Beef cow performance from tropical pastures on semi-arid brigalow lands under intermittent drought

Abstract

The value of perennial tropical pastures for breeding cows was examined over a five year period (1 968-1 973) as part of a long-term land use study on semi-arid fertile brigalow clay soils. Pastures were established to Panicum maximum var. trichoglume (green panic) and Chloris gayana cv. Pioneer (rhodes grass) grown alone or with the legumes Macroptilium atropurpureum cv. Siratro, Glycine wightii cv. Cooper and Medicago sativa cv. Hunter River. These pastures were rotationally grazed at 1.75, 1.24 and 0.87 cows ha-l during 1968-69 and at 1.31, 0.93 and 0.66 cows ha-1 in subsequent years. Pastures received either no fertilizer or 188 kg superphosphate ha-1 year-1 (grass/legume treatments) and in mixture with 250 kg ammonium sulphate ha-1 year-1 (grass-alone treatments). During the first seven months of 1968-69, increased stocking rate from 0.87 to 1.75 cows ha-1 significantly increased cow liveweight (LW) loss from 5 kg to 84 kg and reduced conception rates from 100 per cent to 68.8 per cent. Compared with pure grass pastures (GA), cows grazing grass/legume (GL) pastures lost significantly more LW during this period. These effects occurred during a severe drought which lasted for over a year. As the drought intensified and feed ran out, pasture treatments were individually destocked. A progressive botanical change from GL to pure grass pastures occurred in the GL treatments following the drought. After restocking in late 1969, cows gained an average of 121 kg and recorded an average calving rate of 93 per cent over a 45-month period despite below average annual rainfalls. Stocking rate and to a lesser extent pasture type significantly influenced annual cow LW and gains during mating, gestation and lactation periods, but the actual effects were small. Cows grazing at the highest stocking rate (HSR) of 1.31 cows ha-1 and on GL treatments recorded significantly poorer gains only in the first year after the drought. However, cows grazing GL treatments at HSR required supplementary hay feeding during winter-early spring each year. Although main treatment effects were not significant, a strong interaction between stocking rate and pasture significantly affected time of conception and both conception and calving rates. Application of fertilizer showed only small though consistent advantages for cow LW and fertility responses. It was concluded that sown tropical pasture based on pure green panic allowed high LW and reproductive performances of breeding cows stocked near 0.93 cow plus calf ha-1. This pasture-animal system was resilient to the climatic variability experienced in the semi-arid brigalow environment.