Different Types of Estrus
The description of estrus presented so far is the norm but it does not occur exactly as described in all mares. In a group of 95 mares, the incidence and types of estrus are shown below:
| Overt Estrus |
Incidence |
| Manifest throughout |
47 |
| Proceeded or followed by 1 or 2 days of sub-estrus |
22 |
| Split by 1 or 2 days of sub-estrus |
17 |
| Split by 1 or 2 days of non-estrus |
2 |
| Covert Estrus | |
| Sub-estrus throughout |
6 |
| Non-estrus signs, indistinguishable from diestrus |
1 |
By far the most common form is what we have described already and is listed as manifesting the behavior through a period of 4 - 8 days. As shown, some mares may have a period of sub-estrus before or after. Another group may even have a split estrus with a decrease of behavioral signs in the middle of the estrus period. This latter group may be due to failure of the follicle to fully develop and a second follicle may need to develop leading to the second strong period of estrus behavior. Covert estrus behavior are associated with insufficient estrogen levels and usually will not be a period in which a mare will get pregnant.
