Pathways to improving the N efficiency of grazing bovines

Livestock production has been identified as a major source of nitrogen (N) losses. Therefore, it is important to reduce N output through animal excretions by improving N utilisation by the animal. The objective of this paper is to identify pathways for producing grass-based diets that maximise bovine N utilisation during grazing, based on literature on the interface of plant and animal sciences. The focus is on Western-European perennial ryegrass-based systems under rotational grazing and both beef and dairy production systems are considered. Three pathways have been 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). Matching the diet requirements of grazing bovines through herbage manipulation encompasses the manipulation of carbon (C) and nitrogen (N) contents of growing herbage. These C and N contents vary both spatially within the grass sward and over time. Under grazing conditions, grassland management tools, such as the length of the regrowth period, grazing intensity, fertiliser N application rate and herbage variety are the main pathways to manipulate C and N dynamics. Regrowth length, N application rate and high sugar varieties were shown to be the most promising grassland management tools with respect to manipulating herbage quality and subsequent bovine N efficiency. However, these management tools are interrelated and may show adverse effects on production. Due to the complex nature of interactions, modelling is essential in order to quantify and predict the effect of any combination of herbage management tools under specific circumstances. Areas in which additional research is required are the fractionation of N compounds in herbage as affected by herbage management, and the effect of high sugar varieties on bovine N efficiency under a range of herbage management combination