Manipulating dietary N in perennial ryegrass pastures to reduce N losses in dairy cows in spring

Abstract

Time of day and pasture regrowth interval were assessed as potential management strategies for reducing nitrogen (N) losses. Six groups of six lactating Friesian x Jersey cows were offered new pasture allowance either following morning milking onto pasture which had undergone a short (2 week) regrowth interval (AMS), or following afternoon milking onto pasture after a long (4 week) regrowth interval (PML). The experiment took place over nine days which included a four day adjustment period. Animals were offered the same pasture allowance and both diets contained the same metabolisable energy content. Dry matter intake was not affected by management treatment but milk production was significantly lower (P <0.f05) in the PML group (18.6 kg vs. 20.9 kg MS/cow/day for PML and AMS respectively). Nitrogen content of the forage was influenced by treatment resulting in a lower N intake in the PML, compared with the AMS group (513 vs. 563 g/cow/day respectively). Subsequently reductions in milk urea N and urinary N were recorded in the PML group. The results of this study indicate the potential for taking advantage of short- and long-term changes in plant composition to manipulate N intake and utilisation, but that animal requirements for protein need to be met first.