Use of different carbohydrate sources associated with urea and implications for in vitro fermentation and rumen microbial populations

Abstract

Context Alternative feed sources have been investigated as replacements for green forages and cereal grains traditionally used in ruminant feed. We hypothesised that, when replacing sources of true protein with non-protein nitrogen (NPN) in the ruminant diet, the efficiency of utilisation of the NPN may be affected by the source of energy and that different energy resources used as alternatives to maize may improve efficiency and maximise ruminal fermentation characteristics. Aims The objective of this study was to evaluate the effects of diets containing different carbohydrate sources associated with urea on in vitro ruminal fermentation and rumen microbial profile. Methods Four diets based on Tifton 85 Bermuda grass hay (584 g/kg dry matter) were tested as substrates: cornmeal + soybean meal (C + SM, typical diet), cornmeal + urea (C + U), cassava scraping + urea (CS + U), and spineless cactus + urea (SC + U). The experimental design consisted of randomised blocks with four treatments and five blocks. Five adult Nellore cattle with permanent fistula in the rumen were used as inoculum donors. The semi-automatic in vitro gas production technique was used in two experiments. Quantitative real-time PCR was used to monitor the changes in the rumen microbial community. Key results The diets containing C + U and CS + U decreased (P < 0.05) concentrations of isobutyrate, isovalerate, and valerate after 24 h of incubation, and all diets containing urea decreased (P < 0.05) concentrations of isobutyrate, isovalerate and valerate after 96 h and increased (P < 0.05) acetate:propionate ratio. After 96 h of incubation, the diets containing CS + U and SC + U resulted in a lower (P < 0.05) population of Ruminococcus flavefaciens than the C + U diet, and a lower (P < 0.05) population of Streptococcus bovis than the C + SM diet. Conclusions From our results, a diet containing cassava scraping produces more methane per unit of degraded organic matter, which reduces fermentation efficiency. Diets that contain corn with either soybean meal or urea result in greater degradability with lower gas production rates than diets that contain either cassava scrapings or spineless cactus with urea. Diets containing urea as a total substitution for soybean meal alter the production of short-chain fatty acids and reduce the populations of S. bovis and R. flavefaciens. Implications Use of urea to replace soybean meal in the ruminant diet alters ruminal fermentation and rumen microbial population.