing was recorded at 10-min intervals for 7 h from 07:00 to 14:00 h on day
Table 1 Barley (kg/cow/day) fed at morning and evening milking to treatment groups and given access to a new daily allocation of pasture after morning (07:00 h) or evening (16:50 h) milking in experiment 2 Treatment
Time of access to new daily allocation of pasture
Time of barley (kg/cow) feeding Morning milking
Afternoon milking
Asynch Synch PM
Morning Morning Evening
3 1 1
1 3 3
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2 of experiment 1 and for 17 h from 07:00 to 24:00 h on the last day of experiment 2.
Table 2 Mean nutrient content (g/kg DM) of pluck samples of ryegrass pasture from experiment 2
2.3.4. Estimation on pasture on offer and pasture intake The pasture allocated to each group on a daily basis was estimated from pre-grazing pasture mass on offer above a 5-cm stubble height (actual height), determined by a rising-plate meter (RPM) (Earle and McGowan, 1979) based on a pooled calibration curve for that type of pasture (Fulkerson and Slack, 1994). Basing pasture availability on a post-grazing residue above 5 cm related to previous studies (see Fulkerson and Donaghy, 2001) which indicated that cows could be grazed to a 5-cm stubble height to maximise pasture utilisation by stock without prejudicing pasture regrowth or milk yield/cow. In experiment 2, intake and in vivo digestibility of pasture consumed by six cows in each group representing the total group in terms of age, liveweight, days in lactation and milk component yield, was estimated using plant cuticular alkanes as inert markers (Mayes et al., 1986; Reeves, 1998). Thus, the cows were selected at random and dosed with controlled-release capsules containing C32 and C36 alkanes (Captec (NZ)) 7 days prior to the commencement of the experimental period to allow the alkanes to equilibrate within the rumen contents. Faecal samples were then taken immediately prior to afternoon milking for a period of 8 days and immediately frozen for 24 h, before freezedrying. Previous studies Reeves (1998) showed no significant difference in alkane content of faeces taken in the morning or afternoon. The faecal samples, as well as pre-grazing pasture samples (see Section 2.3.5), were extracted for alkane analysis according to the method of Mayes et al. (1986). The faecal samples were collected in the paddock after being excreted by the cows in over 90% of samples with the remainder collected per rectum. The nutrient content of pasture offered to cows in experiment 2 is shown in Table 2.
Nutrient (g/kg DM)
Treatment group Asynch
Synch
PM
Water soluble carbohydrate Crude protein Neutral detergent fibre Acid detergent fibre
70 308 410 190
74 328 390 170
124 268 390 180
2.3.5. Chemical analysis Pasture samples were plucked pre-grazing to the post-grazing height of previously grazed paddocks three times/week. The pasture samples, together with concentrates fed, were pooled on a weekly basis and then dried in a forced-draught oven at 80 jC for 48 h,
3.1.1. Grazing behaviour Most cows grazed pasture for approximately 2 h after they were given their new daily allocation of pasture in the morning (see Fig. 1). This was followed by a rapid decline in grazing activity and within a 0.5 h all cows had ceased to graze.
then ground through a 1-mm sieve and analysed. Ammonia was determined colorimetrically by the phenol –hypochlorite reaction adapted from the method of McCullough (1967) using sodium nitro-prusside as a catalyst; N by combustion using a Leco FP-428 analyser (Leco Corp., St. Joseph, MO, USA); for neutral detergent fibre (NDF) by the method of Van Soest and Wine (1967); for acid detergent fibre (ADF) using the method of Van Soest (1963) and for WSC by a modification of Technical Industrial method number 302-73A as described by Fulkerson et al. (1998). 2.3.6. Statistical analysis In experiment 1, the pH and NH3 concentration in rumen fluid over time was analysed by analysis of variance with differences between times being indicated by least significant differences (L.S.D.). In experiment 2, the data (milk and milk component yield data) was subject to ANOVA based on the general linear model with data obtained in the week prior to the adjustment day, used as covariate factors. Differences between individual means were tested by L.S.D. A linear regression equation, describing the relationship between urinary N excretion and N intake, was fitted.
3. Results 3.1. Experiment 1: Grazing behaviour and changes in rumen pH and NH3 when cows grazed ryegrass pasture
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daily pasture allocation in the afternoon (PM group) compared to the two groups given their allocation in the morning (Asynch and Synch). 3.2.2. Grazing behaviour Grazing activity, expressed as the summation of % cows grazing each hour, was similar for all groups (see Table 3) but the diurnal pattern of grazing differed. Reference to Fig. 3 indicates that grazing activity was 39% higher after afternoon milking in the PM group than for the other groups at that time of day.
