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 lipids, such as isomers of CLA or fatty acids, in the diet of lactating cows. Not only can energy intake be improved, but positive reproductive responses are noted, such as increased conception and pregnancy rates.
The transition period in dairy cows extends from 3 weeks before calving through the first 3 weeks of lactation and is associated with marked changes in dietary intake and liver metabolic load. Negative energy balance, acting through the combined metabolic signaling of low blood glucose, insulin and IGF-I concentrations along with elevated NEFA, β-hydroxybutyrate, and liver accumulation of triglycerides can delay the increases in LH and FSH necessary for stimulation of ovarian follicles, estradiol production, and ovulation. These various metabolic factors, interactions and responses shift the course of postpartum ovarian activity and negatively influence the resumption of ovulatory cycles.
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 and genotype differences 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.
K. N. Galvao, M. Frajblat, W. R. Butler, S. B. Brittin, C. L. Guard, and R. O. Gilbert. Effect of early postpartum ovulation on fertility in dairy cows. Reprod.Dom.Anim. 45 (5):e207-e211, 2010.
A. Schneider, V. Absalon-Medina, G. Esposito, M. N. Corrêa, and W. R. Butler. Paraoxonase (PON) 1, 2 and 3 expression in granulosa cells and PON1 activity in follicular fluid of dairy cows. Reprod.Dom.Anim. 48:989-994, 2013.
A. Schneider, M. N. Corrêa, and W. R. Butler. Association between growth hormone receptor AluI polymorphism and fertility of Holstein cows. Theriogenology 80:1061-1066, 2013.