Effect of sward surface height and level of herbage depletion on bite features of cattle grazing Sorghum bicolor swards1

Abstract

To maximize herbage DMI, pregrazing sward surface height (SSH) and level of herbage depletion (HD) must be such that variables determining short-term herbage intake such as bite mass (BM) and bite rate (BR) are optimized. The objective of this study was to determine a SSH target and the level of HD as a proportion of the SSH that optimizes BM and BR of beef heifers grazing Sorghum bicolor swards. Two experiments were conducted using 2 S. bicolor swards and 4 beef heifers (25 mo old; 322 kg BW). Experiment 1 compared the effect of 6 pregrazing SSH, 30, 40, 50, 60, 70, and 80 cm, on BM, BR, and jaw movements. Experiment 2 assessed the effect of HD level as a proportion of SSH (0.17, 0.34, 0.50, 0.67 and 0.84) on BM, BR, and jaw movements using the optimal pregrazing SSH defined in Exp. 1. Short-term herbage DMI was estimated using a double-weighing technique and corrected for insensible BW loss. Herbage DMI was subsequently used to calculate the BM. Net eating time and jaw movements for apprehension and manipulation + mastication during grazing as well as total jaw movements were determined using the IGER (Institute of Grassland and Environmental Research) behavior recorders. Bite rate and the number of total jaw movements per gram herbage DMI were derived from jaw movement count and measurements of herbage DMI. The results of Exp. 1 showed low and high SSH constraint the ease of herbage harvesting. Greater BM are maintained until a SSH of 50 cm is reached (P < 0.05) and then decline at greater SSH due to herbage dispersion. The nonbiting jaw movement rate increased at greater SSH whereas BR decreased (P < 0.05). For both variables, the turning point was close to a SSH of 50 cm. Experiment 2 showed that such an optimization of BM and BR was maintained until an HD level of 0.34 was reached (P < 0.05). There was a linear increase in both the total jaw movements per unit herbage DMI and the nonbiting jaw movements rate (manipulation + mastication) subsequent to levels of HD greater than 0.34 (P < 0.05). These studies provide, for the first time, sward feature targets to manage grazing and optimize BM and BR, aiming to maximize the short-term herbage DMI of cattle grazing S. bicolor swards.

Ecophysiology of C4 Forage Grasses—Understanding Plant Growth for Optimising Their Use and Management

“…As grazing progresses and herbage mass decreases towards the end of grazing, the proportions of stem and dead material increase in the consumed herbage as the consequence of the changing plant-part composition of the sward vertical profile [129][130][131]. This pattern of variation in the short-term rate of intake in relation to the decreasing sward height during grazing was formally described by Fonseca et al [127,132] and corroborated by Mezzalira et al [128], indicating the potential for manipulating pre-and post-grazing conditions as a means of optimising herbage production and intake in rotationally managed pastures. In general, more frequent defoliations (at 95% canopy light interception) than traditionally used, associated with moderate grazing severity (post-grazing heights around 50% of the pre-grazing height), result in greater leaf dry matter production [84,93,96,124], higher nutritive value [89,90] and intake rate of animals [127][128][129]132], augmenting animal performance and productivity [133], indicating that rational intensification of grassland use could be an effective way of ensuring sustainability of tropical pastoral systems of animal production.…”

Maintaining post‐grazing sward height of Panicum maximum (cv. Mombaça) at 50 cm led to higher animal performance compared with post‐grazing height of 30 cm

“…In rotational grazing, priority is commonly given to forage harvest efficiency, which ultimately means lower heights of post‐grazing residues and few or no leaves. However, when animals are forced to graze in the lower layers of the pasture canopy, significant reductions occur in forage intake by animals (Fonseca et al., ; ; Mezzalira et al., ).…”

Effect of planting pattern of forage sorghum on forage productivity and heifer performance during finishing phase

“…The decreased participation of leaf blade of forage sorghum in the total forage mass reflects the preferential consumption of leaves by the animals when compared to other plant parts (FONSECA et al, 2013) due to the lower bromatological quality of the stems in relation to leaves (BENVENUTTI et al, 2008). As the cycle advances and the photoperiod decreases, the plants tend to flower differentiation, decreasing the leaf appearance rate, which is lower compared to the consumption of leaves, reflecting the decrease in mass and leaf supply.…”