Fig. 1. Rumen ammonia concentration (mg/l) (a), rumen pH (b), and the percentage of cows grazing (c) at specified times after being given their new daily allocation of ryegrass pasture in experiment 1. L.S.D. ( P = 0.05) for time of sampling: NH3 = 28; pH = 0.11.
3.1.2. Rumen status The NH3 concentration in the rumen of dairy cows rose rapidly, almost doubling ( P < 0.001) within 2 h from the time they commenced grazing. The highest rumen ( P < 0.001) NH3 concentration was in samples taken at 5 and 7 h after grazing had commenced. Rumen pH fell to reach its lowest value between 5 and 9 h from the time of commencement of grazing (see Fig. 1). 3.2. Experiment 2: Grazing behaviour and changes in rumen pH and NH3 in relation to time of feeding pasture and supplements 3.2.1. WSC content The WSC content of afternoon pasture was 52 g/kg DM higher than morning pasture and this led to an in vivo digestibility 8% units higher for cows given their
3.2.3. Rumen status There was a significant ( P < 0.001) effect of treatment on rumen NH3 concentration (See Fig. 2). The NH3 concentration in the PM group was 46 and 37% lower ( P < 0.001) at 3 (174 F 20 and 323 F 24 (mean F S.E.) mg/l) and 7 (212 F 24 and 337 F 33 (mean F S.E.) mg/l) h, respectively, after commencement of grazing than the Synch group. In contrast, the NH3 concentration of the Synch group was 39% lower ( P < 0.001) than the PM group 3 h after evening grazing commenced (341 F 42 and 208 F 36 (mean F S.E.) mg/l, respectively). The mean rumen NH3 concentration of PM cows over the 17h monitoring period was 17% lower ( P < 0.10) than the mean of the other two groups (234 F 23 and 277 F 24 (mean F S.E.) mg/l for the PM, and the other two groups, respectively). There was no significant treatment effect on rumen pH (7.04 F 0.04 (mean F S.E.)), however, there was a significant effect ( P < 0.001) of time on rumen pH with the lowest values at 7 h after commencement of grazing in the morning (6.89 F 0.06 (mean F S.E.)) and 3 h after evening milking (6.7 F 0.06 (mean F S.E.)). Rumen pH was highest at 0 h (7.5 F 0.06 (mean F S.E.)) after grazing commenced. Table 3 Grazing activity (summation of % cows grazing at each hour) recorded at 10-min intervals in experiment 2 Time of day (h)
Treatment groups (summation of % cows grazing at each hour) Asynch
Synch
PM
07:00 – 15:00 16:00 – 24:00 Total
564 379 944
532 394 926
373 539 912
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Fig. 2. Rumen ammonia concentration (mg/l) (a) and rumen pH (b) at specified times after Asynch (.) Synch (n) and PM (D) cows were given their daily allocation of ryegrass pasture in experiment 2. L.S.D. ( P = 0.05) for the time of sampling were: NH3 = 89 mg/l; pH 0.17.
3.2.4. Production, intake and N excretion The milk and component yield is shown in Table 4.
Milk yield was 2.1 l/cow/day higher ( P < 0.001) in the PM cows than the mean of the other two groups. The protein yield of the PM group was also
Table 4 Mean milk (l/cow/day) and component (g/cow/day) yield and pasture nutrient intake for cows grazing ryegrass in experiment 2 Parameters
Milk yield (l/cow/day) Milk fat (%) Milk fat yield (g/cow/day) Milk protein (%) Milk protein yield (g/cow/day) Liveweight change (kg/cow/day) Pasture dry matter intake (DMI) (kg/cow/day)a Pasture (kg/cow/day)b Pasture DMI (kg/cow/day)c Digestible dry matter (DDM) DDM intake (kg/cow/day) N intake (g/cow/day)d
Treatment group Asynch
Synch
PM
25.1 3.84 936 3.28 809 0.25 14.7 13.0 13.7 0.74 11.0 791
24.3 3.54 874 3.25 794 0.24 14.7 13.0 13.8 0.74 11.0 836
26.8 3.39 904 3.19 850 0.6 16.4 13.2 13.7 0.82 13.6 835
** Level of significance P = 0.06. a Estimated by faecal and pasture alkane analysis (Reeves, 1998). b Estimated by rising plate meter (RPM) for the period during which intake was estimated by alkane analysis as per above. c Estimated by RPM over the whole experimental period. d Including concentrate N.
