Energy-protein supplementation and grazing dynamics of growing beef cattle grazing stockpiled native grasslands in winter

Doctoral thesis abstract

Resumo

The weight loss of growing beef cattle grazing native grasslands in winter may be reverted by supplementing with energy and protein. To render this practice profitable, its efficiency -SFE- difference in average daily gain- ADG - between supplemented - S - and control - C animals- ADGchng - supplement response - per kg of dry matter -DM intake- must be controlled. Twenty-five trials carried out in Uruguay between 1993 and 2018 were gathered. The average SFE was 0.21 ± 0.076 kg/ADGchng/kg DM, from an average ADGchng of 0.38 ± 0.180 kg/animal/day. No association was found between SFE and supplementation rate nor type (p < 0.05), but forage allowance negatively affected it while herbage mass positively affected it, yet in a smaller magnitude. Weather conditions during trials affected SFE (p < 0.05), with greater SFE in winters with lower temperatures and more frosts. Another collated analysis using only 15 of the original 25 trials was performed to estimate supplement substitution rates (sSbR) and to assess the existence of different phases and ADGchng evolution patterns. sSbR were 0.3-1.1 kg/kg and were negatively associated with SFE. Three responses were identified (linear, quadratic and Weibull). Quadratic patterns were closely associated with sward biomass and substitution rates, while Weibull were associated with frosts. Finally, a grazing-down experiment was performed comparing S and C animals in their grazing dynamics. No differences were found between the horizontal dynamics, and when the upper grazing horizon was depleted by > 80 %, the subsequent horizon was being depleted by 50 %. Supplementation is a highly reliable technology for improving the performance of growing beef cattle.