High body weight gain and reduced bovine mammary growth: physiological basis and implications for milk yield potential

Available evidence concerning the relationship between growth rate, mammary growth and milk yield in heifers leads to these conclusions: 1) Increased growth rate due to high feeding level before puberty onset can lead to redu ced pubertal mammary growth and reduced milk yield potential. 2) Increased growth rate due to high feeding level after puberty and during pregnancy ha ve no effect on mammary growth and milk yield. 3) Higher body weight gain d ue to higher genetic potential for growth is positively related to milk yie ld. The negative effect of high feeding level before puberty occurs in all breeds, but the level of feeding causing reduced yield varies. Variation in responses between experiments suggests that feeding regimes that support h igh growth rates without negative effect on yield can be developed. A break through most likely will originate from increased knowledge of the physiolo gical relationship between nutrition and mammary development. Our investiga tions suggest that blood growth hormone (GH) is important for mammary devel opment, and that the negative effect of high feeding level on mammary devel opment may be due to reduced blood GH. GH, however, does not bind to mammar y tissue. Experiments with exogenous GH suggest that GH acts on mammary tis sue via IGF-I, but IGF-I is increased by high feeding level - not decreased as GH. This paradoxical relationship cannot be explained by changes in cir culating IGF binding proteins. However, the sensitivity of mammary tissue t o IGF-I is reduced by high feeding level, probably due to the action of loc ally produced binding proteins and/or growth factors. (C) 2000 Elsevier Sci ence Inc. All rights reserved.