Selection for crude protein content in Phalaris tuberosa L. II. Field studies on selected lines

Abstract

Two Phalaris tuberosa lines, previously selected in a controlled environment for high or low Kjeldahl nitrogen content (percentage nitrogen, dry weight basis) in whole tillers at the heading stage, were grown in the field both as spaced plants and in swards. Herbage from spaced plants was examined for nitrogen content and for in vitro organic matter digestibility (IVDOM) at five stages during the first year of growth. Differences in percentage nitrogen between the high nitrogen and low nitrogen lines were maintained at each growth stage (vegetative, preheading, heading, anthesis, mature). At heading, the nitrogen content of whole tillers in the high nitrogen line (1.42%) was 30 % greater than that of the low nitrogen line (1.09 %), and there was considerable residual genetic variation for this character in each line. Part of the difference between the lines at this stage was due to alterations in the relative proportions of tiller components (leaf, sheath, stem, and head) which differed markedly in nitrogen concentration, and in particular to changes in the leaf/stem ratio (0.62 and 0.43 for the high and low nitrogen lines respectively). However, much of the difference was due to changes in the nitrogen content of tiller components, the high nitrogen line having consistently greater contents than the low line. The weight of whole tillers and of all tiller components was negatively related to the nitrogen content. The low nitrogen line was marginally superior in IWOM to the high nitrogen line at most growth stages, and at the mature stage was superior by 4.6 digestibility units. In swards, the two selected lines and a control (unselected) line were subjected to two defoliation frequencies (five and 10 cuts per year) and two levels of applied nitrogen fertilizer (0 and 280 kg N/ha/year, applied in the form of urea after each defoliation), commencing 3 months after sowing in the spring. Measurements of herbage yield and quality (percentage nitrogen, IVDOM) were taken during the year following commencement of treatments, and herbage yield measurements were continued for a further 8 months on the infrequently defoliated swards. Phalaris in the high nitrogen line plots had consistently higher nitrogen concentrations and lower IVDOM levels, recovered more of the nitrogen fertilizer, and showed the greatest herbage yield responses to applied nitrogen. However, over the range of treatments imposed, the low nitrogen line gave the greatest herbage yields.