L.S.D. ( P = 0.05) 0.8 0.14 NS 0.06 36 0.14 NS NS NS 0.03 2.9** NS
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significantly higher ( P < 0.01) despite a small, but significantly ( P < 0.02) lower milk protein content. There were no differences in milk fat yield between groups (913 F 14 (mean F S.E.) g/cow/day), although the milk fat content of the PM group was significantly lower ( P < 0.001). Cows in the PM group also gained significantly more ( P < 0.001) body weight. The pasture DMI of individual cows, estimated by alkane technology, was not significantly different between groups (15.3 F 2.1 (mean F S.E.) kg DM/ cow/day), however, the DDMI of pasture consumed by cows in the PM group was significantly higher ( P < 0.06) than that observed in the other groups (13.6 F 1.0 and 11.0 F 0.5 (mean F S.E.) % DM, respectively) and this reflected the substantially higher in vivo dry matter digestibility (DDM) of 82% compared to 74%, respectively. Nitrogen intake (733 F 23 (mean F S.E.) g/cow/day) was not different between groups but was 280% above recommendations (178 g N/cow/day; SCA, 1990), assuming an N use efficiency of 70% (SCA, 1990).
4. Discussion Friesian milking cows given access to their daily pasture allocation after afternoon milking produced 2.5 l milk/cow/day, 56 g milk protein/cow/day and gained 0.36 kg liveweight/cow/day more than the Synch ‘control’ cows given access to their daily pasture allocation immediately after morning milking. This was despite the fact that the Synch group was fed concentrate and pasture at times predicted to synchronise more closely the availability of N and RFC in the rumen. Milk fat, and to a lesser extent milk protein content, was lower in the PM cows, however, this appears to have been a dilution effect given their higher milk protein yield. The increased productivity of the PM group was associated with an increase of 8% units in pasture DDM and 2.6 kg/cow/day higher DDMI by these cows. These findings are in line with the previous observations (Van Vuuren et al., 1986; Fulkerson and Donaghy, 2001) that afternoon pasture, containing more WSC than morning pasture, should provide forage of higher ME and should therefore be more appropriate in terms of ruminal availability of N and
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RFC (Hoover and Stokes, 1991). In this study, afternoon pasture contained 52 g/kg DM units more WSC than morning pasture. The relatively long hours of unimpeded solar radiation (maximising photosynthesis), accompanied by relatively low temperatures (minimising respiration and growth) in the present study, were conditions conducive to a high rate of WSC accumulation in ryegrass (Fulkerson and Donaghy, 2001). Two recent studies confirm the beneficial effect of high WSC content of pasture on milk production in dairy cows. In a similar study to our own, Orr et al. (2001) reported a 1.3 l milk/cow/day (23.1 vs. 21.8 l milk/day cow) increase in milk production for cows, given their daily allocation of pasture in the afternoon when WSC content of perennial ryegrass pasture was 20.4% DM, compared to morning when it was only 17.5% DM. In vivo digestibility was not measured in this study. In the second study by Miller et al. (2001), milk yield was significantly increased from 12.6 to 15.3 kg/ cow/day when Holstein –Friesian cows were fed a perennial ryegrass variety with a high WSC content (165 g/kg DM) compared to low WSC content (126 g/ kg DM). In this study the DM digestibility of the high WSC variety was 0.71 g/g DM compared to 0.64 g/g DM for the low WSC variety and this is similar to the results of the present study. Interestingly, in this study the N efficiency was significantly increased in the cows fed a high WSC variety of ryegrass. Both the studies of Orr et al. (2001) and Miller et al. (2001) used small numbers of animals. Previous studies (Fulkerson and Dobos,unpublished data), have shown that 60 –90% of the daily pasture allocation was consumed in the first 4 h of grazing. In experiment 2 of the present studies, the PM group’s afternoon grazing activity lasted 39% longer than that observed in the remaining groups at that time, given their daily allocation in the morning (Table 3) and this presumably equated to a greater proportion of pasture DMI consumed at a higher WSC content. The lowest rumen pH in PM cows in experiment 2 was recorded 3 h after afternoon milking, presumably reflecting the higher intake of WSC from pasture. This is supported by the observations of Van Vuuren et al. (1986) who also observed that the rumen pH of dairy cows grazing ryegrass pasture was lowest after evening milking.
