Management targets for maximising the short-term herbage intake rate of cattle grazing in Sorghum bicolour.

FONSECA, L., MEZZALIRA, J.C., BREMM, C., FILHO, R.S.A., GONDA, H.L. AND CARVALHO, P. C.DE F.

LIVESTOCK SCIENCE

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2012 p. 205 - 211

1871-1413

One of the variables in the structure of the sward with the most influence on the short-term herbage intake rate by grazing ruminants is the canopy height. The objective of this study was therefore to characterise the effect of the canopy height on short-term herbage intake rate using Sorghum bicolor cv BR 501. as a reference. Two experiments were conducted at Research Station of the Federal University of Rio Grande do Sul, Brazil, between December 2009 and April 2010. The treatments in experiment 1 were used to determine the influence of the pre-grazing canopy height on the short-term herbage intake and consisted of six canopy heights (30, 40, 50, 60, 70 and 80 cm). The treatments in experiment 2 used the pre-grazing canopy height from experiment 1 thatmaximised the short-termherbage intake rate to evaluate the influence of the severity of grazing down (16, 33, 50, 67 and 84%) on the herbage intake. Both experiments used a completely randomised block design, with two replicates in experiment 1 and three replicates in experiment 2. Four heifers (24±2 months and 306±56.7 kg) were used in experiment 1. Three of these animals were used in experiment 2, which were then 26±2 months and 339±45.5 kg. The short-term herbage intake rate was measured by weighing the heifers preand post-grazing, corrected for insensible weight losses. The number of grazing jaw movements (biting and non-biting) was counted automatically using an IGER Behaviour Recorder. In both experiments, the sward measurements included the pre- and post-grazing canopy height, the pregrazing herbage mass and the vertical distribution of morphological components. In experiment 2, the post-grazing herbage mass was also measured. The results showed that the grazing canopy height that maximised the short-term herbage intake rate was approximately 50 cm. The grazing down protocol showed that the short-term herbage intake rate was constant until the depletion of 40% of the optimal pre-grazing canopy height. After this level of depletion, there was a marked reduction in the short-term herbage intake rate. The results showthat the best targetmanagement height in a Sorghum cv. BR 501 pasture, allowing for high levels of the short-term herbage intake rate, is 50 cm. With intermittent stocking, this level should be considered as the pre-grazing canopy height, and the level of herbage depletion should not exceed 40%.