Timing of herbage allocation in strip grazing: Effects on grazing pattern and performance of beef heifers1

Abstract

The timing of grazing bouts (GB) determines how cattle allot time to meet their nutritional needs. Net photosynthesis and evapotranspirational losses increase herbage nonstructural carbohydrate and DM concentrations, which may lead to longer and more intense GB at dusk. Hence, linking the grazing pattern, plant phenology, and herbage allocation time emerges as an option to manipulate the GB and nutrient intake. The objectives of this work were to analyze grazing behavior and performance of beef heifers when herbage allocation was at 0700 each morning (MHA) or at 1500 each afternoon (AHA). Two pairs of experiments were conducted during the winter and spring examining behavior and performance. Measurements were grazing, rumination, and idling times during daylight hours, and their patterns, as well as bite rate, ADG, change in BCS, and daily herbage DMI. In the behavioral experiments, 8 heifers strip-grazed annual ryegrass (Lolium multiflorum Lam.). The grazing, rumination, and idling times as well as bite rate were measured and also analyzed per time of day. In the performance experiments, 48 beef heifers strip-grazed annual ryegrass in 2 groups according to treatments. Daily DMI, ADG, and changes in BCS were analyzed. The AHA increased daily idling time (P < 0.01) and decreased grazing time (P < 0.01). The AHA concentrated grazing time in the evening, when bite rate was greater (P < 0.01). The daylight rumination time varied by time of day (P < 0.01), but total daylight rumination time did not differ (P = 0.11). With AHA, rumination time and idling time were concentrated in the morning and afternoon. In the performance experiment during the winter, there was a treatment × week effect (P < 0.01) for ADG and change in BCS. Beginning in wk 4, heifers in AHA gained 150 g of BW and 0.0145 points of BCS more than those in MHA (P < 0.05) per day. In the spring, AHA increased ADG by 549 g and 0.0145 points of BCS more than those in MHA (P < 0.05) per day during the entire 6 wk. The herbage DMI (kg/d) did not differ in winter (AHA, 5.0 vs. MHA, 4.5) or spring (AHA, 5.6 vs. MHA, 5.0). These results suggest that timing of herbage allocation alters grazing, rumination, and idling patterns; AHA leads to longer and more intense GB when herbage has greater quality, which improves cattle performance. Copyright © 2006. . Copyright 2006 Journal of Animal Science