We choose the length of our calving season by how long we leave the bulls out. Some leave bulls out until it is convenient to remove them and may get away with a reasonable calving period, until they don’t. A recent call reported a frustration with as many cows yet to calve after 60 plus days of calving as had calved in the first or second 21 day periods. Assuming all those remaining calve in the
4th 21-day period, those calves will be 80 to 100 pounds lighter than the early born calves at a common weaning point. If we assume a sale price of
$1.80, that is $144 to $180 in lost value.
It will be important for the producer to work through possible causes of this issue which may include nutrition, genetics, health, male fertility, management or a combination to prevent future occurrence. Consideration may be given to marketing these as bred females as they may fit better in another producer’s system. There are some management actions that can be taken now to hasten rebreeding.
The first step is to get them into a positive energy balance as soon as possible after calving. They will need the highest quality feed available and whatever additional supplement might be needed to meet requirements. If they are grazing, remember there must be both sufficient quality and quantity to meet needs.
There are a number of factors that control how long it takes before a cow resumes normal estrous cycles after calving. They include presence of the
calf, suckling, nutritional status, cow age, uterine involution and calving difficulty. Nutritional status generally has the longest negative effect. A key component of the endocrine mechanism that controls the resumption of estrus cycles is LH (Luteinizing Hormone).
Immediately after calving a small amount of estradiol in the cow’s system keeps the amount of LH released low. As time passes after calving, the cow becomes less sensitive to the negative feedback of the estradiol and LH release increases. At some point sufficient LH is released to result in ovulation and formation of the first postpartum corpus luteum. An early release of prostaglandin F2α often occurs after this first ovulation, shortening the estrous cycle. Generally, the next cycle will be of normal length and fertility.
Tools we can use to get cows cycling sooner mimic or stimulate parts of this normal process. Progesterone exposure, temporary weaning/calf removal, and bull exposure have all been shown to shorten the postpartum interval to conception. Each of these treatments can cause an increase in LH pulse frequency.
The use of a CIDR (intravaginal insert containing progesterone) or feeding melengesterol acetate (MGA; orally active progestin) have both been shown to induce estrus in previously non-cycling cows. When compared, more cows were cycling sooner after CIDR treatment than MGA feeding. The CIDR does not have the complication of MGA feeding and the need to ensure each animal gets their daily dose. One week of treatment is common in research settings but 5 to 9 days would likely work as well for this purpose.
Changes in response to temporary calf removal have been shown after 48 hours but in some cases up to 96 hours were needed. Younger calves often have greater shrink associated with the separation but most studies have not found a difference in 205 day adjusted weights. No doubt the separation has some degree of stress on everyone including those that are in listening range of the complaining cows and calves. A tight fence, calf-accessible water and access to high quality feed are needed.
Exposure to bulls or androgenized steers or cows have had positive effects on cyclic activity in cows. Direct contact with bulls produced more cycling females than fence line contact. The interaction of all factors controlling the postpartum interval at the time of exposure has an effect on their response. While data are mixed on the timing of the bull exposure, the response does seem to be sooner when cows are exposed later postpartum as com-pared to soon after calving. One study combined bull exposure and temporary weaning after treatment with a progestogen. Presence of vasectomized bulls and temporary weaning resulted in higher first service conception rate and fewer services per conception than bull exposure alone or controls.
For postpartum cows that are thin and having difficulty achieving a positive energy balance, only permanent weaning of the calf may help her resume normal cycles within an acceptable time frame.
To avoid the need to apply these tools, watch body condition score of cows to help plan a weaning time. Develop a production system that ensures cows achieve adequate BCS prior to calving. Monitor your calving distribution each year (by cow age group to the extent possible) and be sensitive to increasing numbers of late calving cows. Use a short breeding season on yearling replacement heifers (no longer than 45 days).