The leaf growth, carrying capacity and dairy production of four summer-growing pastures were measured in a subtropical area of Australia. The growing season was subdivided into ten 4-week periods and the production was estimated for each period. Carrying capacities were determined by rotationally grazing four paddocks of each pasture over the 4-week period, and varying the stocking density so that a target weight of leaf material remained on the pasture at the conclusion of each grazing.
A non-limiting rate of nitrogen fertilizer increased total leaf production of kikuyu by 97% over a control treatment without nitrogen, but the response was not evenly distributed throughout the season. It fell to 50% during autumn when growth on the nitrogen-fertilized pastures was restricted following heavy defoliation during late summer. The use of the tropical legumes, siratro and glycine, in a mixed sward with kikuyu, did not increase leaf production over a kikuyu control pasture. The legumes grew poorly and this appeared to be due to the combined effects of their poor adaptability to the krasnozem soil, a high plant mortality especially during the first winter after sowing, the 4-week grazing interval and strong grass competition.
The carrying capacity of the nitrogen-fertilized pasture was 1·3–5·3 cows/ha at the beginning (September–October) and end (April–mid June) of the growing season, and increased to a peak of 7·4–9·7 cows/ha during February-mid March. Nitrogen fertilizer increased the carrying capacity by an average of 131% over that on a kikuyu pasture without nitrogen. The carrying capacities were similar on kikuyu and on a mixed carpet grass-kikuyu pasture, both without nitrogen, but were less on a tropical legume-kikuyu pasture which was grazed at a lighter grazing intensity during autumn to aid legume persistence.
Reflecting the experimental method of adjusting the stocking density according to the pasture available, differences in dairy production per cow were small relative to differences in the carrying capacity. Consequently seasonal variation in total dairy production/ha mirrored the carrying capacity. Nitrogen fertilizer increased the average production of 4% fat-corrected milk to 13·2 t/ha/40 weeks, an increase of 133% over the control without nitrogen.
The results illustrate the marked seasonal imbalance in growth and carrying capacity for a given pasture, and emphasize the need to use mixed feeding systems to provide a uniform level of nutrition for a fixed number of cows.