Reduction of nitrogen emissions from grasslands through improved nitrogen use efficiency of perennial ryegrass genotypes

The present paper summarizes the data of field and laboratory trials conducted at the Plant Research International in Wageningen, the Netherlands during the period 1999-2000. In this study, differences in nitrogen use efficiency (NUE) between populations of perennial ryegrass (Lolium perenne L.) are determined under limited level of nitrogen (N) availability. Each of 93 segregating entries was individually evaluated in a greenhouse in a hydroponics system aiming at 2.7-3.5 % N in herbage dry matter (DM). The population was examined for root and shoot regrowth, tillering, leaf extension and width in order to screen the genotypes with the most vigorous leaf area regrowth after defoliation. All the entries were also subject to a field study to assess their total N content. Five selections outperformed the corresponding mid-parent values of their source materials by 63-124 % in leaf area increase (LAI) and its components: leaf length (8-28 %), leaf width (8-16 %) and tiller number (16-72 %). Tiller number per plant was the most variable (cv = 29.8 %) component of the LAI index and had a stronger impact (r = 0.88) upon the DM productivity than did leaf length and width (r = 0.50 and 0.48, respectively). Leaf weight ratio of the selections was 3 % lower to 13 % higher than that as a mean of two parents. Harvestable DM yield of the selections was by 70-122 % higher relative to the mid-parent value, indicating a selection potential within the heterozygous original material. The selections revealed their superiority in reduced N contents (2.19-2.27 % in DM) that was reduced by 13 % in terms of the mid-parent value. Inverse relation (r = -0.26) established between the plant N concentration and harvestable DM yield suggests that improvement in nitrogen use efficiency under simulated grazing is mainly the result of better metabolic efficiency arising from more efficient use of proteins for leaf growth. Through plant breeding, perennial ryegrass cultivars can be created capable of higher growth rates at sub-optimum levels of available N