Effect of an associate grass, cutting interval, and cutting height on yield and botanical composition of Siratro pastures in a sub-tropical environment

Abstract

Plots of Siratro and Siratro plus Nandi setaria were cut every 4, 8, 12 or 16 weeks at either 7.5 cm or 15.0 cm above ground over three years. At the end of three years plant density and the residual effect of treatment on yield over ten weeks in spring and summer were measured. The yield of Siratro was reduced by about 30 per cent when grown with Setaria (4.04 and 2.86 t ha-1/yr-1). But total annual yield was increased at all cutting intervals, by an average of 4 t ha-l when the grass was included (1 0.6 and 6.6 t ha-1yr-1). Siratro yield increased by an average of 225 kg ha-l yr-1 for each week extension of the cutting interval. Associated with this increase was a decline in weed yield and an increase in the percentage of Siratro. Cutting at 15 cm reduced total yield by 2.44 t ha-1 yr-1 compared with cutting at 7.5 cm, but only reduced Siratro yield at the 12 and 16 week cutting intervals. In the plots cut to 7.5 cm every four weeks Siratro declined in yield over three years and was replaced by invading white clover. Siratro density was reduced in the grass-legume plots to one third of that in the corresponding legume plots. In both pastures, Siratro density increased linearly as cutting interval increased but was not affected by cutting height. Grass density also increased linearly with cutting interval up to 12 weeks but then declined sharply when cut every 16 weeks. This decline was attributed to the very heavy shading by the associated Siratro. There were large residual effects of cutting interval on total and Siratro yield which were associated with plant density and plant vigour.