Calf-rearing management is a particularly important field in animal production as the calf will be the future dairy or beef cattle. Therefore, a healthy, robust, and productive animal will ensure successful farming. High morbidity and mortality rates elucidate the need and potential for improvements in calf rearing. One of the key factors is the nutrition of the calf. Beginning with the essential colostrum feeding to provide the newborn with nutrients, immunoglobulins, and additional bioactive factors, the further provision of sufficient energy and nutrients will improve the calf´s development in short-term and possible long-term effects.Like all mammals, neonatal calves depend on the supply of liquid feed, i.e., milk or milk replacer (MR), before the rumen development is adequate to absorb and digest solid feed for maintenance and growth. In conventional feeding regimes calves are fed restricted amounts of liquid feed (e.g., 4 6 L/d) for a few weeks, whereas ad libitum intake of milk or MR is at least twice as high and naturally lasts for several month. Hence, restricted-fed calves are not able to express their natural feeding behavior and cannot exploit their full growth potential. Furthermore, feed additives might increase the growth performance and feed efficiency. The short-chain fatty acid, butyrate, is known as a natural growth promoter and affected body growth, development of the gastrointestinal tract, and health in different species (i.e., calves, piglets, and chickens). Therefore, in the critical rearing stage where young stock is prone to health maladies, feed additives are used to improve health and robustness. An improved maturation of the intestinal tissue would contribute to the postnatal development of the calves through a greater and more viable absorptive surface.In the present study, the effects of ad libitum MR feeding and butyrate supplementation on the postnatal development, energy metabolism, and the regulation of growth through the somatotropic axis were evaluated. The trial was conducted with 64 Holstein calves (32 female, 32 male) that were studied from birth until wk 11 of life. All calves received the same amount of colostrum (2.5 kg) from their dam within 2 hours postnatum. Subsequent meals of transition milk were either offered in restricted amounts of 6 L/d (Res; n = 32) or ad libitum (Adl; max. 25 L/d; n = 32) until d 3 of age. From d 4 until d 56, dietary planes continued with MR (12.5 % dry matter) feeding, whereby feeding groups were subdivided and received MR either without (ResB-; AdlB-) or with 0.24 % butyrate supplementation (ResB+; AdlB+). All calves were gradually weaned to 2 L/d from wk 9 to 10, and 2 L MR/d were offered until the end of the trial. Concentrate, hay, and water were freely available.Performance data were determined by daily feed intake and weekly weighing. On four time points in wk 1, then weekly or once every other wk (insulin-like growth factor binding protein; IGFBP) until wk 11 of age, blood samples were taken to evaluate plasma concentrations of specific metabolites, hormones, and the systemic IGF system. Liver samples were collected on d 50 and 80 of age to determine gene expression related to glucose metabolism and to the hepatic IGF system.Calves fed liquid feed ad libitum had lower concentrate intake but higher milk and MR intakes which resulted in elevated metabolic and endocrine plasma levels, e.g., of glucose and insulin. The enhanced preweaning nutrient and energy intake positively affected the IGF system, especially with higher IGF-I and IGFBP-3 but lower IGFBP-2 concentrations. The effect of accelerated anabolic metabolism and stimulation of the somatotropic axis reflected the overall greater growth performance in Adl compared to Res calves. The supplementation of butyrate in MR had no additional effects on feed intake and body growth in the treatment groups. However, butyrate supplementation suppressed glucose status and the IGF system at specific time points throughout the study.In conclusion, butyrate supplementation had no additional effects, but preweaned calves benefit from ad libitum milk or MR feeding that stimulated the intermediary metabolism and growth performance. Furthermore, the intensive feeding practice allowed the expression of a more natural feeding behavior.
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