Effect of different grazing pressure by lambs grazing Lolium perenne and Dactylis glomerata pastures during spring on: 1. Diet quality

This study was undertaken to determine the influence of three grazing pressures [high (HGP), medium (MGP) and low (LGP), corresponding to 30, 50 and 75 g available DM/kg BW/day, respectively] on the quality of herbage consumed by lambs grazing Lolium perenne and Dactylis glomerata pastures in spring. Feed samples collected via oesophageal fistulae at the start and end of a six-week grazing period were analyzed for nitrogen (N), ash, neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL) concentrations, and in vitro digestibility of organic matter (IVDOM). In general, the quality of diets selected by lambs grazing both L. perenne and D. glomerata pastures was lower at the end than at the start of the grazing period. For both pastures the N concentration of herbage consumed by lambs in the HGP treatment was lower than for that in the LGP treatment. For L. perenne HGP resulted in the selection of a diet with higher fibre (NDF and ADF) concentrations and consequently lower IVDOM than in the LGP treatment. In the D. glomerata pasture, higher ADF and lower IVDOM were recorded in the MGP treatment compared to the other treatments. On the D. glomerata pasture the ADL concentration of the selected forage was lower in the HGP than in the MGP or LGP treatments. Forage selected on the L. perenne pasture had generally lower N, ash, ADF and ADL concentrations than those selected from the D. glomerata pasture. However, the IVDOM of the selected L. perenne was higher than that of selected D. glomerata. Keywords: Cocksfoot; chemical composition; diet selection; pasture availability; perennial rye grass South African Journal of Animal Science Vol. 36 (5) 2006: pp.46-4