Relationship between in vivo digestibility and in situ degradability of grass in grazing

Technology Agricultural Department, I.E.S. La Granja 39792 Heras. Cantabria, Spain

Grassland Science in Europe 2011, Vol 16, 88-90.

During the grazing season three Friesian cows, fed on a grassland herbage diet dominated by Lolium perenne and Trifolium pratense, were studied in a metabolism house. A randomly blocked experimental design was used to test the efficiency of in vivo digestibility and in situ degradability values to estimate dry matter (DM) intake and nitrogen (N) excretion in faeces and urine. Significant monthly differences (P < 0.05) were found for N digestibility, degradability, intake and excretion. For each DM digestibility and degradability percentage unit increase, DM ingestion increased by1.77 and 1.91 g DM kg^-1 W^0.75 for the ranges 58.6-77.2% and 60.3-82.3%, respectively. However, in vivo digestibility showed an over-estimation of faeces-N and urine-N excretion by 55% compared to in situ degradability. Overall, the obtained results show that both techniques are reliable for predicting N intake and excretion in dairy cows fed with a grassland herbage diet.

Keywords: grass, N intake, N excretion, estimation, methodology