Timing of Insemination
Since cattle should be inseminated so that viable sperm are at the fertilization site as the unfertilized egg arrives, it is important to estimate the time of ovulation for each cow that is to be inseminated. Ovulation occurs 25 to 32 hours after the onset of standing heat. Standing behavior is the only reliable symptom producers have to determine time of ovulation.
Sperm have to be in the female reproductive tract for approximately six hours before they are capable of fertilizing the egg. This process is termed capacitation. Although live sperm have been found in the female tract up to 48 hours after insemination, sperm viability usually is estimated to be 18 to 24 hours. Improper semen handling or poor insemination technique can dramatically reduce the number of sperm cells available for fertilization and thus can lower the conception rate.
The egg travels very rapidly from the ovulation site to the fertilization site in the oviduct. The fertile life of the egg is shorter than that of the sperm. Ovulated eggs remain fertilizable longer (10–20 hours) than they remain capable of being fertilized and developing into normal embryos (8–10 hours). The likelihood of embryonic death increases as the time beyond this interval increases. Thus viable sperm should be at the site of fertilization awaiting the arrival of the freshly ovulated egg. Breeding either too early or too late allows an aged sperm or an aged egg to interact at the site of fertilization and will result in poor conception. Events and time intervals associated with standing heat and insemination are summarized in Figure 3.
Figure 3. Average time relationships among reproductive events.
Cattle should be inseminated during the last half of standing heat. The a.m.-p.m. rule was developed as a guide. Cows first seen in standing heat in the morning (a.m.) would be inseminated in the afternoon (p.m.) and those observed standing in the evening would be bred the next morning. This system was based on research in which cows were observed frequently (4 to 12 times per day), allowed to interact, and exhibited mounting/standing behavior. Furthermore, insemination was based on standing heat, not secondary signs. Under such conditions, heat detection was very good. But herd managers may not be in a position to accurately predict the latter half of the heat period. Generally it is a challenge just to detect standing behavior. Knowing when to inseminate is another management challenge.
More recent studies conducted by artificial insemination organizations and universities reexamined timing of insemination. In a Virginia study with twice daily heat checks, cows were inseminated either at the end of the heat check period in which they were first observed in heat or at the end of the next heat check period. Using a routine 8:00 a.m. and 8:00 p.m. heat detection system, waiting 12 hours to inseminate resulted in a slight numerical advantage in pregnancy rate over inseminating immediately after heat was first observed (55% vs. 51%). However, this was not a significant difference. Thirty percent of the cows stood to be mounted at the 12-hour heat check after they were first observed in standing heat. These cows had higher pregnancy rates than their herdmates, whether they were inseminated immediately after being first observed in heat or 12 hours later.
Results from a field study in New York showed that near optimal fertility was obtained with a single morning insemination of all cows in heat from the previous evening, including those in heat that morning. This result suggests there may be a benefit to earlier breeding under farm conditions.
Applying this information to a herd situation suggests the following guidelines:
- Best fertility is obtained when cattle are inseminated during the last half of standing heat.
- If a management schedule permits routine heat checks and if it can determine when a heat began and thus predict the latter portion of the heat, the a.m.-p.m. system should be used.
- If the conception rate is unsatisfactory or heat detection is not routine, cows should be inseminated soon after they are first detected in standing heat. Waiting 10 or 12 hours probably results in most of the cows being bred too late.
- Remember, factors other than timing of insemination can affect the conception rate.