Macromineral composition of kikuyu herbage relative to the requirements of ruminants.
In a 2-year study, the macromineral status of kikuyu (Pennisetum clandestinum) grass pastures growing on representative soils in the KwaZulu-Natal Midlands was studied. Herbage samples were collected at 25- to 35-day intervals during the growing season from 12 rotationally-grazed camps located on 3 sites. Seasonal mean herbage calcium and phosphorus concentrations were within the ranges 0.22 to 0.33% and 0.27 to 0.39%, respectively, while seasonal Ca:P ratios ranged from 1.1:1 to 0.63:1. Ca concentrations and Ca:P ratios were lowest in the midsummer months, while P concentrations increased in midsummer. In terms of the requirements of various classes of sheep and cattle, Ca concentrations and Ca:P ratios were frequently well below published critical limits. This finding, coupled with reports of kikuyu containing high levels of oxalate which reduce Ca absorption by ruminants, implies a major Ca insufficiency for livestock pastured on kikuyu. Herbage magnesium concentrations (seasonal mean range: 0.25 to 0.35%) were adequate in terms of the requirements of most classes of ruminants; however, potassium concentrations (seasonal mean range: 2.64 to 4.46%) were well in excess of requirements, and Mg absorption could be severely inhibited by the oversupply of K. In most camps the K:Ca+Mg ratio exceeded the reported safety threshold for grazing animals of 2.2, suggesting a high hypomagnesaemic tetany potential for animals on kikuyu pastures. Sodium concentrations in kikuyu were inadequate in most locations.