Apparent synthesis of thiamin and vitamin B12 in rumen of lactating dairy cows fed alfalfa or orchardgrass silages of different particle lengths
Effects of forage particle length on apparent ruminal synthesis and post-ruminal supply of thiamin and vitamin B12 were evaluated in two experiments. Alfalfa and orchardgrass were harvested and ensiled at two theoretical lengths of cut: 19-mm (long cut, LC) or 10-mm (short cut, SC). Within a forage type, rations containing silages of each length of cut as the sole forage were offered to ruminally and duodenally cannulated lactating Holstein cows in crossover design experiments. Experiment 1 compared diets containing LC and SC alfalfa silage (~47% forage, ~20% forage NDF and ~25% total NDF) offered to 13 cows in two 19-d treatment periods. Experiment 2 compared diets containing LC and SC orchardgrass silage (~50% forage, ~23% forage NDF and ~28% total NDF) offered to 15 cows in two 18-d treatment periods. In Experiment 1, intakes of thiamin and vitamin B12 were greater (P<0.01) for LC than SC (thiamin: 89.4 vs. 77.5 ± 2.13 mg/d; vitamin B12: 118.6 vs. 67.1 ± 2.55 µg/d). Neither duodenal flows of thiamin (P=0.94, 174.2 ± 18.87 mg/d) and vitamin B12 (P=0.17, 8223 ± 713.0 µg/d) nor apparent ruminal synthesis of these vitamins (thiamin, P=0.15, 90.9 ± 17.82 mg/d; vitamin B12, P=0.16, 8131 ± 711.0 µg/d) were affected by treatment in Experiment 1. In Experiment 2, there was no effect of treatment on the intake of vitamins (thiamin, P=0.79, 71.9 ± 2.55 mg/d; vitamin B12, P=0.85, 111 ± 3.9 µg/d), duodenal flows (thiamin, P=0.80, 156 ± 9.5 mg/d; vitamin B12, P=0.88, 6887 ± 476.0 µg/d), or their apparent ruminal synthesis (thiamin, P=0.82, 84.1 ± 8.14 mg/d; vitamin B12, P=0.88, 6725± 473.0 µg/d). The reduction of legume particle length decreased the dietary supply of thiamin and vitamin B12. However, forage particle length, in the studied range, did not affect apparent synthesis of thiamin and vitamin B12 in rumen or the supply of these vitamins to the sites of absorption.
Keywords: dairy cow, thiamin, vitamin B12