Pathways for improving the nitrogen efficiency of grazing bovines

Livestock production has been identified as a major source of nitrogen (N) losses in agro-ecosystems. N excreted in dung and urine contributes to environmental N pollution either as ammonia and N oxides in air, or as nitrate in soil and ground water. Therefore, it is important to reduce N output through animal excretions by improving N utilisation by the animal. Bovine N utilisation can be increased substantially through changing the composition of the diet. In many parts of Europe, a large proportion of the bovine’s diet consists of grass taken up by grazing. Manipulating the nutritional composition of grazed grass poses a complex challenge, since it is hard to control the diet under grazing as this depends on grassland management and environmental factors. The objective of this research was to investigate the efficacy of grassland management tools for manipulating herbage quality and to assess the subsequent effect on the N efficiency of grazing cows. In the literature review, three pathways were identified through which more efficient N utilisation by grazing bovines can be achieved by manipulation of the chemical composition of the grass forage: 1) matching protein supply to animal requirements, 2) balancing and synchronising carbohydrate and N supply in the rumen, and 3) increasing the proportion of rumen undegradable protein (RUP). Under grazing conditions, grassland management tools, such as the length of the regrowth period, defoliation height, fertiliser N application rate, and growing high-sugar grass cultivars, are the main tools to manipulate herbage quality and subsequent bovine N efficiency. A field experiment was carried out to study the effect of those grassland management tools on the chemical composition of lamina and sheath material. These results were used to design a model for predicting the efficacy of herbage management tools for affecting the quality of herbage ingested by cattle under strip-grazing management. This model was validated and connected to the Cornell Net Carbohydrate and Protein System model in order to evaluate the effectiveness of a) the grassland management tools and b) the herbage quality pathways on the N utilisation of grazing dairy cows. Within the modelled scenario the concentration of crude protein (CP) in the ingested dry matter (DM) was the main factor affecting N utilisation. Model predictions indicated that herbage should be managed to achieve a CP concentration of 130−150 g / kg DM in order to maximise the efficiency of N utilisation for milk production and minimise the proportion of N excreted in urine. Both N application rate and rotation length were shown to be effective tools for affecting the CP concentration of the intake and subsequent cow N utilisation. However, there was no effect of the high-sugar cultivar and defoliation height on cow N utilisation. Assessment of the effectiveness of the three herbage quality pathways for improving bovine N utilisation resulted in the following conclusions: 1) N utilisation is strongly related to the daily N intake (g / day), however, this seems more connected to the N concentration of the ingested DM (g / kg DM), rather than the actual daily N intake. Therefore, the effect is more related to the balance between energy and N (pathway 2). 2) The balance between N and energy is the most important herbage quality factor for improving bovine N utilisation. In contrast, the synchronisation between the release of energy and N seems to have little effect. 3) The proportion of protein in the form of RUP is not much affected by the herbage management tools, and is therefore not an effective pathway for improving the N utilisation of grazing cows. It is recommended that the model will be extended to include a herbage yield and intake component. This would allow the model to be used to design herbage management systems to optimise N utilisation on a yearly basis.