The use of herbage nitrogen status to optimize herbage composition and intake and to minimize nitrogen excretion: an assessment of grazing management flexibility for dairy cows

Abstract

AbstractA framework for managing rotationally grazed pastures for dairy cattle which enables the cows’ energy and protein requirements to be met while simultaneously limiting the amount of N excreted in order to reduce N losses is described. The first objective is achieved by ensuring that lamina mass and the N concentration of herbage do not limit herbage intake or feeding value. The second objective is achieved by limiting N fertilizer supply or increasing the interval between defoliations to reduce the N concentration of herbage. Lower and upper thresholds for the N concentration of herbage and lamina mass were estimated from published data. The method is illustrated using two vegetative regrowths (beginning and end of spring) in a cutting experiment with two fertilizer treatments, 0 or 120 kg N ha−1 (−N and +N), and early or late cutting. Decreasing N supply led to a reduction in grazing management flexibility, i.e. the defoliation interval ranges which were compatible with the required sward characteristics (minimum lamina mass and N concentration of lamina) for herbage intake and to meet the protein requirements of dairy cows. Aiming for the upper threshold N concentration of herbage increased the minimum interval between defoliations only for the +N treatments. Nevertheless, grazing management flexibility remained the highest for the +N treatments.