Reduced Expression of Estrus In High Producing Cows – Why?

Posted: June 5, 2005

There is no simple answer to why reproductive performance is lowering for high producing cows.

Why is reproductive performance lower for high producing cows? There is no simple answer. Reproduction is a complex system affected by multiple management factors and many interrelated physiological mechanisms. It has been known for many years there is a close association between dry matter intake (DMI) and milk production. Some very early research with sheep and swine demonstrated that increasing dry matter intake (DMI) caused an increase in metabolic clearance rate (MCR) of progesterone. Estrogen and progesterone are steroid hormones critical to reproduction and the liver is the major site of steroid metabolism. More recently, research from Wisconsin has shown that lactating dairy cows have about twice the basal liver blood flow (LBF) of nonlactating cows. How might this relationship impact on reproductive performance?

The Wisconsin group examined these basic physiological relationships in lactating and nonlactating dairy cows. Liver blood flow and MCR of progesterone increased soon after feeding and this increase persisted longer at higher DMI. The MCR of estrogen also increased after feeding and remained elevated over a 4.5-hour blood-sampling period. The researchers concluded that there is a dramatic increase in clearance of estrogen and progesterone following feeding and this is related to changes in liver blood flow. In lactating cows, a constant high DMI appears to increase LBF and the MCR of progesterone and estrogen. It was suggested that the decrease in circulating levels of hormones could have a significant effect on reproduction in high producing cows with high DMI.

The next chapter of this story describes the association between level of milk production and duration of estrus. Holstein cows (267) with rump-mounted radiotelemetric transmitters were monitored for estrus mounting activity beginning 50 days postpartum. Blood samples were obtained weekly on a subset of cows to determine the concentration of estrogen. There were 323 estrous periods monitored. Average milk production for the ten days prior to these heats was 103 and 73.7 lbs/d for the high and low producers, respectively. Duration of estrus (6.2 h vs. 10.9 h), number of stands to be mounted (6.3 vs. 8.8) and total time standing to be mounted were significantly less for the high producers. Higher producers had lower concentrations of estrogen. Milk production was negatively correlated with duration of estrus and estrogen concentrations. It should be noted that in this study estrus was determined via radiotelemetry which has been shown to be more efficient and accurate in detecting estrus than visual heat detection. The differences may have been more dramatic if estrus was determined by visual observation. The researchers speculated that a lower peak concentration of estrogen and a more rapid clearance of estrogen after estrus may be responsible for shorter and less intense heats in high producing cows (Lopez et al., Animal Reproduction Science, 2004). This would result in a lower heat detection rate among higher producing cows.

This concept explains in part why heat detection rates have declined as production has increased over the years. What can dairy producers do to compensate or minimize this effect? Obviously more frequent observation for estrus within the high production group and use of heat detection aids would be effective. Realizing the challenge of achieving good heat detection rates many producers have adopted estrous synchronization programs using timed insemination in an attempt to increase submission rates for insemination.

This was a study comparing expression of estrus among high and low producing cows within the same herd. These results do not imply that high producing herds have lower heat detection rates than low producing herds. Most large DHIA summary reports illustrate that as a group higher producing herds have higher heat detection rates than lower producing herds. Implementing management strategies to improve heat detection and submission rates for insemination are critical to maintaining good heat detection rates and reproductive performance in high producing cows.

Michael O’Connor, Dairy and Animal Science Extension