Genetic analysis in the pasture grass Setaria sphacelata. II. Chemical composition, digestibility and correlations with yield

Abstract

The progeny of a 7 x 7 diallel cross between plants randomly selected from a segregating tetraploid population (Setaria sphacelata var. sericea and var. splendida) was studied at Lawes, south-east Queensland, over a two year period, together with the parent clones. Plants were harvested at 6-week intervals (9-10 weeks in winter), dry weight was recorded, and family rows were bulked for analysis of in vitro digestibility (DMD) and for multi-element analysis. Concentrations of nitrogen, phosphorus, magnesium and calcium tended to be low in summer, and potassium concentration decreased during the two years of the experiment. In contrast magnesium, calcium and sodium increased. Genetic variance for digestibility was detected for only two of 14 harvests. Additive genetic variance was statistically significant, and generally consistent over harvests for nitrogen, potassium, sodium, calcium and magnesium. Family by season and family by year interactions were of a low order, showing that the expression of genetic variance was not strongly influenced by seasonal effects. Additive variance effects were erratic for phosphorus, sulfur, manganese, copper, zinc, silicon, iron, aluminium and boron, and selection for higher or lower concentrations of these elements would be unlikely to be successful. Nitrogen, potassium, calcium, magnesium and sodium were negatively and consistently correlated with dry matter yield. Over all harvests r was - 0.64, - 0.16, - 0.77, - 0.30 and - 0.44 respectively. With the possible exception of potassium, selection for higher concentrations of any of these minerals would result in a decrease in yield. Sodium was negatively correlated genetically with nitrogen, potassium, calcium and magnesium on an individual harvest or a season basis. For the total of all 15 harvests values for rg were -0.76, -0.88, -0.21 and -0.73 respectively. Nitrogen, potassium, calcium and magnesium were consistently positively correlated (for all harvests rg - 0.69 for all combinations), and selection for high concentration of any of these elements would result in an increase in the others, but a decrease in sodium. Significant genetic variation was detected for the mineral ratios K:(Ca + Mg) and calcium: phosphorus. However, these ratios are not known to adversely affect production in setaria pastures, and manipulation by breeding is not warranted. These examples show that there is the potential for breeding for mineral ratios in tropical forages.