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Although PM cows received the same quantity of concentrates as the Synch cows at morning milking, rumen NH3 levels of the PM group were 37 and 39% lower than their Synch counterparts at 3 and 7 h following morning milking, respectively, and this presumably reflects their lower pasture N intake over this period. Of further interest was the lack of difference between the PM and Asynch rumen NH3 levels 3 h after afternoon milking despite the latters’ increased grazing activity and presumably pasture intake. Hence, we can speculate that the more stable NH3 levels in the PM group resulted from a higher intake of WSC. This same phenomenon has previously been demonstrated in sheep (Trevaskis et al., 2001), when urinary allantoin excretion was increased by as much as 50% in sheep fed kikuyu grass harvested in the afternoon compared to morning. However, the increased milk production (2.5 l/cow/day) and liveweight gain (0.36 kg/cow/day) observed in the PM group in experiment 2 could be accounted for entirely by their 2.6 kg/cow/day increase in DDMI. This would provide an additional 31 MJ ME, more than the extra 26 MJ ME predicted to be required (assuming 4.63 MJ ME required to produce 1 l of milk and 34 MJ ME for 1 kg of liveweight gain; SCA, 1990) for the extra milk production. The first objective of the present study was to test the hypothesis that feeding RFC at a time when rumen NH3 levels peaked would improve N-utilisation, MPS and consequently animal productivity. The benefit of synchronising N and RFC has previously been shown in sheep (Trevaskis et al., 2001) when timing of an intra-ruminal infusion of sucrose was controlled relative to a once-daily meal of pasture. In the present studies, rumen NH3 peaked at 7 and 9 h after the initial grazing bout and this confirms previous observations by Van Vuuren et al., 1986), and those by Khalili and Sairanen (2000) and Soriano et al. (2000) which were published subsequent to the present study. Based on the pattern of change in rumen NH3 concentration, we hypothesised that the degree of synchrony between the availability of pasture N and supplemental carbohydrates in the rumen would be greater in cows fed a high proportion (3:1) of their 4 kg/cow/day crushed barley at afternoon milking ( c 8 h after grazing commenced), than morning milking (when NH3 was lowest). In turn, MPS (Trevaskis et al., 2001), and possibly milk production, could be
expected to be increased. The results of experiment 2 (see Fig. 3.) indicate that rumen NH3 was lowest after afternoon milking in the Synch group indicating success in improving the synchrony between N and RFC. However, despite substantial reduction in rumen NH3 in the Synch group, there was no response in production associated with improved diet synchrony. There are several reasons why an improvement in diet synchrony may have failed to improve productivity under the conditions of the present experiment. Firstly, milk production averaged 24.7 l/cow/day for the Asynch and Synch groups and was probably not sufficient to require additional metabolisable protein (MP). The pasture N and ME content of the diet in experiment 2 would be predicted to be sufficient for cows producing up to 25 milk l/cow/day (Beever and Siddons, 1986). In confirmation of this, Kolver et al. (1998) detected no change in milk production or liveweight gain when MPS was increased by synchronising the release of carbohydrate-rich supplements with the availability of pasture N in the rumen. They also attributed this lack of response, in part, to an already sufficient supply of MP from the asynchronous diet for cows producing in excess of 30 l/cow/ day. Recent studies by Hongerholt and Muller (1998) and McCormick et al. (1999) also showed that provision of additional protein post-ruminally as supplemental undegraded protein, did not improve milk production in cows producing c 30 l/cow/day and supports the conclusions of Kolver et al. (1998). Consequently, any additional MPS arising from an improvement in diet synchrony would be expected to be catabolised and excreted. In fact, Peyraud and Delaby (2001) have shown that the MP supplied from good quality ryegrass pasture can support milk yields as high as 28 l/cow/day. Secondly, the CP intake of cows in the present study (732 g N/cow/day) amounted to 280% of their daily recommended requirements (178 g N/cow/day; SCA, 1990). Assuming ryegrass protein is 85% digestible in the rumen (Corbett, 1987), the 622 g N degraded there would require 7.7 kg of fermentable carbohydrates to provide what is considered as the optimal 1:12.5 ratio between N and RFC (Hoover and Stokes, 1991). As the diets of the Asynch and Synch cows provided only 4.9 kg of RFC, it is conceivable that the benefits (of diet synchrony)
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Fig. 3. Percentage of cows grazing in the PM (a), Synch (b) and Asynch (c) groups in experiment 2.
were negated by an extreme imbalance in the ratio of N to RFC. This was evidenced by the extremely high urinary N excretion of these cows. In experiment 2, there was nearly a unit of N excreted in the urine for each unit of N ingested. With such an excessive N intake, the timing of input of RFC to the rumen microbial population would have been irrelevant but the additional RFC could have benefited whenever it was given. The lowest levels of rumen NH3 recorded in experiment 2 (see Fig. 3) were at least three times higher than what is commonly considered necessary for efficient MPS ( c 50 NH3 –N mg/l; Slyter et al., 1979). It seems reasonable to speculate that the benefits of diet synchrony will become more pronounced as rumen NH 3 becomes marginal to rumen microbial requirements and this is substantiated in our results in sheep
(Trevaskis, unpublished data). This situation is unlikely to be experienced in cows grazing ryegrass pastures in a vegetative stage of growth and fed moderate amounts of cereal-based concentrate. The benefits of a simple adjustment in grazing management to provide the herd’s daily pasture allocation after afternoon milking rather than the morning seems very worthwhile. Whether there are benefits lost by going to a 24-h rather than 12-h allocation of pasture may need to be tested.
Acknowledgements The authors would like to thank Kim McKean and Madelaine Hall for their careful and diligent assistance. The financial support provided by the Dairy
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