The effects of level of nutrition before and after 8 months of age on subsequent milk and calf production of beef heifers over three lactations

Abstract

One hundred and five Hereford heifers were reared on varying amounts of the same diet of grain and hay to grow at either high (H: 0·91 kg/day), medium (M: 0·67 kg/day) or low (L: 0·55 kg/day) rates between 2 and 8 months of age. Between 8 and 14 months, intake was restricted for half of the H heifers (HL: 0·14 kg gain per day), ad libitum for half of the L heifers (LH: 0·97 kg gain per day) and moderate for all other heifers (HM: 0·57; MM: 0·58; LM: 0·55 kg gain per day). These treatments resulted in a range of live weights at 8 (weaning) and 14 (premating) months commonly seen in commercial production. Heifers were mated with Hereford bulls at 14 months of age and subsequently grazed at pasture. Milk yields (I/day) at 30 days post partum and the 240·day sex-adjusted calf weaning weights (kg) for HM, HL, MM, LH and LM heifers in the first lactation were 4·1, 218; 4·9, 236; 4·3, 221; 5·7, 241; and 6·3, 259 respectively (P < 0·01). Calves from HM and MM cows were significantly smaller and leaner than those weaned by LM cows. Milk yields 30 days post partum and 200·day sex-adjusted calf weaning weights for the second lactation averaged: HM, 5·2 and 202; HL, 6·8 and 217; MM, 7·0 and 222; LH, 6·6 and 219; and LM, 7·0 and 229, respectively; and for the third lactation: 5·1 and 211; 6·5 and 225; 7·2 and 223; 7·4 and 230; and 7·6 and 237, respectively. The continued lower milk yield of HM cows (P < 0·05) was offset by a trend (NS) to higher fertility and lower calfmortality, particularly in the 1st and 2nd years of production. Over the three lactations studied, the number of calves weaned per opportunity and the total actual weight (kg) of calves weaned per cow kept averaged: HM, 0·86 and 573; HL, 0·66 and 465; MM, 0·68 and 466; LH, 0·70 and 512; and LM, 0·67 and 507, respectively.The high growth rates commonly achieved by autumn-born Hereford heifers under commercial grazing conditions may reduce their subsequent milk-yield potential. This potential appears to be more sensitive to level of nutrition before 8 months of age than between 8 and 14 months.