The Importance of Body Condition on Fertility

Published on Thu, 05/17/2018 - 10:32am

 The Importance of Body Condition on Fertility

 By Jaclyn Krymowski for American Dairymen

 Most agree that body condition scoring (BCS) is one of the single most important tools to evaluate the overall health and nutritional status of a herd. What some may not know, is how BCS is almost just as important an evaluator for fertility. A cow’s body fat reserves are an important resource for both her lactational and pregnancy demands, especially at parturition through her lactational peak. Monitoring BCS should be part of routine procedure performed at breeding, dry off, and freshening.

In the U.S. the BCS scale for dairy cattle runs from 1-5 and goes by increments of .25. The scale begins with 1 being an extremely thin cow and 5 being an extremely fat one, 3 is considered the most “average.” Body condition can be thought of as an estimate on where the animal stands in being in her overall energy balance, being either positive or negative. Whenever an animal deviates from the ideal condition for her nutritional demands, levels of fat metabolism are impacted and are correlated with negative energy balance which reduces fertility among other issues.

The impact
Many studies have proven the impact of negative energy balance on poor ovulation and conception rates. One study reported that cows losing less than 0.5 points on their condition during the first 30 days post-calving took an average of an additional 30 days to first ovulation. Cows that lost 0.5-1.0 BCS took 36 days, and those over 1.0 took 50 days. Overly condition cows also tend to have longer calving intervals.

A 1989 study in the Journal of Dairy Science also explored the loss and gain of BCS and its relationship in the first 5 weeks after calving and the subsequent breeding. The animal that averaged a BCS of 3.7 at calving took 27 days to first ovulation, 28 days to first heat, 68 days to first service, and had a 65% first service conception rate. Cows with a 4.1 calving BCS took 31 days to first ovulation, 41 days to first heat, 67 days to first service, and had a 53% first service conception rate. Cows with a 4.5 BCS took 42 days to first ovulation, 62 days to first heat, 79 days to first service, and had a 17% first service conception rate. This summary illustrates how the cows with the higher BCS at time of calving also lost the most condition in the following month. This directly impacted their longer intervals to first ovulation, first heat, and poorer conception rates.
In young virgin heifers, body condition is an important detail even before they are large enough to breed. Heifers that are too thin and unthrifty growers run the risk of not having sufficient resources to reach puberty at the optimal 11-13 months of age. Even if small under-conditioned heifers can reach puberty in a timely fashion, she will be a much higher risk for dystocia and metabolic diseases. Likewise, heifers that are too fat are more difficult to get bred. Studies have also shown that heifers that are above the recommended BCS at puberty may produce less milk as a cow and have calving difficulties.

Remember to monitor
The rule of thumb for optimal lactating animals is a BCS from 2.5-3.75, increasing as the dry-off period approaches. As a cow approaches dry-off, specifically last 2-3 months of lactation, she begins to accumulate additional condition as milk production decreases. Cows that have been dry too long or are overfed when they’re dry are especially at risk for over-conditioning. Under-conditioning is often related to an animal that faces some sort of feed restriction either during lactation or the dry period. These can be weaker cows competing with stronger herdmates. Some animals simply struggle more than others to maintain and restore their fat reserves. An overbearing parasite load is also a major culprit for unthrifty animals.
The optimal condition for cows at calving time is approximately within the 3.0-3.5 range. Cows within this range seem to have the most optimal milk production in the following lactation compared to their counterparts above or below. This range also provides an allowance for the weight that cows will inevitably lose within the first few months after calving. This way, a cow will fall back to a 2.5-3.5 range during her lactation and will steadily gain again as she approaches her next dry-off period.