Effect of live weight gain before or during pregnancy on mammary gland development and subsequent milk production of Australian Holstein-Friesian heifers

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Abstract
Two groups of 90 Holstein-Friesian heifers were grown at 0.60 kg/day liveweight gain up to 110 kg liveweight and then grown at either 0.18, 0.62 or 0.94 kg/day for 15 weeks commencing at about 4 months of age. All heifers were then grown at 0.59 kg/day, except for a period of 15 weeks commencing at 4.5 months of pregnancy when the heifers grown at 0.62 kg/day from 110 kg liveweight were grown at either 0.22, 0.59 or 1.06 kg/day. Groups of heifers were milked or slaughtered either at mating (300 kg liveweight) or 2 weeks after calving and the chemical and histological composition of the udders determined. Mean daily yields (kg) of milk fat and solids not fat (SNF) during the first 10 weeks of the first lactation were significantly (P< 0.05) higher for heifers grown at 0.18 (0.70, 1.58) and 0.62 kg/day (0.61, 1.42) than for those grown at 0.94 kg/day (0.49, 1.19) before mating. During the complete first lactation, there was a trend for higher milk fat yield (kg) and concentration (g/kg) of heifers grown at 0.18 kg/day (122, 44) compared with those grown at 0.62 (119, 41) and 0.94 kg/day (114, 41) before mating. During the first 10 weeks of the second lactation, the mean daily yield (kg) of milk fat was significantly (P < 0.05) higher for the heifers grown at 0.18 kg/day (0.79) than for those grown at 0.62 (0.68) and 0.94 kg/day (0.74) before first mating. Rate of liveweight gain of the heifers during pregnancy had no significant effect on the production of milk or milk components. There were generally no significant differences in the composition of the udders from heifers grown at different rates of liveweight gain before mating or during pregnancy. It was concluded that, although there were no detrimental effects of rearing heifers at 0.18 kg/day before mating on production during the first lactation, the economic implications of at least a 6-month delay in commencement of lactation compared with heifers reared at a rate of liveweight gain of 0.62 kg/day must be considered.