Open cows that aren’t cycling normally can hamper successful reproduction. This article, the first in a series on cyclicity, defines the terms anovulation and anestrous. Though similarly problematic, the causes and potential corrective actions for anovulation or anestrous conditions in a dairy cow are different.
If you think about it, ovulation is a pretty remarkable event, even though it is a routine occurrence in mature heifers and cows. A fluid-filled follicle on the ovary has grown to the point that it is ready to respond to hormonal signals that cause it to rupture and release an oocyte (egg) into the oviduct, where fertilization can potentially take place if properly-timed insemination has occurred. Without ovulation, reproduction is not possible.
Estrus is another routine event in a normally cycling cow. This short phase of the reproductive cycle, when cows are receptive to mounting activity, typically begins one day prior to ovulation and is an important indicator for breeding, whether that job is done naturally or artificially. Unless dairy producers are relying entirely on natural service and/or timed AI, observance of estrus by some means is critical in a reproductive management program.
Let’s look at two terms often used when these activities are not occurring and a dairy cow isn’t cycling—anovulation and anestrus. These two conditions may be either positive or negative indicators of the reproductive status in a cow. A pregnant cow is generally anovular and anestrous—a positively good thing! On the flip side, these can also be conditions which hamper reproductive success in non-pregnant dairy animals.
Anovulation
Anovulation simply defines the situation when cows are not ovulating. Follicles may or may not reach an appropriate size or responsiveness for ovulation. If no follicles are developing to the point of releasing an oocyte and transforming into a corpus luteum (CL), then the reproductive process cannot be completed.
In fresh cows, first ovulation often occurs by 30 DIM, but it may not be accompanied by any sign of estrus. It is also common for first ovulation to be delayed for a longer period of time, persisting well beyond the voluntary waiting period. At first insemination 23% of dairy cows are anovular, on average. Even at later breedings more than a quarter of the cows that are open and eligible for breeding aren’t ovulating normally.
When anovular cows finally ovulate, conception rates for any corresponding insemination are lower and the rate of pregnancy loss is higher when compared to cows that were cycling normally and had significant progesterone in their circulation prior to estrus.
Heritability of anovulation is 0.17, higher than most other reproductive traits, which have heritability measurements in the range of 0.03 to 0.07. This suggests that there may be opportunity for genetic progress to lessen the impact of anovulation on fertility in dairy herds.
Anestrus
Anestrus is a related term with a significantly different meaning than anovulation. It defines the situation when animals are not showing estrus—they are not standing to be mounted, not being measurably more active, and not providing any other signs of a good heat.
A cow might be anestrous because she truly is not cycling, but it is also frustratingly common for a cow to not show estrous behavior even when she is cycling normally. There are several potential causes for either situation, ranging from nutritional issues to facility design to employee management. One possible reason for supposed anestrus in cycling cows may simply be a lack of farm personnel who are observing and recording heats. There might be signs of estrous behavior that are being missed.
Like anovulation, the incidence of anestrus is particularly high during the first month after calving, but it can continue to be a significant challenge for several months.
Though similarly problematic, the causes and potential corrective actions for anovulation or anestrous conditions in a dairy cow are different. Though specific recommendations won’t be addressed in this article, it’s generally worth the effort to understand what is happening at an individual cow level and, even more so, at a wider herd level. There may be management changes that could alleviate some of the challenges and reap significant benefits.
Source: Penn State Extension