Effect of feeding before puberty and during gestation on milk production potential and body development of dairy replacement heifers

This thesis consists of studies concerning the effect of feeding before puberty and during gestation on pre-pubertal mammary growth, subsequent milk yield and body development of replacement heifers. Publications I and II investigated the effect of high (daily gain 850 g) or low (daily gain 650 g) levels of feeding with urea or rapeseed meal as protein supplements on growth and mammary development of pre-pubertal Finnish Ayrshire heifers. It was shown that heifers on the high feeding level had less mammary parenchymal tissue compared with heifers fed lower levels of feeding. The amount of mammary parenchymal tissue was positively correlated with plasma growth hormone (GH), but not plasma IGF-I concentrations. Furthermore, no correlation existed between plasma GH and IGF-I concentrations. On a high feeding level Finnish Ayrshire heifers under 220 kg live weight had a higher growth rate when rapeseed meal rather than urea was used as protein supplement on a hay-barley based diet. For pre-pubertal slaughter heifers it appears that urea is not suitable source of supplementary nitrogen for hay and barley based diets. Instead, protein source had no effect on pre-pubertal mammogenesis at either feeding level. The effect of feeding intensity during gestation on subsequent milk yield was investigated in publication IV. It was shown that during the first six months of gestation daily gains of 800 g, or higher, had no effect on subsequent milk yield but resulted in greater fat deposition and reduced postpartum intake potential. On the other hand, during the last trimester a high level of feeding (live weight change over 800 g/d) was advantageous in attaining maximal milk production. The effect of feeding on body development of pre-pubertal (publications I and III) and pregnant heifers (publication IV) was examined. It was observed that pre-pubertal heifers fed on a low level of feeding had higher wither heights at puberty than heifers fed more intensively. Results suggest that pre-pubertal wither height is determined more by age than live weight. Heart girth was shown to be a good predictor of pre-pubertal live weight. During pregnancy high (gain 800 g/d) compared with moderate (gain 650 g/d) planes of nutrition had no effect on body size (wither height, body length) but increased heart girth, hip width and body condition score of primiparous cows at parturition. Live weight at parturition and daily gain before and after breeding had positive genetic correlations with first lactation milk yield within field data reported in publication V. Therefore, it appears that genetic selection for higher milk production will gradually lead to higher genetic growth potential. Such changes need to be taken into account in future recommendations of daily gain acceptable for pre-pubertal dairy replacement heifers. Based on the current studies it was suggested that for pre-pubertal Finnish Ayrshire heifers daily gains above 650 to 700 g have detrimental effects on pre-pubertal mammogenesis. Dietary protein source has no effect on mammary growth. During the first six months of gestation a moderate feeding level is recommended to avoid excessive fat deposition and ensure maximal postpartum intake. However, during the last trimester intensive feeding is necessary in attaining maximal milk production.