It is never too early to be looking ahead; in fact it is a necessary management skill. For the cow/calf operation, calving season means looking ahead to the breeding season. Reproductive efficiency is measured by the timeliness of getting a cow bred back and producing a healthy calf within a 12 month period. How well this is done determines the number of calves that will be marketed each year, thus directly impacting upon the gross income of a cow/calf operation.

During the Managing for Dynamic Change beef school this past February, Dr. Les Anderson, an Extension Beef Specialist from the University of Kentucky, addressed the topic of reproductive efficiency. In this article I will summarize some of the information he presented.

Dr. Anderson said that reproductive efficiency is governed by two factors: length of the anestrous period and then conception rate at estrus. Anestrus is the lack of or absence of estrus. Estrus is defined as the period of ovulation when the cow is in heat or sexually receptive. This is the period within the estrous cycle when conception is possible. The estrous cycle is that reoccurring period, generally 21 days, of physiological changes induced by reproductive hormones leading up to ovulation. Obviously then, the goal is to limit the anestrus period and get the cow cycling again as soon after calving as possible.

Anestrus occurs after each calving and can vary in length from as little as 14 days up to 180 days. The factors that play a role in determining the length of the anestrus period are the presence of the calf, the body condition score of the cow, the age of the cow, the number of days since calving, birthing difficulties, calving season, and general health of the cow. Some of these factors can be managed to reduce the anestrus period and some can't be. Consider age. First calf heifers require 20-30 more days after calving to resume estrous cycles as compared to their older herdmates. On the other hand, body condition score (BCS) can be managed.

The BCS scale is a 1 to 9 grading system where 1 = emaciated and 9 = extremely obese. It is essentially an estimate of available energy stores. One basic nutritional principle is that there is a hierarchy of nutrient use. Energy as a nutrient is used to first meet body maintenance requirements, then development needs, followed by growth requirements, then lactation needs and, if sufficient energy is still available, then reproductive requirements can get met. Dr. Anderson showed a slide in his presentation that indicated in order to get 70 to 80 percent of the cows cycling by the onset of breeding season, a BCS of 5.5 to 6.0 at calving is necessary. The nutrition of the cow and BCS at calving is related to management. Higher BCS reduces the length of the anestrous period which increases the percent of cows that are cycling by the onset of the breeding season.

The presence of the calf is what initiates the anestrus period. Dr . Anderson said that while the exact mechanism is unknown both the nursing of the calf and the mere presence of the calf at the cow's side are involved with the initiation of anestrus. From a management standpoint, removal of the calf from the cow for a 48 hour period has been show to induce estrus and start the estrous cycle. Dr. Anderson conceded that while this is possible, the management involved is probably not practical for the average cow/calf farmer.

Another management tool that can be used in anestrus cows to induce estrus and improve conception is the CIDR device. The CIDR is basically a synthetic progesterone. A CIDR device inserted for seven days in problem cows with long anestrus periods resulted in estrus and pregnancy rates of greater than 90 percent.

The conception rate at estrus is affected by BCS, the environment, health of the cow, disposition of the cow and the bull. From the previous discussion in this article on BCS, it stands to reason that thin cows and heifers will have lower conception rates. Dr. Anderson said that conception rates decrease 5 to 20 percent when the BCS decreases during the period from calving to the beginning of breeding season. Additionally, there is a penalty for over-conditioned cows. Conception rates are lower for cows and heifers that have a BCS above 7.5.

The main environmental factor affecting conception rate is heat stress. In our area a contributing factor is endophyte infected pastures. When this forage is consumed, alkaloids produced by the endophyte reduce the ability of the animal to dissipate heat, resulting in increased body temperature, which equals heat stress. Conception rates due to heat stress can reduce the conception rate down to the 30-35 percent range. Other environmental factors that can reduce the conception rate include mineral imbalances and excessive crude protein above 22-23 percent.

Health stresses that reduce the conception rate can be reduced by a complete vaccination program, meeting mineral and nutritional requirements, and deworming young, thin cows.

While it is not a factor that might easily come to mind when thinking about conception rates; disposition of the cow should be considered. Dr. Anderson said that the impact of disposition on conception rate is well established. Hormones released during stress reduce the conception rate.

Finally, the bull must be factored in to conception rates. Dr. Anderson said that often cows shoulder the blame for fertility issues when the problem may in fact, be the bull. Bulls should have a breeding soundness exam 30 days before the beginning of the breeding season and only those bulls that easily pass the exam should be considered. Dr. Anderson used an example of a farmer who had purchased a high dollar bull and when that bull failed the breeding soundness exam, he persisted in re-testing the bull until he had a marginally successful examination. The end result was an overall conception rate with that bull of around 30 percent. The lesson is to set high standards and stick to them.

While there are factors that are beyond the control of the cattleman, an understanding of the main factors that influence reproductive efficiency and then application of proper management to factors that are controllable will reduce the risk of reproductive failure and, increase reproductive performance in the cowherd.

Source: Ohio State University Extension.


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