Varietal differences in in vitro dry matter digestibility in Setaria, and the effects of site, age, and season

Abstract

Six Setaria introductions were grown in small plots at three sites differing in soil type and location in south-eastern Queensland, and fertilizer and irrigation water were applied at similar rates. All plots of each introduction were genetically identical. Three different cutting regimes were imposed, such that at 12-week intervals during the 2 years of the experiment 4, 8, and 12-week regrowth was available for comparison of in vitro dry matter digestibility.

There was an overall difference in digestibility between varieties of 6.6 percentage units, and the ranking order was usually the same for all sites, seasons, and ages of regrowth. Three of the introductions were consistently superior to cv. Nandi, the only commercial cultivar included in the present study. The two Setaria splendida introductions ranked second and sixth, although they were both higher in ash concentration than the other introductions, and this was associated with high potassium content. Differences between varieties in digestibility were not associated with flowering differences, despite large differences in flowering behaviour.

The maximum mean difference between sites was 4.9 units and this was associated with differences in ash content. Difference in ash content, however, accounted for only 23% of the digestibility difference. The high ash content was associated with high concentrations of potassium. There was no association between nitrogen content and digestibility. The introduction lowest in digestibility had the highest nitrogen content.

Digestibility was higher in winter than in summer at all sites, and the extent of this difference increased with age of regrowth. Higher digestibilities were obtained in younger regrowth.