Nitrogen use efficiency and morphological characteristics of Timothy populations selected for low and high forage nitrogen concentrations

Improving timothy (Phleum pratense L.) N use efficiency (NUE) through genetic selection aims at producing greater or similar forage dry matter (DM) yields with less N fertilizer while maintaining N concentration close to the optimal level required for ruminant nutrition. Two populations of timothy, arising from divergent selection for high (N+) and low (N-) forage N concentration, and a reference population, 'Champ', were studied under controlled conditions with N rates of 1, 5, ill, and 20 mg N plant(-1) wk(-1). The populations Nand Champ produced more forage DM yield than Nf. This difference in forage production was the result of changes in biomass partitioning between shoots and roots because, at the whole plant level, no differences in total biomass (shoots + roots) were found. On the basis of total biomass, there were no population differences in NUE and N accumulation efficiency (NAE). For a given level of forage DM yield, N+ had a greater N accumulation than N- and Champ and, therefore, a greater N concentration. The greater forage N concentration of N+ was not due to a greater leaf N concentration but to a greater proportion of leaves. The population N+ also had a greater proportion of roots than N-. The forage insoluble N concentration of N+ was greater than that of N-, while NO3-N concentrations of the populations were similar. The population N+ had a greater tiller density and specific leaf area (SLA) than N-. Differences in forage DM yield and N concentration between two populations selected for low and high N concentrations were mainly due to the modification of C and N partitioning between shoots and roots, and between leaves and stems. Our results highlight the role of biomass partitioning in improving grass NUE or N concentration.