Relationships of Negative Energy Balance with Fertility

Calve...Produce high amounts of milk...Eat lots of feed...
Recover from calving...Start cycling normally.

We ask a lot of our cows these days. Conception rate in large commercial dairy herds stands at only 35-40% for mature cows as compared with 51% in first lactation cows and normally 65+% in virgin heifers. These differences within a herd indicate that fertility declines with each calving until cows reach maturity.

Negative Energy Balance (NEBAL) results when cows use more energy in a day than they consume from their feed. Beginning about one week before calving, feed intake (DMI) slows resulting in a NEBAL that will continue to increase over the next 2-3 weeks, reaching its highest point about two weeks postpartum. After the dairy cow calves, the extent of NEBAL is apparent from degree of body condition score (BCS) loss. Cows with more severe NEBAL, lose more body condition during the first 30 days of lactation and experience longer intervals to first ovulation. Differences in DMI are directly related to NEBAL and the timing of first ovulation. Cows with the lowest efficiency ratio (milk production/DMI) have the shortest interval to first ovulation. On the other hand, the most efficient cows, the cows producing more milk with less feed (using more body reserves), have longer delays to first ovulation.

Monitoring NEBAL in dairy herds is done by observing changes in BCS. Greater NEBAL/BCS loss during the first 30 days postpartum often delays first ovulation. Significant numbers of cows (28-50%) continue to have ovulations suspended beyond 50 days in milk and into the breeding period. Obviously, cows that fail to resume ovulating are infertile, however cows with delayed first ovulation will lack the benefit of multiple ovarian cycles and will express lower fertility to insemination. There is strong agreement among many studies that conception rate decreases with increased BCS loss. For example, conception rate decreases about 10%/0.5 unit BCS loss. The results of 11 large studies indicate that low BCS (< 2.5) at first AI extends the days open by 12 days. Comparing differences in BCS loss during early lactation shows the following: 0.5 to 1 BCS loss = +3.5 days open; >1 BCS loss = +11 days open. Cows that do not ovulate until after 50 days of lactation are at significantly higher risk of not becoming pregnant and, therefore being culled from the herd.

Progesterone is essential for pregnancy after breeding and must be adequately present in the blood to support embryo development and survival. The levels of progesterone increase over the first three reproductive cycles in postpartum cows with less improvement in cows with greater NEBAL. Lower progesterone levels normally observed in high producing cows probably also reflects increased metabolism by the liver. Progesterone has an important influence as early as 5-7 days after insemination. Researchers have attempted progesterone supplementation after insemination which has had mixed success. Another possible carryover effect of early NEBAL may be that the eggs are damaged by harmful conditions within the ovarian follicles during their development over a period of 60-80 days. An additional study found a high incidence of poor embryo quality in normal healthy high-producing cows in early lactation as compared to non-lactating cows. While these results support concerns about early NEBAL affecting eggs, results of another study showed that early embryo development is compromised even later during mid-lactation by the ongoing metabolic effects associated with low BCS <2.5. Therefore, these collective results indicate an unfavorable impact of NEBAL on egg competence for embryo development, but metabolic effects are not limited to follicles during early lactation and may be applied continuously during high milk yield.

Negative Energy Balance occurring early during lactation delays the timing of first ovulation and causes other, with carryover consequences on fertility during the breeding period. These effects include reduced or sub-optimum levels of progesterone in blood that influence fertility through alteration of uterine function and inadequate rate of early embryo development. In addition, NEBAL may detrimentally impact the quality of the egg that is released after ovulation. Reducing NEBAL is always beneficial, but very difficult to achieve in cows being managed for high milk yield. Maintaining higher feed intake during the transition period is most important for achieving better energy status during the early weeks of lactation.

Taken with permission from Dr. Ron Butler,
Professor of Animal Science & Physiology, Cornell University, NY

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Posted Jan 10th

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