Effect of forage: concentrate ratio on ruminal metabolism and duodenal flow of fatty acids in beef steers

The objective of this study was to determine the proportion of forage in the diet which would maximize duodenal flow of unsaturated fatty acids in beef steers supplemented with linseed oil. A second objective was to determine how diets differing in forage content, but equal in nitrogen (N), energy and lipid supply, would affect the duodenal flow of C18:1 and conjugated linoleic acid (CLA) isomers. Eight Hereford×Friesian steers (533±13·6 kg), prepared with rumen and duodenal cannulae were offered one of four forage:concentrate (F:C) ratios: F80C20; F60C40; F40C60 and F20C80 on a dry-matter (DM) basis. All diets were offered at 0·013 body weight and designed to be isonitrogenous and isoenergetic with total lipid made up to 0·06 DM intake with linseed oil. The experimental design was a replicated incomplete 4×4 Latin square with three periods. Increasing the concentrate component in the diet from 0·20 to 0·60 reduced rumen pH from 6·58 to 6·37 and caused a small but significant shift in volatile fatty acid molar proportions, decreasing the non-glucogenic ratio. Rumen ammonia-nitrogen concentration was also significantly reduced with increasing concentrate, from 156·8 to 101·0 mg N per l on F80C20 and F20C80, respectively. Microbial nitrogen (MN) and the efficiency of microbial protein synthesis was significantly elevated as forage level decreased from 51·6 to 72·4 g/day and 17·2 to 27·3 g MN per kg organic matter apparently digested in the rumen, respectively for F80C20 and F20C80. Intake and duodenal flow of C18:1n-9 and C18:2n-6 were significantly higher with increasing concentrate level in the diet whereas C18:3n-3 intake and flow was not different, averaging 143·6 and 6·37 g/day, respectively. There were no differences in the flows of total C18:1 trans or CLA (47·7 and 1·79 g/day, respectively) across the diets. However, although not significantly different in duodenal flow there were trends (P<0·1) for an increasing proportion of trans-10 and a decreasing proportion of trans-11 when increasing the concentrate in the diet. Biohydrogenation of C18:2n-6 decreased from 0·91 to 0·85 when increasing concentrate in the diet from 0·20 to 0·40 but further increases had no effect. F:C ratio had little effect on the flow of unsaturated fatty acids, C18:1 trans and CLA to the duodenum of beef steers, and this may relate to the ability of the rumen to buffer the large changes in concentrate intake.