Effect of fertilizer, root and shoot competition on the growth of Siratro (Macroptilium atropurpureum) and green panic (Panicum maximum var. trichoglume) seedlings in a native speargrass (Heteropogon contortus) sward

Abstract

The effect of root and shoot competition and the application of fertilizer on the growth of Siratro (Macroptilium atropurpureum) and green panic (Panicum maxinzum var. trichoglume) seedlings under supplementary irrigation in an existing native grassland dominated by Heteropogon contortus was studied. The seedlings were grown in both the presence and absence of root competition and varying levels of shoot competition. Root competition was prevented by the use of steel tubes while shoot competition was varied by: clipping the native pasture to a height of either 3-5 cm or 10-15 cm every 7-10 days, leaving the native sward unclipped, and unclipped sward with wire-netting guards fitted around each seedling to prevent overtopping and facilitate light penetration. Seedling growth was assessed by determining dry weights after 29 days and also by measuring plant heights, leaf and tiller numbers and the leaf widths at 7-day intervals during this period. Application of superphosphate and nitrogen fertilizer increased the growth of both species, the initial effects being evident within 1-2 days of seedling emergence. However, at both fertilizer levels the growth of both species was primarily affected by root competition for nutrients. Where fertilizer was applied and no root competition was present heavy shading in the lower 5-10 cm of the sward canopy restricted seedling growth by reducing tiller and leaf production. The removal of shoot competition increased seedling yields fivefold whereas the removal of root competition and the removal of both root and shoot competition increased yields by 25- and 45-fold, respectively. Siratro and green panic yields were the same in the absence of root competition but Siratro yields were 11 times those of green panic where root competition was present.