Managing Postpartum Anestrus to Maximize Fertility this Fall By Pedro L. P. Fontes, Ph.D. UGA Reproductive Physiologist and Beef Extension Specialist

MANAGING POSTPARTUM ANESTRUS TO MAXIMIZE FERTILITY THIS FALL

By Dr. Pedro L. P. Fontes, Assistant Professor and Extension Specialist, University of Georgia

In a cow/calf state such as Georgia, fertility is the main driver of profitability in beef herds. Unfortunately, there is not a silver bullet that producers can utilize to increase pregnancy rates. Instead, to optimize pregnancy rates, producers should consider managing as many factors that are known to influence fertility as possible. The proportion of cows in postpartum anestrus in the beginning of the breeding season is one of the main factors known to influence reproductive performance. After parturition, cows undergo a period of anestrous that is defined as the lack of regular estrous cycles and estrus expression. Multilocation studies evaluating the percentage of cows cycling in the beginning of the breeding season indicate that, on average, 50 percent of beef cows are in anestrus when the breeding season starts. Interestingly, the number of cows cycling in the beginning of the breeding season can range from 17 to 70 percent, depending on the operation and their management practices (Day et al., 2004).

The two main factors influencing the variation between herds in the percentage of cows that are cycling in the beginning of the breeding season are: (1) the interval between calving and the beginning of the breeding season (days postpartum); and (2) the cow/herd nutritional status. Fortunately, by implementing proper management practices, producers can control both the number of days postpartum and the body condition score of the cowherd in order to maximize the number of cows cycling. For example, by having a controlled and short (~60 days long) breeding season and implementing proper culling practices, producers can increase the proportion of cows calving in a narrow window of time. The proportion of cyclic cows at the beginning of the breeding season increases as the number of days postpartum increases (Stevenson et al., 2015). By having a narrow calving window, the number of days between calving and the beginning of the breeding season will increase on average, leading to an increase in the proportion of cows that are cycling in the beginning of the breeding season.

Interestingly, managing cows in a narrow breeding window also helps producers manage their nutritional program. For example, a herd with a 120-day breeding window will have cows that are reaching peak lactation (~60 days postpartum) and cows with 180 days postpartum (6 months) in the same group. The nutritional requirements of these cows differ substantially, which makes it difficult for producers to optimize the efficiency of their nutritional programs. For example, if producers balance their supplementation strategies

based on cows in peak lactation, they might spend extra resources in cows that are in late lactation. Alternatively, if producers develop a supplementation strategy based on the requirements of cows with approximately 180 days postpartum, cows that are in peak lactation will lose condition, which will ultimately decrease their likelihood to start cycling before the beginning of the breeding season, and consequently their ability to breed back.

Another important component of managing cow herd postpartum anestrus is getting cows to calve in an adequate body condition score. A good rule of thumb is to have cows at a body condition score of 6 at calving. Cows will lose condition as they start increasing milk production during the postpartum period. This is particularly important in replacement heifers that are having their first calf and are still growing during the postpartum period. Another important component of pregnant cow body condition score is the fact that cow nutrition during gestation can impact the performance of her subsequent offspring through a process called fetal programming. In fact, in the context of commercial cow/calf operations, calves that are sold at weaning spend more time in the uterus of their dams (for 9 months of gestation) than actually nursing it after parturition (approximately 8 months from birth to weaning). During gestation, the fetus relies exclusively on the dam to grow and develop. Neglecting cow herd nutrition during late gestation has been shown to decrease nutrient delivery to the fetus and negatively impact the performance of the offspring from birth to harvest (Moriel et al., 2021). Therefore, by having cows calving in adequate body condition score at calving, g producers are not only optimizing their chances of cows breeding in the subsequent breeding season, but also optimizing growth and performance of their future progeny that is still in the uterus.

Another tool in the toolbox to manage postpartum anestrous is the use of progesteronebased estrus synchronization protocols. These protocols have the ability to induce cyclicity in cows that are in postpartum anestrus. In fact, some studies show similar pregnancy rates to fixedtime artificial insemination between multiparous cows in anestrous and multiparous cows that are cycling when a progesterone device was used (Figure 1; adapted from Stevenson et al., 2015). Interestingly, while progesterone supplementation can also help first-calf heifers to start cycling and breed back in the subsequent breeding season, first-calf heifers that resume cyclicity prior to the beginning of the estrus synchronization protocol have greater pregnancy rates compared with firstcalf heifers that were in anestrus. These results indicate that progesterone supplementation can effectively induce cyclicity. Yet, cows in extremely metabolically challenging conditions (i.e., first-calf heifers) respond less effectively than cows that are closer to resume cyclicity. Therefore, utilization of estrus synchronization can effectively induce cyclicity; however, it is not a substitute for proper nutritional management of the cow herd.

80 % 70 Cycling Anestrus r a t e s t o A I , 4 5 6 0 0 0 a b n a n c y 2 3 0 0 P r e 10 0 Primiparous Multiparous Parity

n = 662 a n = 907 a

a,b: P < 0.05 n = 4 199

n = 2 732

Figure 1. Pregnancy rates to fixed-time artificial insemination (AI) in primiparous and multiparous cows based on cyclicity at the beginning of the breeding season.