An evaluation of Kikuyu-clover pastures as a dairy production system .1. Pasture and diet

An experiment used 42 lactating Holstein-Friesian cows to evaluate kikuyu (Pennisetum clandestinum) cv. Whittet, Haifa white clover (Trifolium repens cv. Haifa) and Safari clover (T. semipilosum cv. Safari) pastures at 3 stocking rates (2.5, 3.75 and 5 cows/ha) and a range of nitrogen fertiliser levels (0, 150, 300 and 600 kg/ha N). These perennial pastures were compared with an annual ryegrass (Lolium spp.) production system stocked at 5 cows/ha that received 400 kg/ha N. All pastures were irrigated over the 3 years of the experiment. The diet selected from pasture was evaluated using oesophageally fistulated cows and the botanical and chemical composition of their diets related to botanical and chemical components in the pasture on offer. The experiment was conducted in a tropical upland environment, where the average annual rainfall is 1279 mm and summer dominant. Total pasture on offer was generally highest in late spring and summer and lowest in late winter. Clover on offer was always highest in November (520 +/- 29) kg/ha DM with means of 280 (+/- 20), 230 (+/- 13) and 290 (+/- 12) kg/ha DM in February, May and August. Clover yields were highest at 2.5 and 3.75 cows/ha where no nitrogen fertiliser was applied and Haifa was the dominant clover throughout. Clover on offer declined over the 3 years. Annual ryegrass pastures yielded approximately 14 t/ha DM from May-December. Pasture quality was high with the crude protein % of kikuyu increased by stocking rate and level of N fertiliser (range 12-18%). The crude protein % of ryegrass averaged 24%, while that of clover ranged from 23-26%. Clover % in the diet (CLD) was positively related to both clover % in the pasture (CLP) and clover DM on offer (CLY, kg/ha DM): CLD = 0.91 +/- 1.725 CLP (May and November) and CLD = 7.92 + 1.725 CLP (August) (r(2) = 0.44; P < 0.05). In August, clover % in the diet was related to clover on offer by the equation: CLD = 5.69 +/- 0.058 CLY (r(2) = 0.42; P < 0.05). The conversion rate of kikuyu in the pasture to kikuyu in the diet was similar for all treatments (regression coefficient 0.6), but where 600 N was applied, there was a higher overall kikuyu % in the diet. The best predictor of crude protein % in the diet (CPD) included kikuyu leaf % (KLD) + clover % (CLD) in the equation: CPD = 6.34 +/- 0.199 (KLD +/- CLD) (r(2) = 0.60; P < 0.01). Dietary clover % declined over the 3 years in all treatments and seasons except for treatments 2.5/0N and 3.75/0N in spring when clover in the diet was 30-40%. This decline over time was related to a reduction in clover on offer in the pastures over the same period. In general, season had the dominant effect on the relationships between pasture and diet parameters.