Abstract

This study, conducted on the north coast of New South Wales, determined the optimal time to defoliate kikuyu (Pennisetum clandestinum Hochst ex chiov.) in terms of pasture quality in relation to the number of leaves per tiller. It also compared the quality of well-managed kikuyu grass and ryegrass (Lolium perenne L.) pasture to determine the nutrients in kikuyu likely to limit milk production. The crude protein (CP), P, and K concentrations of individual leaves declined; Ca and Mg increased; and Na levels remained constant with increasing age. These changes in individual leaves were reflected in the quality of kikuyu available above the 5-cm stubble height. The proportion of green leaf available above the 5-cm stubble height declined whilst the proportion of stem and dead material increased markedly after 4.5 leaves per tiller had regrown. These changes were reflected in a substantial decline in organic matter digestibility (OMD) and CP of herbage. Between 0900 and 1500 hours, water-soluble carbohydrate (WSC) levels in kikuyu above the 5-cm stubble height increased at a rate of 5 g/kg dry matter (DM) per hour, reaching maximum levels during the mid afternoon. Kikuyu fertilised at a high rate of N fertiliser contained significantly higher nitrate levels than that receiving moderate levels (3.12 v. 0. 67 g/kg DM, respectively). There was a linear increase in the levels of nitrate as the CP of forage exceeded 230 g/kg DM. Kikuyu and ryegrass samples plucked to simulate forage consumed by milking cows were obtained in the midst of the growing season for each species. Kikuyu had a lower OMD than ryegrass (733 v. 842 g/kg DM) but contained higher levels of neutral detergent fibre (600 v. 400 g/kg DM). High levels of CP and low content of WSC in kikuyu compared with ryegrass resulted in a much lower ratio WSC: CP (0.09 v. 0.36, respectively). Kikuyu contained 50% less Ca than ryegrass, much of which would be expected to be bound to oxalates and thus unavailable to grazing stock. The concentration of Na in kikuyu was extremely low compared with ryegrass (0.15 v. 3.67 g/kg DM, respectively). The most desirable stage of regrowth of kikuyu on which to graze lactating dairy cows, in terms of a compromise between forage quality and quantity, appears to be at 4.5 leaves/tiller. However, even at this stage, various innate nutrient deficiencies would appear to restrict milk production to 15-16 L milk/cow.day.