Abstract
The objective of this study was to determine the effects of the administration of recombinant bovine somatotropin (rbST) throughout lactation to high-yielding subfertile (conception > 270 days postpartum) Holstein cows undergoing extended lactations (≥480 d) on milk production and reproductive performance. The study used two large adjacent commercial herds with similar management in a hot area of northern Mexico (25° N). Cows in one herd (n=2341) received a 500-mg dose of rbST every 14 d until dried off, starting 60 days postpartum. The other herd served as control (n=984). Across parity, rbST-treated cows produced 20% greater milk yield than control cows in 305-d lactations. Across parities, milk yield from 305 d postpartum to the end of lactation was 2734 kg higher in rbST-treated cows than control cows. Across parities rbST-treated cows produced 4777 more kg of milk during the entire lactation (mean 605 d) than the control group (mean 572 d). Conception rates (CR) at first service were only 3.2% and 5.9% (p<0.05) for control and rbST-treated cows, respectively. Overall CR was 43.5% and 61.6% (p<0.05) for control and treated cows, respectively. Control cows required 2.4 more (p<0.01) services per conception than rbST-treated cows. It was concluded that in this hot environment, the use of rbST suits dairy producers because it substantially improves both milk yield and reproductive performance in subfertile high-yielding Holstein cows. By markedly increasing lactation length and lactation persistency farm efficiency is greatly improved due to extended herd life.
Highlights
The steady genetic selection of dairy cows has brought about cows with high milk yields but reduced reproductive performance under high-input modern production systems (Berry et al, 2016)
The extended lactations in the present study were not the result of a conscious decision to postpone the first insemination after calving, but to the incapability of cows to become pregnant before 200 days postpartum
This study reiterates that the use of recombinant bovine somatotropin (rbST) allows each cow to produce an average of 20% additional milk in 305-d lactations which derives from the homeorhetic function of this hormone regulating nutrient partitioning and increasing milk production and improving the efficiency of milk synthesis (Bauman, 1999). rbSTinduces an increase in blood growth hormone and insulin-like growth factor-1 (IGF-1) concentrations which improves milk yield through increased lipolysis, nutrient prioritization to the mammary gland, and proliferation and maintenance of epithelial cells of the mammary gland (Bauman, 1992)
Summary
The steady genetic selection of dairy cows has brought about cows with high milk yields but reduced reproductive performance under high-input modern production systems (Berry et al, 2016). Heat stress compromise physiology and production of dairy cows because of the physiolo gical adaptations employed to prevent hyperthermia (Rhoads et al, 2009). Both thermal stress and high milk yield reduce fertility due to failure of the ovary to appropriately interact with the hypothalamus and pituitary during the periconception period (Lopez et al, 2005) which leads to reduced expression of estrus (Boer et al, 2010) and defective oocytes/embryos (Leroy et al, 2008). Dairy cows going through the transition period shift from positive to negative energy balance which trigger a cascade of postpartum disorders (Vergara et al, 2014) and immune suppression (Hammon et al, 2006)
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