6.2 Material and methods
6.4.4 Implications for digestion in the rumen
The degradation kinetics of boli material were studied using the in sacco method, and the soluble ‘A’ fraction is a measure of nutrient release by mastication. However, the extent of release by washing in sacco bags were substantially less than that washed through cheese cloth using the method of Boudon and Peyraud (2001) and presented in Table 6.6. There are two possible causes for this difference. Firstly, the calculation assumes that the quantity of NDF in the bolus is the same as the quantity of NDF in the feed eaten, which is unlikely as some fibre will be solubilised during ingestion. Burke (2004) demonstrated that the soluble fraction (A) of NDF for chicory, lucerne and ryegrass was 0, 40 and 21%, so while Boudon and Peyraud (2001) did not feel the assumption would introduce a bias, the values for soluble NDF observed by Burke (2004) and Chaves et al. (2006) were greater for ryegrass and lucerne than that assumed by Boudon and Peyraud (2001). Thus these calculated values may overestimate N loss. The second reason for the disparity was that the larger pore size in cheese cloth would have allowed more material to pass through when washing compared to the nylon bags which had a very small pore size of 35 microns. However, the trends for N release and N solubilisation determined by both methods were similar between the three forages evaluated and, thus the in sacco degradation results support the conclusion that lucerne herbage releases more N during comminution than chicory and ryegrass. This finding is largely due
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to the greater N concentration in lucerne, where most N is stored in weak leaf tissue that is susceptible to rupture (section 6.4.3). Released N is very rapidly hydrolysed in the rumen (Mangan, 1982; Weisbjerg et al., 1998), if this N is not used by the microflora it is excreted. The greater initial release of N from lucerne could result in poor utilisation N by cows, which may in part explain the greater excretion and high concentrations of N in urine of cows fed lucerne compared to cows fed diets without lucerne (Waghorn et al., under review).
In this study, DM degradation rate of masticated ryegrass (k = 0.22%/h) appeared to be more rapid than for lucerne or chicory (Table 6.6) and this value is greater than that for fresh macerated ryegrass observed in section 5.3.4 (0.04 %/h) and reported in the literature (Burke et al., 2000; Chaves et al., 2006). It is difficult to explain this data. One suggested reason is that the lesser proportion of DM in the A fraction of boli compared to macerated material meant that some of the potentially soluble material was degraded in the rumen, which would likely have been very rapid leading to the high k value demonstrated in this study. Or perhaps the greater amount of saliva added to ryegrass boli may have improved the hydration and microbial colonisation of ryegrass boli particles facilitating more rapid degradation. Further investigation would be required to support this.
Total potential degrability (P) of herbage DM and N was least in lucerne compared with chicory and ryegrass. These data support the findings of Burke (2004) (discussed in section 2.6), and are likely associated with the morphology and chemical composition of the herbage, i.e. that lucerne contains fibrous and lignified stem material whereas the other species did not. However, this study focussed on the effects of ingestive mastication on comminution and nutrient release, to what extent the animal compensates for the lesser degrability of lucerne herbage by subsequent ruminative chewing was not evaluated.
Care is required if the intention is to extrapolate these data to cattle grazing outdoors. Rate of DM intake was greater in this study than demonstrated by others investigating lactating dairy cows grazing perennial ryegrass and white clover (mean 60 vs. 40 g DM/min) (Marotti, 2004; Rutter et al., 2004). The disparity is likely associated with the hunger state of the animals used, as they had
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been fasted overnight, and because they were fed indoors cut fresh herbage which would have facilitated higher intake rates. Boudon et al. (2006) compared the ingestive behaviour and its effect on perennial ryegrass herbage that was fed indoors or grazed by cattle and reported that cattle fed indoors had 128% greater DMI rate, heavier boli with less saliva added per gram of DM, larger particle size in the boli and reduced loss of nitrogen during ingestive mastication when compared with the cattle grazing. Therefore, it is likely that greater release of N during ingestive mastication would occur from these forages when grazed due to the additional damage to herbage tissue from the forces applied during prehension and biting of herbage.
6.5
Conclusion
Evaluation of swallowed boli showed that the extent of comminution of herbage and nutrient release after ingestive mastication differed between chicory, lucerne and ryegrass. Boli from cows ingesting chicory contained more herbage with larger particles and less saliva than boli from cows ingesting ryegrass. The proportion of nutrient release was not solely related to the extent of comminution of the feed, but rather the toughness of the herbage and the amount of nutrient. Soluble carbohydrate is more readily released from the feed than nitrogen, and variation in the amount of released nutrient exist between forages. This may go some way to improve our understanding of how forages are utilised.
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7
Simulating ingestive behaviour, N metabolism and N
excretion of cows grazing swards fertilised with increasing
rates of nitrogen
7.1
Introduction
The efficiency of nitrogen (N) utilisation in dairy cows is low. That is, typically only 15 – 30 % of ingested N is captured in product (i.e. body tissue or milk) and the remainder excreted as waste, mostly in urine (Astigarraga et al., 1994; Delagarde et al., 1997; Astigarraga et al., 2002). Nitrogen that is excreted in urine, and on to pasture, is a source of environmental pollution because the concentration of N in a urine patch often exceeds plant capacity for uptake (up to 1000 kg N/ha (Di and Cameron, 2007)). Then, that urinary N is largely converted to nitrate in the soil, which is highly susceptible to leaching (section 2.2). Several studies have demonstrated that N excreted in urine is associated with N intake of livestock (Kebreab et al., 2001; Tas et al., 2006), which is a function of the N concentration in herbage and the amount of herbage eaten (Waghorn et al., 2007). As demonstrated in section 5.4.2, reducing N fertilisation rate decreases herbage N concentration but has little effect on soluble carbohydrate and structural fibre concentrations in non-legume species. It is for this reason that reducing the rate of N fertiliser applied to forage swards has been proposed as a means for improving N use efficiency (NUE) and reducing urinary N excretion from grazing ruminants (Hoekstra et al., 2007a; Dijkstra et al., 2011).
Conversely, reducing N fertiliser applied to swards can decrease herbage growth rate and sward height (section 5.4.1, (Belesky et al., 2000; Binnie et al., 2001; Martin et al., 2017). Furthermore, in some forages (e.g. ryegrass), reducing N fertiliser increases the toughness of herbage (section 5.4.1). These are important considerations for farmers as reductions in sward DM production can influence stock carrying capacity, and a reduction in sward canopy height and increases in herbage toughness can alter livestock grazing behaviour and intake (Ungar and Noy-Meir, 1988; Laca et al.,
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1992). Thus, an optimal fertiliser rate for forage species is sought that balances herbage productivity with livestock N use efficiency.
Key elements required to measure the NUE of livestock fed different forages, such as feed intake and ingestive behaviour, production, and N excretion, are difficult and expensive to measure (Gregorini et al., 2013a). Therefore, the use of mechanistic models to predict these processes is a valuable exercise, and a critical first exploratory approach to assess the potential merits of different forages. The purpose of this study was to use the ‘MINDY’ dairy cow model to explore outcomes of increasing fertiliser N applied to swards of four common forage species on N metabolism and partitioning, and the ingestive behaviour of grazing dairy cows. It was hypothesised that the optimal N fertiliser rate to reduce urine N excretion and maintain or improve milk production from dairy cows would vary among forages. The aim of the exercise was to explore how varying N fertiliser rates applied to swards of chicory, lucerne, plantain and ryegrass, would be predicted to effect dairy cow production and N excretion.