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.

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.

Previous studies on winter supplementation of growing cattle grazing stockpiled native Campos grasslands suggest that forage allowance (FA), herbage mass, and weather conditions before and during the supplementation period could all affect supplement feed efficiency (SFE)—that is, the difference or change in average daily gain (ADG) between supplemented (S) and control (C) animals (ADGchng, kg) per unit (kg) of supplement dry matter (DM) intake. In this study, we analyse data from fifteen collated winter supplementation trials carried out in Uruguay between 2004 and 2018. The working hypotheses of this research paper were: (i) that average substitution rates are positive, and (ii) that ADGchng is not constant throughout the supplementation period and that its variation may be attributed to sward, animal or weather variables. There were two main objectives: (i) to estimate the average supplement substitution rate (sSbR, kg forage, f, dry matter, DM intake reduction: kg supplement DM intake) and potential herbage intake substitution rate (hSbR, kg fDM intake reduction: kg fDM intake of control animals), and its association with SFE, and, (ii) to assess the existence of different phases and supplementation response patterns and its association with other relevant variables. Estimated substitution rates were always positive (sSbR = 0.3–1.1 kg/kg; hSbR = 0.1–0.3 kg/kg) and were negatively and moderately associated with SFE. Supplementation proved to be a dynamic process where three possible supplementation responses over the supplementation period were identified (linear, quadratic and Weibull). While linear patterns did not appear distinctly associated with any particular set of variables, quadratic models were mostly associated with herbage biomass and substitution rates, whereas Weibull models were the clearest in their association with frosts. Regardless of the response pattern, at the beginning of the trials it was the animals’ body weight and supplement quality that most influenced supplement response, whereas towards the end, supplementation intake, supplemented animals’ ADG and forage quality played a more relevant role. The estimated parameters and response patterns are expected to be used as inputs in decision support systems for livestock farmers in the future.

Stockpiling forage (i.e., deferring grazing) is one way to provide forage for livestock during intervals of low pasture growth, but there are trade-offs as nutrient content declines with increasing forage maturity. Phosphorous (P) concentration, crude protein (CP) content and organic matter digestibility (OMD) were evaluated in two C3 and four C4 grasses native to the South American Campos grasslands. These were: Bromus auleticus (BROAUL) and Nasella neessiana (NASNEE) as C3 grasses and Andropogon lateralis (ANDLAT), Mnesithea selloana (MNESEL), Paspalum dilatatum (PASDIL), and Paspalum notatum (PASNOT) as C4 grasses. The grasses were grown in pots during five stockpiling periods (450, 900, 1350, 1800 and 2250 degree days, approximately 20, 40, 60, 80 and 100 days). As the forage deferment increased, the nutritional value decreased more in C4 than in C3 grasses. Short rest periods (approximately 40 days) are recommended for PASDIL and MNESEL, and medium rest periods (approximately 80 days) for ANDLAT and PASNOT. However, the C3 grasses BROAU and NASNEE maintained high P and CP concentrations and may be the most appropriate option for long rest periods (≥100 days). This information is important to manage different Campos grassland communities for the optimal rest period according to the dominant species.

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. La pérdida de peso durante el invierno del ganado en crecimiento sobre campo natural puede ser revertida mediante la suplementación con energía y proteína. Para que esta práctica sea rentable, es necesario maximizar la eficiencia en el uso del suplemento —EUS—, diferencia en la ganancia media diaria —GMD—, entre animales suplementados —S— y testigo —T—, RespSup —respuesta a la suplementación— por kilo de materia seca —MS. Se compilaron 25 ensayos realizados en Uruguay entre 1993 y 2018. La EUS fue de 0,21 ± 0,076 kg RespSup/kg MS, promediando una RespSup de 0,38 ± 0,180 kg/an/día. No se encontró asociación entre EUS y tasa de suplementación ni tipo de suplemento (p < 0,05), pero la asignación de forraje la afectó negativamente, mientras que la disponibilidad la afectó positivamente, si bien en menor magnitud. Las condiciones del clima afectaron la EUS (p < 0,05), con mayores EUS en inviernos de temperaturas más bajas y con más heladas. Otro análisis conjunto se realizó con 15 de los 25 ensayos originales para estimar las tasas de sustitución (TS) y determinar la existencia de distintas fases en la evolución de la RespSup a partir de sus patrones de evolución. Las TS fueron de 0,3-1,1 kg/kg y fueron asociadas negativamente con EUS. Tres tipos de RespSup fueron identificadas: linear, cuadrática y Weibull. Las cuadráticas estuvieron asociadas con la disponibilidad y tasas de sustitución, mientras que las Weibull estuvieron asociadas con las heladas. Finalmente, un experimento de tipo grazing-down fue realizado comparando animales S y T en su dinámica de defoliación. No se encontraron diferencias entre la dinámica horizontal, y cuando el horizonte superior fue consumido en > 80 %, el horizonte subsecuente había sido consumido en un 50 %. La suplementación invernal sobre campo natural es una tecnología altamente confiable que mejora el desempeño del ganado de carne en crecimiento.