This research evaluated the effect of supplementation with cracked corn grain (0 or 1% of liveweight) on the performance of Hereford steers grazing a mixed pasture of legume and grass at 3 forage allowances (3, 6 or 9 kg DM/100 kg liveweight). The experiments were conducted in West Uruguay over 2 consecutive summers, with 72 Hereford steers (282 ± 15.3 kg; n = 36/year) randomly allocated to 1 of the 6 treatments in a factorial arrangement. Cattle were supplemented in individual pens and weighed every 14 days. Grain intake did not differ (P>0.05) between forage allowances. Both supplementing and increasing forage allowance reduced (P<0.01) forage utilisation. Year effect was significant (P<0.01) for liveweight gain and grain intake, but neither the 2- nor the 3-way interaction of year with forage allowance or supplementation was significant (P>0.05). Increasing forage allowance only improved (P<0.01) liveweight gain of non-supplemented cattle (0.299, 0.483, 0.667 kg/day for forage allowance of 3, 6 or 9%, respectively). Hence, response to supplementation decreased with forage allowance, increasing the grain (kg)/gain (kg) conversion ratio. These results provide a quantitative basis on which to decide upon the best feeding management option for summer.

O objetivo deste trabalho foi avaliar o desenvolvimento de novilhas de corte no sistema de pastejo rotativo "ponta" e "rapador" durante o primeiro (Ano-1) e segundo (Ano-2) invernos/primaveras pós-desmama até o final do primeiro acasalamento aos 22-24 meses de idade. Setenta e seis bezerras de corte mestiças foram mantidas durante Ano-1 e Ano-2 em pastagem nativa melhorada de azevém (Lolium multiflorum L.) e trevo vesiculoso Yuchi (Trifolium vesiculosum cv. Yuchi). No período de verão/outono e acasalamento os animais foram mantidos em campo natural, em lote único. Os tratamentos Ponta (TP): grupo consumindo a metade superior da pastagem disponível; Rapador (TR): consumindo a metade inferior da pastagem foram testados. A evolução do peso vivo (PV) em cada etapa do experimento e ganho de peso médio diário (GMD) por período foram analisados. O peso vivo de TP diferiu do TR ao final do Ano-1 (195,6 vs. 168,1 kg), ao final do Ano-2 (316,3 vs. 274,1 kg) e ao início (318,5 vs. 275,7 kg) e final (341,5 vs. 304,4 kg) do acasalamento, respectivamente. O maior PV foi atribuído ao maior GMD obtido por TP sobre TR durante o uso da pastagem melhorada, com GMD de 0,790 vs 0,563 kg/dia e 0,753 vs. 0,522 kg/dia, no Ano-1 e Ano-2, respectivamente. Houve inversão no GMD com melhor desempenho do TR em relação ao TP (0,377 vs. 0,303 kg/dia) no acasalamento. O sistema de recria de novilhas para reposição, favorecendo determinado grupo de animais (TP), é uma das alternativas para obter pesos vivos satisfatórios, tornando-as aptas à concepção aos 22-24 meses de idade.

No abstract

Supplementing growing cattle grazing native subtropical Campos grasslands during winter improves the low, even negative, average daily weight gain (ADG) typical of extensive animal production systems in Uruguay. Nonetheless, to render the practice profitable it is crucial to control supplement feed efficiency (SFE), that is, the difference in ADG between supplemented and control animals (ADGchng) per unit of supplement dry matter (DM) intake. Little has been studied specifically on how SFE varies in these systems. The objective of this study was to quantify the magnitude and variation in SFE of growing beef cattle grazing stockpiled native Campos grasslands during winter and assess putative associations with herbage, animals, supplements, and climatic variables. We compiled data from supplementation trials carried out in Uruguay between 1993 and 2018, each evaluating between one and six supplementation treatments. The average ADG of unsupplemented and supplemented animals were 0.13±0.174 and 0.49±0.220 kg/animal/day, respectively. In both cases, ADG decreased linearly as the proportion of green herbage in the grazed grassland was lower, but the ADG of unsupplemented animals was further reduced when winter frosts were numerous. Estimated SFE were moderately high, with an average of 0.21±0.076 ADGchng/kg DM, resulting from average ADGchng of 0.38±0.180 kg/animal/day in response to an average supplementation rate of 1.84±0.68 kg supplement DM intake/animal/day (0.86±0.27 %body weight). No association was found between SFE and supplementation rate nor type (protein vs. energy-based; p&gt;0.05), but forage allowance negatively affected it, and herbage mass positively affected it, yet in a smaller magnitude, suggesting that a balance is needed between the two to maximize SFE. Weather conditions during trials affected SFE (p&lt;0.05), with greater SFE in winters with lower temperatures and more frosts. Daytime grazing time was consistently lower in supplemented animals compared to their unsupplemented counterparts, whereas ruminating time during the day was similar, increasing as the proportion of green herbage decreased. Herbage intake estimated from energy balance suggested the existence of some substitution effect. This agrees with the moderately high SFE and with the total digestible nutrients-to-protein ratio of these subtropical humid grasslands being higher than in semiarid rangelands and dry-season tropical pastures but lower than in sown pastures.

Se evalúo el efecto de la sombra asociado a la restricción en el tiempo de acceso al pastoreo sobre la performance animal y conducta en pastoreo durante el período estival. Treinta y dos novillos Hereford (313,2 ± 36,2 kg) fueron asignados al azar a dos manejos de pastoreo sobre praderas, en dos veranos consecutivos: pastoreo restringido (PR, entre las 10:00 y las 17:00 h los animales eran retirados de la pastura para un corral con sombra); o pastoreo libre (PL, los animales permanecían en la parcela, sin acceso a sombra). Parcelas independientes por tratamiento fueron pastoreadas en franjas diarias con una oferta de forraje de 6 kg MS/ 100 kg peso vivo (PV). El manejo PR mejoró la ganancia de PV (0,875 vs. 0,998 kg/día; P=0,0197) sin afectar el consumo ni la selectividad (P>0,10). La actividad de pastoreo fue mayor en PR (P<0,001) en perjuicio de las actividades de rumia y descanso (P><0,01), pero sin diferencias en la tasa de bocado (P>0,10) con relación a PL. Esta respuesta habría contribuido a mantener el consumo estable a pesar del menor tiempo de acceso a la pastura. Una reducción estimada de 27,3% en las exigencias de EMm, explicaría la mejora en el balance energético en PR con relación al PL.