Effect of feed hydration on the dynamics of in situ ruminal digestion

Lucerne haylage (Medicago sativa), maize silage (Zea mays), ground maize, soyabean meal, and a totally mixed diet received hydration for 7 days or no hydration to test treatment effects on the dynamics of in situ dry matter (DM) and nitrogen digestion in the rumen. Duplicate samples of all feed and treatment combinations were placed in the rumen of two cannulated lactating Holstein cows for 0, 1, 2, 4, 8, 16, 32 or 64 h. Soluble, degradable, and indigestible fractions, fractional digestion rates and lag times estimated for DM differed among feeds but were unaffected by hydration. On average, 13% more DM and 19% more nitrogen were solubilized or rinsed from hydrated bags at time 0. Nitrogen disappearance was also unaffected by hydration except that the soluble and degradable fractions were higher and lower, respectively, for hydrated feeds. Although feed dry matter and nitrogen disappearance were increased by hydration at time 0, the effects of hydration on the kinetics of in situ digestion in the rumen were minor. Other factors (e.g., feed composition, particle size, microbial attachment, and rumen environment) are likely more important determinants of the kinetics of digestion in the rumen than hydration.