Native grasslands are the main source of food for livestock in the Campos region of South America. These forage resources are heterogeneous in species composition, grazing management, and soil fertility within a context of variable climate, all of which are factors that affect forage crude protein content over time and space. Despite the importance of protein in livestock nutrition, there is a gap in the knowledge of how fertilisation, sward height, and soil water availability influence the crude protein content of these grasslands. We used data from a long-term fertilisation experiment to construct a structural model aiming to identify the main factors influencing forage crude protein content of a basaltic native grassland in northern Uruguay. The structural model revealed that both fertilisation and the increase in soil water availability (through the improvement of the nitrogen content of green leaves) are the main pathways by which forage crude protein content increases. This new approach (which identifies and quantifies the main factors that drive forage crude protein content of native grasslands) could be used to support prediction models for forage protein content in order to improve grazing livestock nutrition of Campos native grasslands.

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>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<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.

Las restricciones nutricionales de los terneros de recría sobre campo natural durante su primer invierno es un problema común en Uruguay, dada la insuficiente calidad y cantidad del forraje. La suplementación invernal del ganado de carne utilizando granos o sus subproductos y pastoreando sobre pasturas nativas, ha sido estudiada previamente, pero muy poca atención se le ha dado a el acceso restringido de animales a una base forrajera mejorada adyacente, como lo puede ser un verdeo de avena. Treinta y seis terneros machos castrados pastoreando sobre campo natural durante su primer invierno fueron asignados al azar a uno de los siguientes tratamientos: (a) Diario (D) con acceso a una parcela adyacente de avena por 4 horas al día (28 horas/semana); (b) de Lunes a Viernes (LaV) con acceso al mismo verdeo por 5 horas 36 minutos por día (28 horas/semana); (c) Testigo (T), en el que los animales no tenían acceso alguno al verdeo de avena. El experimento fue dividido en dos fases: la fase I en donde los tratamientos fueron aplicados (invierno) y la fase II, luego que los mismos fueran aplicados (primavera), a los efectos de evaluar un eventual arrastre de efectos. El acceso restringido a la avena afectó la disponibilidad y altura del forraje del campo natural en ambas fases evaluadas (P

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Dentro de los factores que limitan el mayor desarrollo de la Cadena Cárnica Ovina del Uruguay, se encuentra la marcada estacionalidad de la oferta de animales. Esta última se asocia principalmente a problemas de déficit en la cantidad y calidad del forraje que normalmente se da en verano, seguido por el otoño. Este artículo resume, sistematiza y prioriza trabajos de investigación sobre la recría y engorde de corderos durante los meses de verano y su eventual prolongación a los meses otoño. Se resumen los parámetros de producción animal registrados en investigaciones realizadas en el exterior y a nivel nacional. De la información local se destaca el estudio de varios factores, tales como la base forrajera utilizada, sistemas de pastoreo, nivel y tipo de suplementación, uso del riego, carga animal y género, sobre la productividad de corderos. Se concluye que existe una amplia gama de opciones y recomendaciones tecnológicas generadas por la investigación nacional, disponibles para asesores técnicos y productores, para evaluar la conveniencia productiva y económica de la implementación de sistemas acelerados de recría y engorde estival de corderos en el Uruguay.