Influence of the critical phase of stem elongation on yield and forage quality of perennial ryegrass genotypes in the first reproductive growth

The phenological stage at harvest time is crucial for various forage production traits in many forage grasses. Heading date is considered as a good compromise for attaining high yields combined with high forage quality. However, the effect of heading date is inconsistent and it is therefore assumed that the critical phase of stem elongation, i.e. the phase between the onset of jointing and heading, may contribute to explain the relationship between phenological development and forage production traits. To quantify this influence in the first reproductive growth, a field trial was conducted over two growing seasons (2012-2013) using single plants of perennial ryegrass (Lolium perenne L.) ecotype populations, sampled from eight old permanent grassland swards in Northern Germany. The genotypes were divided into four phenological categories (early/late jointing; short/long critical phase), with respect to the length and onset of their critical phase. Single plants were harvested at their respective heading date and dry matter yield and various forage quality traits were determined. It was hypothesised that an extended critical phase leads to a benefit in forage production traits and could be used as a selection criterion. The study revealed, with reference to the whole plant material studied, some strong phenotypic and genotypic correlations between the length of the critical phase and the forage production traits. Significantly negative correlations were found for in vitro digestibility of organic matter, and the contents of neutral-detergent fibre, lignin, crude fat and crude protein. Significantly positive correlations were found for dry matter yield, cell-wall digestibility and the content of water-soluble carbohydrates. However, when quantifying the influence of the critical phase within the given phenological categories, more differentiated relationships were detected, which indicate potential for selection for high quality forage.