Restricted feeding and highconcentratediets are potential strategies for growing dairyheifers. Ruminal manipulation with additives such as Saccharomyces cerevisiaeyeastculture (YC) has been shown to alter digestibility when added to this type of diet. An experiment was conducted to investigate the ruminal fermentation and in situ digestibility of diets with 3 different levels of forage to concentrate (F:C) fed at restricted intake without and with YC addition. Three cannulated post-pubertal Holstein heifers (age 18.0 ± 1.2 months; body weight 449.6 ± 19.7 kg) were feddiets consisting of corn silage as the sole forage source in a 3 period (35-day) Latin square design. Heifers were feddiets for 21 days with no YC addition, followed by 14 days where YC was added to the diet (1 g/kg as fed basis). Low (LC), medium (MC), and high (HC) concentratediets (20, 40, and 60% concentrate) were fed once daily on a restricted basis to provide 0.22 Mcal ME/kg empty BW0.75. Rumen fluid was sampled on days 18 and 32 of each period, and rumen contents were evacuated on days 21 and 35 of each period. An in situ study was done on days 14 to 17 and on days 28 to 31. Mean ruminal pH was not different between dietary treatments and no YC effect was detected. Mean total volatile fatty acids (VFA) and ruminal ammonia-nitrogen (NH3-N) concentration was also not different among diets with different F:C. Molar proportions of acetate were decreased, and propionate were increased; while the acetate-to-propionate ratio was decreased as the concentrate level increased from LC to HC. Total VFA, propionate, and acetate as well as isoacids concentration increased, yet NH3-N concentration decreased with YC addition in all diets. From these results we conclude that feeding HC diets in restrictedamounts had minimal effects on rumenfermentation rate between different F:C diets. The addition of YC modified NH3-N and volatile fatty acid concentrations in the rumen in all 3 diets in this study, presumably through alterations in end-product production and utilization.
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