The utilization of maize silage for intensive beef production: I. The effect of level and source of supplementary nitrogen on the utilization of maize silage by cattle of different ages

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Abstract
SUMMARY: Fifty-four British Friesian castrated male calves, initially 3, 6 or 9 months of age (107, 180 and 249 kg initial live weight, respectively), were individually fed for 83 days on maize silage (27·9% dry matter (D.M.), 10·7% crude protein in D.M.), offeredad libitum. Silage was offered either alone or supplemented with cobs of dried lucerne (21% of total D.M. intake). Three levels of urea (0, 1 and 2% of silage D.M.) were added to the silage before feeding.Total D.M. intakes averaged 23·0, 23·4 and 21·6 g/kg live weight (LW) for the 3-, 6- and 9-month-old animals, respectively. Addition of urea increased silage intake by 11% in the 6-month-old group but there was little effect in the 3- and 9-month-old groups. Lucerne supplementation reduced silage D.M. intake from 22·0 to 18·4 g/kg LW (P< 0·001) and increased total D.M. intake by 1·4 g/kg LW (P< 0·001).Live-weight gain (LWG) of the cattle fed on silage alone increased (P< 0·001) with increasing age of animal. The main effect of urea was to elevate (P< 0·001) LWG from an average of 0·79 (no urea) to 0·94 kg/head/day (2% urea). However, it appeared that most of this effect was confined to the 6-month-old group. The effect of lucerne on LWG decreased with increasing age of animal (P< 0·001). Inclusion of lucerne in the diet significantly reduced the response to urea (P< 0·05).Feed conversion efficiency (LWG/100 Mcal DE intake) decreased (P< 0·01) with increasing age of animal but increased with urea addition from an average of 4·7 (no urea) to 5·3 kg LWG/100 Meal DE intake (2% urea). The response to lucerne supplementation in terms of efficiency was greatest in the 3-month-old group and thereafter declined markedly with increasing age of animal.The results of this experiment indicated that cattle older than 6 months of age (180 kg LW) could achieve a rate of growth of 1·0 kg/head/day on maize silage supplemented solely with urea, but that younger animals required supplementary lucerne to support a high rate of live-weight gain.