Herbage and animal production responses to fertilizer nitrogen in perennial ryegrass swards. I. Continuous grazing and cutting

The effects of fertilizer N application on herbage intake and animal performance under continuous grazing management with dairy cows, and on herbage accumulation under a weekly and an approximately 4-weekly cutting regime were studied in 1986-1988 in resown Lolium perenne cv. Wendy grassland on a silty loam soil in Oostelijk Flevoland, Netherlands. 250-700 kg N/ha was applied annually under grazing and from 0 to 700 kg N/ha was applied under cutting. At an assumed marginal profitability of 7.5 kVEM per kg N applied the optimum N application rate was on average 511 and 308 kg/ha per year for 4-weekly cutting and continuous grazing, respectively (1 kVEM = 6.9 MJ Net Energy for lactation). However, especially under grazing, there was a great variation in response to N between years which could be related to soil N availability, length of the growing season and sward quality. Throughout the experimental period the mean tiller density in the grazed swards was hardly affected by the level of N application. However, there were temporary differences in openness of the sward which increased with the level of N application, leading to a loss of productivity as a result of impeded N uptake. Herbage N was poorly converted into animal products. The average efficiency of use of ingested N at 250 kg N was 23%. Higher rates of fertilizer N decreased N use efficiency (19% at 700 kg N/ha per year) but markedly increased N excreted per ha