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a Agriculture and Agri-Food Canada, 2560 Hochelaga Bd., Sainte-Foy, QC, G1V 2J3, Canada. Contribution no. 639 Agriculture and Agri-Food Canada
belangergf{at}em.agr.ca
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, 10, and 20 mg N plant-1 wk-1. The populations N- and Champ produced more forage DM yield than N+. 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.
Abbreviations: DM, dry matter • LERleaf, leaf elongation rate of an entire leaf • LERlin, leaf elongation rate during the linear portion of leaf growth • LNAR, leaf N accumulation ratio • LWR, leaf weight ratio • N-, population selected for low forage N concentration • N+, population selected for high forage N concentration • NAE, N accumulation efficiency • NCE, N conversion efficiency • NUE, N use efficiency • SLA, specific leaf area
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