Effect of Level and Degradability of Rapeseed Meal in Rations for Dairy Cows: 2. Diet Digestibility, Dietary Nitrogen Partition and Urinary Purine Derivatives Excretion

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Abstract
The effects of level and degradability of rapeseed meal on ration digestibility, N balance, urinary purine derivatives excretion, milk production and milk composition in dairy cows were studied. A total of 24 multiparous lactating diary cows were selected from a production experiment and divided into two 12-cow groups according to their lactation stage (LG1, 14th-16th week of lactation; and LG2, 26th-30th week of lactation). Within lactation groups, cows were fed four different rations (3 cows per treatment). Four concentrate mixtures with two levels of protein (19% and 14% of CP on DM basis) and two different rumen degradabilities (72% and 52%) of the dominating protein feed rapeseed meal were fed to the cows together with pasture silage and hay. Apparent total tract digestibilities were not affected by the treatments. N balance was positive and similar between diets in both lactation groups. N partition data were similar between treatments on LG1, while on LG2 a tendency for a more efficient use of the dietary N was detected on lower protein degradability diets. Urinary excretion of purine derivatives remained unaffected by the treatments in LG1, but was higher in cows fed low-protein degradability diets on LG2 associated with a higher DM intake. A positive correlation was observed, in both lactation groups, between urinary purine derivative excretion and digestible organic matter intake. Milk production tended to increase with increasing protein intake in LG1, and with increasing DM intake in LG2. Milk composition was not significantly affected by the diets in any of the lactation groups.

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