STRESS IMPEDES REPRODUCTIVE PHYSIOLOGY OF DAIRY ANIMALS UNDER SUBTRIPICAL CONDITIONS - A REVIEW - M. S. Qureshi (Journal of Animal and Plant Sciences, 2012)
Dairy animals in the tropics face numerous challenges under tropical environments. They have to maintain their primary body functions within optimum ranges during the extreme seasonal variations, especially the thermal stresses during summer. The malnutrition and prevalence of diseases exert further stresses. Under such circumstances the animal has to re-visit its priorities for the body functions and survival remains the topmost priority over growth, productivity and fertility. Reproduction gets the last priority for nutritional partitioning and hence it is the first to be affected by any type of stress. The local breeds of dairy animals are well adapted to local environment of temperature variation, feed availability and diseases prevalence. However the stress factors do affect the reproductive physiology and lactation, adversely. Resistance to stress is reduced with the genetic improvement coupled with productivity enhancement of animals. The genetically improved animals exhibit repeat breeding probably due to delayed ovulation under stress conditions. There is a need to investigate the relationship among various stressors like higher milk yield, abnormal levels of blood metabolites and ambient temperature, with reproductive physiology. Relationship of stress indicators like reactive oxygen species and enzymes with pituitary, adrenal and ovarian hormones needs exploration. The stress comprises nutritional, health and thermal factors and is revealed by the inability of an animal to cope with its environment, a phenomenon that is often reflected in a failure to achieve genetic potential. The author has reported reproductive and productive disorders associated with crossbreeding in cattle. An important cause of reproductive problems in crossbred cattle in Philippines was under-nutrition, particularly at critical periods of the cow's reproductive life, reflected in the slow recovery from loss in body weight and condition score during the early postpartum period and the increased plasma BHB values at peri-partum period in some cows, indicative of negative energy balance, and the flat lactation profile. It has been suggested that a variety of endocrine regulatory points exist whereby stress limits the efficiency of reproduction. Negative feedback effects appear to operate mainly at the pituitary level during transport but at the hypothalamus during hypoglycemia. The endocrine evidence showed that stressors interfere with precise timings of reproductive hormone release within the follicular phase. Stressors also delay the onset of the luteinizing hormone (LH) surge. The author has concluded that excess intake of crude protein, associated with higher serum urea levels and low energy intake, associated with poor body condition, are the key factors for low reproductive efficiency. It may be corrected by adopting a proper feeding strategy. We also reported a decline in milk with advancement pregnancy was slight up to a point which was declared as joining point; thereafter the decline was much greater, showing a pregnancy stress on milk yield. Milk production stress was found to lower milk progesterone concentration in buffaloes. This review concludes that dairy animals need an optimum range of metabolites, body condition score, nutritional status and management conditions to express reproductive cyclicity at its best level.
Keywords: journal article; Dairy; cows; cattle; buffalo; reproduction; stress; nutrition; body condition