Nitrogen use efficiency in intensive grassland farming

This thesis describes the effects of fertilizer nitrogen on herbage yield under rotational and continuous grazing of perennial ryegrass swards with beef cattle and dairy cows, and under cutting only on both a sand and a silty loam soil. Furthermore effects are described of nitrogen input and grassland management on yield of perennial ryegrass swards after severe winters on both soils and the effects of dung and artificial urine on nitrogen uptake and herbage accumulation on a sand soil.

Differences in both the apparent nitrogen recovery and the response of grassland production to fertilizer nitrogen applied could be related to treatment (cutting versus grazing), soil type (loam versus sand), length of the growing season, the amount of soil inorganic nitrogen in spring, sward quality and, in case of grazing, recycling of excretal nitrogen. The calculated economical optimum fertilizer application rate was on average 430 and 5 10 kg N ha ^-1yr ^-1for 4-weekly cutting on the sand soil and the loam soil, respectively. Under grazing and on a wholefarm scale (integrated grazing and mowing for silage) on the sand soil the optimum rate was some 200 kg ha ^-1less. Under grazing, especially on sand, nitrogen enhanced sward deterioration due to treading, poaching and especially urine scorching, leading to a loss of productivity. On loam, nitrogen had on average a much smaller deleterious effect. Tiller densities were highest under continuous grazing. Efficiency of use of ingested nitrogen varied from 23 to 31 % for dairy cows and was less than 10% for beef cattle at a fertilizer level of 250 kg N ha ^-1yr ^-1. Efficiency decreased with increasing N rates.

Divergent relationships were found between the level of nitrogen input in the preceding year and spring herbage growth after a severe winter. Differences were only temporary. In case of a negative relationship, reductions in first cut dry matter yields were smaller in frequently defoliated (weekly cutting and continuous grazing) than in less frequently defoliated swards (4-weekly cutting and rotational grazing). Positive effects of urine and dung on nitrogen uptake and herbage growth on the sand soil were only observed in a low N sward (250 kg N ha ^-1yr ^-1) and were confined to 15 cm from the edge of the dung and urine patches.