Published online by Cambridge University Press: 02 September 2010
Thirty-six British Friesian entire male calves were reared from 100 kg live weight (LW) to slaughter at 450 kg LW. They were offered maize silage ad libitum plus supplements of minerals and a protein concentrate or urea in three treatment diets: protein concentrate (P), urea (U) and (P/U) in which protein and urea each provided 50% of the supplementary N up to 200 kg LW followed by urea alone. Levels of N × 6·25 in the diet dry matter (DM) were 16% from 100 to 200 kg LW, 14% from 200 kg to 300 kg, and 12% thereafter. Barley was given to all the cattle over 12 mo of age at 1% of LW daily.
Live-weight gain in the 280-day period prior to the introduction of barley averaged 899, 684 and 580 (±31·0) g/day for P, P/U and U, respectively, and was greater for P than for the other two treatments (P<0·05). After the introduction of barley, gains were similar between treatments, averaging 1275, 1333 and 1270 (±43·2) g/day, resulting in mean ages at slaughter of 15, 16·5 and 17 mo for P, P/U and U, respectively.
Substitution of protein concentrate by urea was reflected in a reduced content of metabolizable energy (ME) in the diet and a reduced intake of DM and ME in period 1. Although the contents of ME in the diets were similar in period 2, when barley was given to the cattle, there was little evidence of compensatory growth in treatment P/U and U compared with treatment P. Live-weight gains predicted from ME intake showed good agreement with actual gains. However, reduced intakes and gains by treatments P/U and U compared with treatment P meant that the target weight and age at slaughter of 450 kg and 15 mo of age was met only by treatment P. For the complete system the cattle consumed an average of 2·3 t DM per head of which maize silage DM comprised 78%. Efficiency of feed use averaged 15·5 kg live-weight gain per 100 kg total DM eaten.