Taking another look at conception rates

Heather Schlesser
Over the last two decades, the 21-day pregnancy rate in our dairy herds has increased largely due to the increased use of synchronization protocols.

The other day I had the opportunity to participate in an online training with Dr. Paul Fricke, dairy reproduction specialist, from the University of Wisconsin – Madison.  I learned that over the last two decades, we have increased the 21-day pregnancy rate in our dairy herds. 

The goal in 1998 was to have a 20% conception rate. Now, in 2020, a 20% conception rate is on the low end of what we expect. 

This increase in pregnancy rate is largely due to the increased use of synchronization protocols. While higher pregnancy rates are possible, many herds are not achieving this benchmark even when utilizing synchronization protocols.

Dr. Fricke’s talk highlighted that this difference in pregnancy rates is partially due to the different types of animals we have in our barns.  Three main classes of animals exist on dairy farms. 

The first class of heifers and cows are those that gain weight after they give birth. The second are cattle that maintain weight after they give birth. The final class are those that lose weight after they give birth.

Cattle that maintain or gain body weight after giving birth rebreed quicker and have higher pregnancy rates. Those that lose weight after giving birth take longer to rebreed and have lower conception rates. 

The Britt hypothesis, which was developed in 1992 by Dr. Britt, Professor Emeritus at North Carolina State University, suggests that the follicle that develops during a period of negative energy balance has some negative aspects imprinted on it.  We now know that these negative aspects are due to the production of non-esterified fatty acids (NEFAs). High NEFA levels affect embryo quality which ultimately impacts conception rates. 

What traits dictate if your cattle will gain, lose, or maintain weight after giving birth?  The main factor appears to be her weight, which can be assessed as body condition score (BCS), before she gives birth. 

Animals with lower BCS tend to eat to obtain the energy they need for milk production and maintenance. Cattle with higher BCS tend to metabolize fat stores instead of eating to meet their needed energy demands. Unfortunately, the metabolism of stored fat is what produces NEFAs and negatively affects embryo quality. 

This research tells us that we need to take another look at the BCS of dry cows and first lactation heifers before they give birth. The traditional goal has been for these cattle to have a BCS between 3.25 and 3.75 at two months before calving. 

We now know that cattle at these BCS will metabolize stored fat and lose weight after giving birth. Therefore, we need to lower our target BCS. She must not be over-conditioned when she calves in. We need her to want to eat after giving birth to meet her energy demands. 

The new recommendation is that cattle have a BCS between 2.5 and 3 at dry off and calving, to increase pregnancy rates. Watching cattle BCS is important because if it gets too low milk production and animal health will be compromised. However, as we have learned being over conditioned is not good either. 

It is important to keep in mind that BCS is not the only factor that can affect pregnancy rates. Body Condition Score is only one part to the puzzle. If you are calving in animals with a BCS between 2.5 and 3, and still have a 21-day pregnancy rate less than 20%, consider consulting your dairy herd veterinarian and Extension agent for ideas on how to improve your farm’s reproductive success.

Heather Schlesser

Schlesser has served as the Dairy Agent for Marathon County since 2012. 

UW Extension