Measuring Phenotypic and Genetic Variances and Narrow Sense Heritability in Three Populations of Annual Ryegrass (Lolium multiflorum Lam.)

Abstract

Annual ryegrass, also known as Italian ryegrass is a cool season bunch grass supposed native to Italy and belongs to family Poaceae. Annual ryegrass is a short duration grass chiefly used for pasture and silage in dairy and beef cattle production. Due to low availability of live forage during early winters, beef and cattle producers rely on stored forage to meet nutritional requirements of animals, so increasing early winter productivity would be a great benefit for the beef industry. In order to increase early winter productivity, a recurrent selection project for improved winter dry matter productivity was initiated in which plants were evaluated and selected on the basis of dry matter yield 750 GDD post transplanting. As a common practice in forage breeding programs for cross-pollinated, non-domesticated species, we simply assumed that there is genetic variation for the traits of interest and no assessment of heritable variation was made before the start of the recurrent selection program. Yet it is of interest to investigate phenotypic and genetic variances and covariances in base and selected populations. To measure genetic variation and heritability for dry matter yield three half-sib populations, representing three cycles of recurrent selection were selected. In year 2008/09 the trial simulated a sward and was conducted only at the Plant Breeding Unit (PBU) of the Alabama Agricultural Experiment Station’s E.V. Smith Research Center in Tallassee, Alabama while in year 2009/10 the spaced planted trial was conducted at two locations in Alabama: PBU and the Alabama Agricultural Experiment Station’s Wiregrass Research and Extension Center at Headland (WGS). The experimental design for each trial was a randomized complete block (r = 2) with a split-split plot randomization restriction (SSP). Three harvest schemes were employed. Two schemes were based on accumulated thermal degree-days (GDD) and the third based on heading date. Maximum likelihood methods were used to calculate variances and covariances, which were then used to estimate heritability for dry matter yield. First and second cut data for the first and second harvest scheme enabled us to evaluate genetic variation for productivity under autumn/winter conditions. The third harvest scheme with harvest at heading enabled us to evaluate the effect of maturity differences on genetic variation for yield. In year 2009/10 only two cuttings were done. First harvest was done at 500 GDD and second harvest was done at maturity. Considerable genetic variation among the three populations was observed. Generally the heritability values have been observed higher for year 2008/09 than 2009/10, for corresponding harvests. In both years the heritability values decreased in most of the cases with the each subsequent cycle but differences among the values were not significant, since the standard error values overlapped. Similarly heritability within harvest scheme also decreased with subsequent cuts within each population, but differences among values were also not significant. The trend for the values within harvest scheme can be explained due to increase in temperature in each subsequent cut, since the populations were produced from the selection for high winter forage yield. In year 2009/10 heritability values at Tallassee were observed higher than at Headland which may be due to higher mean temperature at Headland. Overall moderate to high heritability estimates for dry-matter yield were observed indicating sufficient genetic variability for further improvement.