By Glenn Selk, Oklahoma State University Extension Cattle Reproduction Specialist
Producers that are wanting to improve the genetic makeup of their beef herds very often turn to artificial insemination (AI) as a tool to accomplish that goal. Many times, these producers have very high expectations as they begin the first season of artificial breeding. Perhaps they have heard other producers tell of situations where "near-perfect” pregnancy rates resulted from THEIR artificial insemination program. Everyone wants to get every cow or heifer bred as they start the labor and expense of an AI program. However, the rules of biology do not often allow for 100% pregnancy rates in most situations.
First of all it is important to understand several terms.
Estrous response rate: the percentage of cows found to be cycling in response to an estrus synchronization protocol. In other words, if we put 100 cows through the working chute and give them estrous synchronization drugs, and only 80 of those cows responded to the estrous synchronization products, then we have an "estrous response rate” of 80%.
Perhaps some of the cows were not "ready” because they were later calving or they were in poorer body condition. If we are breeding only after they are detected in heat, then only 80 of the original 100 cows would be bred to AI. The long wet winter of 2009-2010 may have an impact on the body condition of cows going in to the estrous synchronization protocols and adversely impact the percentage of cows responding to the synchronization products.
Conception rate: the percentage of the cows that were actually inseminated that were palpated and found to be pregnant 60 or more days later. In other words, of the 80 cows in the above example, that were found in heat and inseminated, IF we later found that 70% of those "settled” or became pregnant, we would have found 56 cows pregnant.
Pregnancy rate: the percentage of cows that were initially started on the estrous synchronization protocol that actually became pregnant. In the above example, 56 of the original 100 cows became pregnant to the AI program resulting in a pregnancy rate of 56%.
Therefore, the Estrous response rate x Conception rate = Pregnancy rate.
In this example: 80% Estrous response x 70% Conception = 56% Pregnant. The above example is hypothetical, yet very much close to the expected outcome of a successful synchronization and AI program. If heat detection is incorporated as part of the system, then it becomes another very important part of the equation.
Below is a brief summary of just a few of the many trials conducted to study synchronization methods. As you look at this table, observe that similar results occur within the same study (or ranch). There is more difference expressed between operations than between the synchronization methods chosen. Note that most pregnancy rates vary between 35 and 60%.
Pregnancy rates (%) in five different beef and dairy studies using three different methods of synchronization