The effects of grazing management on pasture and bull production over the late-autumn to early-spring period : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science at Massey University
The Massey University Tuapaka farm has been operating a bull beef enterprise for 15 years. During this period, late autumn grazing management has been based on a rotation length of 50 days in order to save pasture for the winter. However this intent is seldom achieved, leading to low pasture cover at the end of the winter and low liveweight gain in early-spring, which in turn compromised final bull slaughter weights The objective of this research was to design and evaluate winter grazing management systems based on pasture condition targets and to compare the outcome with the normal Tuapaka management. The aim was to maximise pasture growth rate and animal liveweight gains in late-autumn in order to winter heavier bulls at higher pasture covers than the traditional Tuapaka grazing system. Wintering heavier bulls at the target pasture cover (1800-2000 kg DM/ha) the winter grazing management would focus on maintaining pasture cover around 2000 kg DM/ha throughout the winter in order to reach early-spring with an average pasture cover around 1700-1800 kg DM/ha. This way the grazing management in this period would again target sward conditions for high net pasture accumulation and liveweight gains. The trial was conducted at the Tuapaka bull unit, Massey University, from 1 April to 30 September 1997. It compared three contrasting managements, involving a total of 165 bulls stocked at 2.6 bull/ha. Treatment I followed a predetermined grazing plan based on a predicted the average pasture growth rate, the animal requirements needed to achieve performance targets, and on pre- and post-grazing pasture mass targets (2700-2800 kg DM/ha and 1500-1600 kg DM/ha for late-autumn and early-spring, and 3000-3200 kg DM/ha and 1100-1200 kg DM/ha for winter). Treatment 2 was managed according to the same pre- and post-grazing targets, except in this case post-grazing covers were monitored daily, and the bulls shifted when the post-grazing targets were achieved. No supplement was considered for Treatments 1 and 2. Treatment 3 followed traditional Tuapaka management, based on 50 day rotation over the autumn and winter, and 30 day rotation in August and bulls set stocked in September. Hay was fed as required in winter at the rate of 120.6 kg DM (pasture equivalent) per hectare. The initial pasture cover and bulls liveweight did not differ between treatments. Results showed a significant difference in average pasture cover (P <0.1) over the autumn, although no difference was found in pre and post-grazing cover, apparent daily dry matter intake, and net herbage accumulation. Over the winter, Treatments 1 and 2 pasture cover did not differ, and both were significantly (P <0.001) higher than Treatment 3. Pre-grazing cover was significantly (P <0.01) different between all treatments, while there was no difference between Treatments 1 and 3 in post-grazing cover and apparent daily dry matter intake, and both were lower (P <0.001) than Treatment 2. Net pasture accumulation of Treatment 1 did not differ from Treatments 2 and 3, however there was a significant difference (P <0.05) between these two last ones. In early-spring, Treatments 1 and 2 pasture cover and post grazing cover did not differ and were higher (P <0.001) than Treatment 3. Pre-grazing pasture cover was different (P <0.01) between all treatments. Net pasture accumulation of Treatments 1 and 2 did not differ and both were higher (P <0.05) than Treatment 3, while the average apparent dry matter intake of Treatments 1 and 3 were similar and both were different (P <0.05) from Treatment 2. The overall liveweight gain (0 84 ± 0.02, 0.87 ± 0.02, 0.74 ± 0.01 kg LW/head/day for Treatments 1, 2, 3 respectively) and liveweight (354.7 ± 3.54, 359.8 ± 3.65, 335.6 ± 3.27 kg LW) did not differ between Treatments 1 and 2 but both were significantly (P <0.001) different from Treatment 3. Total pasture production was significantly different (P <0.10) between Treatment 1 (6147 kg ± 369.34 DM/ha), Treatment 2 (7062 kg ± 319.86 DM/ha), and Treatment 3 (5277 ± 334.08 kg DM/ha). The total pasture production of Treatments 1 and 2 were 16 and 30% higher than Treatment 3. The extra pasture production per 100 kg DM/ha of increase in pasture cover was 1.64 and 3.38 kg DM/ha/day for Treatments 1 and 2 respectively. It was concluded that it is possible to improve both pasture production and bull beef performance when grazing management is based on pre and post-grazing pasture targets, and the practical implications of this were discussed.