The modern dairy cow has been genetically selected for high milk production. Many cows now average more than 10 gallons of milk/day with peak yields of 120 lbs/day. High milk production places a heavy metabolic burden on the cow to satisfy the energy and protein requirements for milk production. During early lactation, the energy requirement for milk synthesis is 3.5-fold greater than for body maintenance. Reproductive performance is challenged during this period, since nutrient demands of lactation have a higher priority for available energy than does reproduction. Consequently, the overall fertility in dairy cattle has declined as milk yields have increased.
Selection for high milk production in dairy cows has been associated with a decline in fertility. Nutrition, metabolic demands, and management are the major factors in this interaction. Under traditional management, success towards establishing the next pregnancy in lactating cows is benefitted when estrous cycles become reinstated early after parturition. However, the magnitude of negative energy balance during the early weeks of lactation is a major constraint to this process and acts to depress gonadotropic hormones necessary for ovarian activation. In high producing cows as body reserves are mobilized to provide energy for lactation, ovarian cycles are delayed with a consequent reduction in fertility.
The effects of lactation on ovarian function are studied by both endocrine and nutritional approaches. Blood hormonal profiles monitored at frequent intervals demonstrate the importance of rapid "bursts" of hormone release that signal development of follicles on the ovary. Growth of these follicles and their evolution to maturity and ovulation is monitored by ultrasonography. This ultrasound technique provides a powerful tool for assessing interaction among follicles and the success of ovulation. In turn, quantifying a cow's intake of nutrients and nutrient balance clearly demonstrates the interaction between nutrition, metabolism and the reproductive processes. Strategies to maintain and improve energy intake during the transition period around parturition and early lactation are being investigated.
One avenue investigated has been the inclusion of supplemental fat, such as isomers of CLA or trans-fatty acids, in the diet of lactating cows. Not only can energy intake be improved, but positive reproductive repercussions have also been noted, such as increased conception and pregnancy rates. The mechanism through which CLA affects reproduction may involve improved ovarian follicular steroidogenesis and increased circulating concentrations of IGF-1. It is also a possibility that the peroxisome proliferator-activated receptors (PPARs) play a role in the effects of fatty acids, especially CLA, on reproduction. Elucidating the mechanism by which CLA isomers beneficially act upon ovarian follicles, oocytes and corpus luteum progesterone secretion is an important area of current research.
Reproduction in lactating cows depends on the delicate balance between prioritizing nutrient use for lactation versus establishment of a new pregnancy. As genetic progress places even greater metabolic demands on the cow, new research is needed to successfully formulate the necessary diets to support both processes, as well as to understand what metabolic parameters directly regulate reproductive processes.
Selected Recent Publications:
M. J. de Veth, D. E. Bauman, W. Koch, G. E. Mann, A. M. Pfeiffer, and W. R. Butler. Efficacy of conjugated linoleic acid for improving reproduction: A multi-study analysis in early lactation dairy cows. J Dairy Sci. 92:2662-2669, 2009.
C. O. Lemley, S. T. Butler, W. R. Butler, and M. E. Wilson. Short communication: insulin alters hepatic progesterone catabolic enzymes cytochrome P450 2C and 3A in dairy cows. J Dairy Sci. 91:641-645, 2008.
E. Castaneda-Gutierrez, B. C. Benefield, M. J. de Veth, N. R. Santos, R. O. Gilbert, W. R. Butler, and D. E. Bauman. Evaluation of the mechanism of action of conjugated linoleic acid isomers on reproduction in dairy cows. J Dairy Sci. 90:4253-4264, 2007.