By Andrew Sandeen
Whether specifically thought about or not, reproductive success depends on ovulation. We never get to see it happen, and there is no guarantee it will happen when we think it should, but its occurrence and associated management practices have a great impact on efficiency in a dairy herd.
What is ovulation?
The core of it is simple. Ovulation is the process whereby an ovarian follicle releases an oocyte (AKA egg or ovum). The numerous activities surrounding the event of ovulation are an impressive interplay of hormonal changes, shifting blood supply networks, and controlled movement of the ovulated oocyte into the oviduct, where it can potentially be fertilized by viable sperm cells.
Follicles are selectively developed in a wave-like pattern throughout a fertile heifer/cow's life. Most of these follicles, including the oocytes contained within them, die off, but a small number develop to the point of ovulation. The hormonal milieu during a follicular wave determines which follicles will ovulate.
The typical cascade of events leading towards ovulation begins with at least one large follicle on one of the two ovaries responding to a surge of luteinizing hormone (LH) from the pituitary gland, which typically occurs after progesterone has declined towards the end of the estrous cycle. Following the preovulatory LH surge, but before ovulation, follicular cells start to change in their structure and function. Shortly before ovulation, blood flow stops in a small area on the ovarian surface over the top of the bulging follicle where ovulation will take place.
During ovulation, the wall of the follicle ruptures, causing fluid and the single oocyte contained inside to be expelled. The remaining cells of the follicle collapse into many folds and transform into a corpus luteum (CL), which will soon begin secreting progesterone. Meanwhile, the oocyte moves towards the oviduct where, if fertilized, it will become an embryo and then migrate to the uterus to continue its development until the end of pregnancy.
When does ovulation happen?
The most reliable predictor for determining the timing of ovulation (not visible) is the onset of standing estrus (visible, or detectable with technology). The average amount of time that elapses between the onset of estrus and ovulation is about 28 hours. Knowing the expected timing of ovulation helps us to pinpoint an optimal time for insemination, allowing for both the oocyte and sperm cells to be in the right place at the right time.
Fortunately, timing from the onset of standing estrus to ovulation correlates closely to the timing between the onset of threshold activity as measured by cow activity monitors and ovulation. Comparing these two methods for predicting the timing of ovulation, Stevenson et al., (2014) found that ovulation occurred 26.4 hours after the onset of standing estrus and 24.6 hours after reaching threshold activity.
Performing artificial insemination (AI) 4 to 16 hours after the onset of estrus or threshold activity matches the best conception risk for dairy cows. However, breeding a little bit earlier or later can still provide reasonable success, as often seen with once-a-day breeding programs.
How can we manage ovulation?
Early research focused on follicle development and CL management guided the development of protocols that synchronize follicle growth, control luteolysis (regression of the CL), and predict the timing of ovulation so AI can be performed at a prescribed time.
A common treatment in timed AI protocols is GnRH, which is administered to stimulate an LH surge and cause ovulation. Ovulation in response to GnRH treatment depends on the presence of a follicle responsive to LH, which cannot be assumed, because in the wave-like pattern of follicular growth there are periods of time when follicles are small and still developing. Therefore, when GnRH is administered at random stages of the estrous cycle, the resulting ovulation rate is usually 50 to 60%. After presynchronization and a good, timed AI protocol, ovulation occurs about 85% of the time (Bisinotto et al., 2014). Overall, GnRH is valuable to have in the reproductive management toolbox for many dairy herds.
Cows treated with GnRH between days 5 and 9 of the estrous cycle have the best ovulatory response. Because of this, when using prostaglandin F2α (PGF) treatments for presynchronization, an interval of 11 days from the final PGF treatment to the first GnRH treatment of an Ovsynch timed AI protocol is considered ideal, recognizing that a 14-day interval is sometimes easier to manage on the weekly calendar. Presynchronization approaches that include GnRH have proven to sometimes be more effective than protocols only using PGF because of the impact of GnRH on stimulating ovulation and progesterone production.
How about twins?
When multiple follicles ovulate at one time, twins become likely. About 95% of the time, twins from dairy cows are dizygotic, meaning they came from two different follicles and are not identical. The average incidence of multiple ovulations occurring at one time in lactating dairy cows ranges from 10 to 22% (Macmillan et al., 2018). The incidence in heifers is very low, occurring less than 2% of the time. The presence or absence of progesterone in the circulation is key. Double ovulation occurs more frequently in cows with low circulating progesterone during development of a preovulatory follicle, whether because of an anovulatory condition after calving or because timing targeted the first wave of follicular growth at the beginning of the estrous cycle when progesterone was just starting to increase.
What happens if ovulation fails?
Anovulation is a term used to define the situation when cows are not ovulating. This may be a short-term issue, or it may be a longer-term challenge. Oftentimes, follicles in anovular cows will not reach an adequate size for ovulation. If no follicles are developing to the point of releasing an oocyte and transforming into a CL, then the reproductive process cannot be completed.
The first ovulation after calving often occurs by 30 days after calving, but it may not be accompanied by any estrous activity. It is also common for first ovulation to be delayed for a longer period of time. Approximately 20 to 40% of cows are anovular at the end of the voluntary waiting period or at first insemination (Sandeen, 2015).
When anovular cows finally ovulate, conception rates for any corresponding insemination are lower and the rate of pregnancy loss is higher as compared to cyclic cows with significant progesterone in their circulation prior to estrus.
To summarize, ovulation is a 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 into the oviduct, where fertilization can potentially take place if properly timed insemination occurs. It’s both fascinating and critically important for successful reproduction programs.Source : psu.edu