Differences in the nitrogen use efficiency of perennial ryegrass varietiesunder simulated rotational grazing and their effects on nitrogen recovery and herbage nitrogen content

Eight varieties of perennial ryegrass (six new varieties and two old ones) grown at five levels of applied fertilizer (100, 200, 300, 400 and 500 kg o f N ha(-1)) were cut monthly during two growing seasons (March to October i n 1997 and 1998) and their herbage dry-matter (DM) yield and nitrogen (N) c ontent were determined. Herbage leaf content and the N content of young ful ly expanded leaves were also measured in 1997, and monthly recovery of appl ied N was determined in both the first and second harvest years by using N- 15. The rank order of varieties was similar for annual yield of DM and N at all five fertilizer levels. Proportional differences between varieties in DM y ield were greatest in the first cut of each year, the late-heading candidat e variety Ba12151 out-yielding the old late-heading variety S23 by more tha n 70%. However, differences in annual DM yield were much more modest than i n early spring yield, up to 10% in 1997 and up to 21% in 1998. The relative ly small differences in total annual DM yield were attributed to only a sma ll proportion of the applied N being recovered during a single regrowth per iod, most of the remainder becoming available for uptake in subsequent regr owth periods. There were small but highly statistically significant differences among var ieties in the N content of their leaves, leaf N content being inversely rel ated to yield of DM and N. This lends further support to the hypothesis tha t the metabolic cost of protein synthesis and turnover is a key factor cont rolling genetic variation both in leaf yield and in annual DM and N yield u nder frequent harvesting. Seasonal variation in herbage N content was much greater than differences among varieties in mean N content over all harvest s. In May of both years at all applied fertilizer levels, herbage N content fell below the 20 g N kg(-1) DM level required by productive grazing anima ls.