RESUMO: A demanda por proteína animal tem aumentado consideravelmente. Diante da situação faz-se necessário tecnologias que permitam acréscimos na produtividade com vista na sustentabilidade. Dessa forma, considerando os sistemas em pasto serem a forma mais barata de produzir carne, a suplementação deve ser considerada ou as condições de subnutrição aceitas. As gramíneas tropicais possuem uma estacionalidade de produção, bem como uma variação nos componentes nutricionais, sendo a suplementação do pasto capaz de minimizar as características da baixa qualidade das forrageiras durante o período seco, e o diferimento das pastagens torna-se uma ferramenta para melhorar a disponibilidade do pasto. A suplementação animal no período seco permite maior crescimento de microrganismos ruminais devido ao aumento de nutrientes que sao disponibilizados via suplementação, aumentando o consumo e digestibilidade da forragem, consequentemente melhorando o aproveitamento dos alimentos disponíveis. Objetivou-se avaliar a suplementação na época da seca, bem como suas causas e efeitos, por meio de uma revisão bibliográfica. Palavras-chave: nutrição, diferimento, rendimento econômico .

Although Brazil holds one of the largest commercial cattle herd in the world, about 209 million head (IBGE, 2011), it is observed that the average stocking rate is very low, resulting in lower potential productivity of the livestock sector. Among the various factors that contribute to this low productivity, can highlight the seasonality in food supply as an aggravating factor for the productive sector. Basically, in all regions of Brazil, seasonality is characterized by variations in the availability and quality of forage in response to changes in climatic conditions which do not allow forage plants have uniform growth throughout the year. The choice of forage grass to form a pasture, must be very careful to increase their productivity of dry matter and crude protein content, with seasonal balance and acceptability by animals. The pasture management aims to achieve balance between the performance / quality of forage produced, maintaining the desired botanical composition. Associated with these factors has been able to obtain good results by field of animal production. To obtain proper pasture management, it is necessary to know beyond the physical, structural and anatomical characteristics of forage species, the amount of forage offered to animals and nutritive value, but also the amount of forage being consumed by the animal and nutritional value the forage actually consumed.

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The objective of this experiment was to evaluate the effects of crude glycerin (CG) and virginiamycin (VM) diets on ruminal fermentation and microbial population of feedlot Nellore cattle. Eight rumen fistulated bulls (BW = 600 ± 34 kg; 26 ± 3 months) were used in a replicated 4 × 4 Latin square (21-d periods) with 2 × 2 factorial arrangement of treatments: diets without virginiamycin (VM−) or virginiamycin at 25 mg/kg DM (VM+) combined with diets without crude glycerin (CG−) or CG (80% glycerol) at 100 g/kg DM (CG+). The sugar cane bagasse was used as the exclusive roughage in the proportion of 200 g/kg in dry matter of diet and crude glycerin replaced corn in the diet formulation. Ruminal samples were collected immediately before feeding and at 3, 6, 12 and 18 h post feeding on days 20 and 21 of the sampling week. Data were analyzed in a replicated 4 × 4 Latin square with a 2 × 2 factorial using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). There were no CG × VM interactions for any variable measured (P ≥ 0.10). The intake of DM had a tendency to be greater in CG+ than CG− diets (P = 0.07). Apparent total tract digestibilities of nutrients were similar among diets (P ≥ 0.10). Diets with CG or VM had similar values of pH (mean = 6.15; P ≥ 0.10). Data showed that CG or VM did not affect the concentration of total VFA (116.92 mM; P ≥ 0.10). The proportion of propionate increased 27.5% in CG+ diets, regardless of VM inclusion (P = 0.01). Acetate:propionate ratio was lower in CG+ compared to CG− diets (3.58 vs. 2.12; P ≥ 0.10). Valerate and butyrate proportion was greater in CG+ than CG− diets (P < 0.05). The inclusion of VM or CG did not alter the relative abundance of fibrolityc bacteria (P ≥ 0.10). Total protozoa counts (P = 0.052) and Metadinium spp. (P = 0.058) had a tendency to decrease in VM+ than VM− diets (P < 0.10). Crude glycerin had positive effects on rumen fermentation products and can replace virginiamycin with increment of Megasphaera elsdenii abundance. However, combining virginiamycin and glycerin does not affect positively rumen fermentation and growth of bacteria that metabolize lactate. 1. Introduction Feedlots diets of beef cattle are generally rich in grains with high starch content. The great concentration of starch present in