The utilization of maize silage for intensive beef production: 3. Nitrogen and acidity as factors affecting the nutritive value of ensiled maize

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Abstract
Twelve rumen-fistulated British Friesian castrated male calves, initially three months of age and 77 kg live weight (LW) were fed, both ad libitum and at a restricted level of 21 g organic matter (OM)/kg LW, on either maize silage (26% dry matter, 8.6% crude protein in D.M.) alone or with 2% of urea in the silage D.M., added at the time of feeding. At each level of urea (0 or 2%) silage pH was either increased to pH 5·45 by the addition of sodium bicarbonate, or left unchanged (pH 3·95) in a change-over design within each urea level. Addition of urea increased voluntary silage D.M. intake by 8% (P < 0·05) and partial neutralization increased consumption by 12% (P < 0·01). However, the response to urea only occurred when the silage was partially neutralized.At restricted intake, addition of urea increased rumen ammonia nitrogen from 3·17 to 13·41 mg/100 ml (P < 0·001) and blood urea nitrogen from 2·20 to 10·06 mg/100 ml (P < 0·001). There was no significant effect of addition of urea or of bicarbonate on rumen pH, VFA concentration or energy digestibility. However, the addition of urea significantly increased N retention (P < 0·01). Partial neutralization of silage was associated with an increase in venous blood pH from 7·366 to 7·396 (P < 0·001), and urine pH from 7·43 to 8·42 (P < 0·001).It is suggested that both low nitrogen content and acidity limit the nutritive value of ensiled maize, and that the effects of addition of urea and of partial neutralization may be metabolic rather than ruminal.

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