The release of nitrogen and carbohydrate from herbage during comminution

Abstract

In forage-based livestock production systems, dietary N is often in excess supply. In addition, the efficiency with which ruminants utilise dietary N utilisation is low, leading to substantial losses of N to the environment, particularly in urine. There are considerable political and societal pressures for farmers to reduce N losses, yet still maintain productivity. To enable reductions in N excretion and improve N utilisation, the flow of N through ruminants needs to be understood. While much is known about the ruminal degradation of dietary N, less is known about the release of N from herbage during ingestion. As herbage N is readily fermentable, the extent of release of N during ingestion has implications for ruminal ammonia concentration and concomitantly, N excretion. The aims of this PhD research programme were to quantify the variation between forages in N release during comminution of herbage, explore the characteristics of herbage that influence N release, and consider implications for N excretion. Two laboratory based experiments were conducted to examine the release of N during mechanical maceration of herbage, as a proxy for the effect of ingestive mastication on herbage by dairy cows. The first compared N release from herbage of five commonly used forages: perennial ryegrass (Lolium perenne L.), white clover (Trifolium repens L.), lucerne (Medicago sativa L.), chicory (Cichorium intybus L.), and plantain (Plantago lanceolata L.), collected in February, May, August, and November of 2015. The amount of N (as crude protein, CP) released during maceration of herbage varied 15-fold, with lucerne releasing the most (11.5 g/100 g DM) and plantain the least (0.8 g/100 g DM). Crude protein release from forages ranked as follows: lucerne > ryegrass = clover > chicory > plantain. Investigations revealed that no single characteristic (i.e. herbage physical properties or chemical composition), measured in this study, was strongly associated with CP release of all forages evaluated. However, forages that tended to have greater herbage CP and non-protein N (NPN) concentrations tended to release more N during maceration. This finding led to the second laboratory experiment that explored the effect of increasing rates of N fertiliser application (0, 100, 200 and 350 kg N/ha/y) on herbage characteristics and release of N. Herbage was collected from swards of perennial ryegrass, lucerne, chicory and plantain in May (autumn) and November (spring) 2015. Increasing N fertiliser rate applied increased leaf length (up to 140%), herbage mass (up to 320%), and CP and NPN concentration in perennial ryegrass, chicory and plantain. Increasing N fertiliser rate doubled the amount of CP released from ryegrass herbage across both seasons, and chicory in autumn, but increased CP release from plantain 6-fold in autumn. Increasing N fertiliser rate applied to ryegrass, chicory and plantain swards also reduced the ratio of fermentable carbohydrate to CP in herbage, which is thought to increase loss of N in urine. The greatest increase in the amount of CP released and greatest decline in the ratio was observed at N rates exceeding 200 kg N/ha/y, coinciding with reductions in herbage mass and leaf growth response to increased rates N fertiliser application. This study suggests rates equivalent to 200 kg N/ha/y ought to be the upper limit of N fertiliser applied to swards of ryegrass, chicory and plantain. The third experiment conducted sought to determine the effect of ingestive mastication by mature dairy cows on fresh cut herbage from three forages (perennial ryegrass, lucerne and chicory). The ranking of herbages for CP release was similar to that determined in the first two laboratory based experiments, where 7.5, 5 and 3 g CP/100 g DM was released from lucerne, perennial ryegrass and chicory, respectively, during ingestive mastication. Release of CP was associated with the concentration of CP in herbage (R2 = 0.53) supporting the findings of the previous laboratory experiments. When N intake rate was calculated, delivery of N to the rumen was greatest in cows fed ryegrass (0.79 g N/min) compared with lucerne and chicory (0.54 and 0.22 g N/min), which would have implications for microbial utilisation in the rumen. Despite the slower intake rate of chicory compared to lucerne and ryegrass (46.5 vs. 64.7 and 70.7 g DM/min) the extent of comminution was less in chicory compared to the other forages (36 vs 44 and 41% of particles reduced to < 4 mm), but the difference was not associated with any of the herbage characteristics measured in this study, nor was comminution shown to be associated with CP release when data from the three forages were combined for analysis. This research programme concluded with a simulation study exploring milk-solids (MS) production and urinary N excretion from cows grazing swards of perennial ryegrass, lucerne, chicory and plantain grown under increasing rates of N fertiliser (0, 100, 200, 350 and 500 kg N/ha/y) using the data collected in the second experiment. Predictions of MS production and N excretion by the MINDY model for cows grazing chicory, ryegrass or plantain, suggests that rates of N fertiliser of 200 kg/ha/y could be the optimal point for MS production before total daily N excretion increases substantially. As at this application rate, DMI and MS production predicted for cows grazing chicory was near maximum (15.9 kg DM/cow/d; 1.24 kg MS/cow/d) with little gain achieved through additional N fertiliser but a 100 g N/cow/d increase in urine N excretion (42% increase) was predicted as N applied increased from 200 – 500 kg N/ha/y. In the case of plantain and ryegrass, while MS production was predicted to increase beyond the 200 kg N/ha/y fertiliser rate, the amount of N excreted also increased by 65 and 103 g N/cow/d, respectively (50 and 64%). The simulations also suggest that the addition of N fertiliser to lucerne swards has little effect on MS but increases urine N excretion, and is thus not recommended. Overall, the findings of this research program confirm the hypothesis that the release of N from herbage during comminution differs between forages. This finding can aid explanation of literature that has demonstrated differences in the N excretion by cows fed diets of different forages, and can help understanding of N flow in ruminants. The study also suggests that in autumn, there is potential to manipulate N release from perennial ryegrass, chicory and plantain herbage during maceration and to reduce N excretion from cows grazing diets of these forages through altering the amount of N applied to these swards. This information could be used to inform fertiliser management practices, and development of diets for dairy cows to reduce urinary N loss.