The variation in herbage nitrogen content and in vitro organic matter digestibility (IVDOM) among Phalaris species and hybrids, within an Algerian ecotype of P. tuherosa and among plants derived from a broadly based P. tuberosa breeding population, was examined. Variation was extensive in each of the three populations, and appears adequate for plant breeding purposes. In each population, IVDOM was negatively related to heading date. Another feature common to all populations at the heading stage was a negative relationship between WDOM and nitrogen content, demonstrable particularly when other factors having similar effects on both characters were held constant. In mature herbage of the P. tuberosa ecotype there was a positive genetic correlation between nitrogen content and IVDOM, but IVDOM at maturity was negatively related to nitrogen content at heading. Annual Phalaris species had higher average IVDOM values at heading (mean 70.5 %) than perennial species (mean 66.1 %) when data were adjusted to a common heading date. Although there was considerable intraspecific variation, only the annual species were sufficiently superior to the best P. tuberosa strains to warrant consideration for hybridizing with P. tuberosa. Since such hybrids have reduced perenniality, and are cytologically unstable, interspecific hybridization appears to have the least value as a source of herbage quality for a P. tuberosa breeding program. In addition, annual species were lower in nitrogen content (mean 1 .15 %) than perennials (mean 1.70 %) at a common heading date. Estimated heritabilities of IVDOM and nitrogen content of mature herbage of the P. tuberosa ecotype were 0.78 and 0.59 respectively, for a family mean based on replicated eight-plant rows. Genetic statistics indicated that a substantial response to selection for improved digestibility of mature herbage would be accompanied by a correlated increase in IVDOM at heading, a slight increase in nitrogen content at maturity, a decrease in nitrogen content at heading, little change in heading date, and perhaps some increase in the weight of individual tillers. Response to selection for increased nitrogen content would be small, and would be accompanied by a reduction in tiller weight. Individual plants derived from the broadly based population ranged from 56 to 76% IVDOM at the heading stage. This population, because of its agronomic superiority, is considered to be the most suitable source of genetic variation on which to base a breeding program.
Recurrent evolution of cross-resistance in response to selection for improved post-infection survival in Drosophila melanogaster
