Nitrogen response and stocking rate of Pennisetum clandestinum pastures: I. Pasture nitrogen requirement and concentration, distribution of dry matter and botanical composition

Abstract

SummaryAmmonium nitrate was applied to a kikuyu (Pennisetum clandestinum) dominant pasture at annual rates of 0, 134, 336 and 672 kg N/ha. Each nitrogen (N) rate treatment was continuously grazed at three stocking rates (SR) designed to apply similar grazing pressures at each N level and covering a range from 2·2 to 16·6 Angus weaner cattle/ha.The N status of these pastures was periodically assessed 8–10 weeks after seasonal N applications by temporarily excluding stock and measuring D.M. and N response to additional ammonium nitrate application. Pasture growth response to additional N was constant and independent of SR and of annual N rate over the range 0 to 336 kg N/ha, despite wide variation in pasture availability. N concentration of plant shoots was increased by high SR and rate of N uptake was not significantly affected by SR. Pasture growth rate was positively related to annual N rate.Herbage yield of kikuyu leaf, stem and litter on offer, was negatively related to SR but weight of underground organs was unaffected. Increasing SR increased the proportion of leaf and the rate of tillering. Pasture availability was positively and linearly related to N rate from 0 to 336 kg N/ha and maximal at 336 kg N/ha; the size of the litter fractions and underground organs was generally independent of N rate. N% in leaf and stem was reduced at low SR; N% of leaf, stem, litter, rhizome and root was positively related to N rate which was more influential than SR. The effect of SR on the size and relative distribution of the N in plant tops and litter fraction is discussed.Kikuyu dominance was positively related to N rate and independent of SR. White clover invaded at No and N131 levels especially at a high SR.