This experiment investigated the influence of plane of nutrition on mammary development in heifers. Prepubertal and postpubertal heifers were fed a ration 60:40 concentrate to forage in restricted or ad libitum amounts. Dry matter intake of the heifers on restricted feeding was 60% that of heifers fed ad libitum. Average daily gain of heifers on restricted feeding was 613 g compared with 1218 g for heifers fed ad libitum. Ad libitum feeding during prepuberty lowered mammary secretory tissue weights 23% and deoxyribonucleic acid content 32% compared to restricted feeding. In contrast, there was no difference in growth of mammary secretory tissue between postpubertal heifers fed restricted or ad libitum amounts. Composition of mammary parenchyma was not affected by plane of nutrition in prepubertal or postpubertal heifers. From these data, we suggest a critical period for total mammary cell number in heifers during which mammary growth is affected adversely by a high plane of nutrition.
In replacement heifers, high levels of feeding resulting in high growth rates in the prepubertal period can cause severe reduction of the milk production potential. This has been demonstrated in many experiments; however, there are a number of experiments where this effect is not seen. In many cases, the reason for the lack of effect seems fairly obvious (short treatment periods, high pretreatment growth rates, small growth rate differences between treatment groups, variation of growth rates within treatment groups, treatment periods outside the critical period, etc.). However, in a few experiments absence of treatment effect cannot be explained in this way. This demonstrates that our knowledge on the effect of nutrition during rearing on the future milk yield of heifers is incomplete and that it may be possible to develop high growth rate feeding regimens for heifers. Experimental evidence suggests that the observed negative effects of feeding level on subsequent milk are due to impaired mammary development. Development of suitable high growth rate feeding regimens therefore requires understanding of the influence of nutrition on the physiological regulation of mammary development. Available data suggest that the growth hormone-insulin-like growth factor I axis is involved, but it is not clear how. It is likely that understanding of the role of insulin-like growth factor binding proteins is important. Alternative hypotheses involve possible effects of growth factors and modifications of mammary tissue sensitivity to hormones and growth factors.
Milk yield of the dairy cow follows a pattern termed the lactation curve. We have investigated the cellular background for this pattern. Seven mammary biopsies were obtained from each of 10 cows: at the end of lactation (d 347, equal to d 77 before next parturition); during the dry period at d 48 (4 d after dry off); 16 d before parturition; and during lactation at d 14, 42, 88, and 172. The fraction of proliferating (staining positive for Ki-67) alveolar cells was higher during the dry period (8.6%) than during lactation (0.5%). The fraction of apoptotic (staining positive by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) alveolar cells was higher immediately after dry off (0.37%) and in early lactation (0.76%) than during other periods (0.15%). The enzyme activities of fatty acid synthetase, acetyl CoA-carboxylase, and galactosyl transferase were approximately 12-, 11-, and 4-fold higher, respectively, during lactation than during the dry period. In conclusion, mammary cell proliferation is substantial in a period near parturition but otherwise low, and apoptosis is elevated at dry off and in early lactation. The increase in apoptosis in early lactation may be due to discarding nonfunctional or senescent cells or to removal of a surplus of newly synthesized cells. The activity of selected enzymes central for milk synthesis is probably not limiting for milk production.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.