The Cow-Calf Manager

Livestock Update, December 2003

John B. Hall, Extension Animal Scientist, Beef, VA Tech

First Heat After Calving May Not Be Fertile

Weaning a heavy calf from almost every cow is essential to profitability of the cow-calf operation. Getting a high percentage of cows bred is the first step towards achieving this goal. However, nutritional deficiencies, long calving seasons (or no calving season), calving difficulty, and a nursing calf can decrease pregnancy rates. Understanding the basics of cow reproduction is essential in developing management strategies to improve reproduction. Over the next few months, this column will concentrate on different aspects of beef cow reproductive biology as well as techniques to improve reproduction.

Post-calving (postpartum) anestrous
After calving, it takes 60 to 90 days for cows to resume cycles. This period is called postpartum anestrous. In first calf heifers, postpartum anestrous lasts longer than mature cows. It normally takes 90 to 120 days for first calf heifers to resume cycles.

Biologically postpartum anestrous makes sense for several reasons. First, heat cycles and reproduction take a great deal of nutritional energy. However, immediately after calving the most important use of nutrients is to make milk for the calf, so shutting off the reproduction system makes sense. Second, because pregnancy lasts 9 months in cattle getting pregnant "too soon" would result in calves being born earlier each year. Back when cattle were wild, the delay in rebreeding was important to ensure calves were born when weather and grass were favorable to calf survival. Also, it gives the reproductive system time to repair and shrink back to its normal size after calving. Finally, the interplay of hormones that cause estrous cycles has been shut off for nine months and it take time to bring these hormones back to normal levels.

The length of the postpartum interval is influenced by several factors the most important are nutrition and presence of the calf. From time to time, this column has discussed the role of these two factors in beef cow reproduction and we will revisit these in the coming months.

Cycles resume ­ false starts?
After the first heat, about 25 to 30% of cows have a "short cycle". A normal cycle lasts 19 to 23 days in beef cattle. However, short cycles last only 12 to 15 days. During short cycles progesterone, the hormone that supports pregnancy, does not stay elevated for enough days to allow implantation to occur.

In 70 to 80% of beef cows, there is a brief (2 to 8 day) increase in progesterone that occurs before the first heat (Figure 1). This increase in progesterone appears to be necessary part of the resumption of normal cycles. A recent study reported that 81% of cows that had a rise in progesterone before first heat had cycles of normal length whereas only 36% of cows that did not have increased progesterone had normal cycles (Looper et al., 2003). Cows that have short cycles do not become pregnant.

The rise in progesterone before the first heat "primes the pump" so that the first cycle after the first postpartum heat is of normal length. It appears that progesterone induces production of the hormones from the brain that cause follicular development and ovulation. Also, the rise in progesterone may be important to synchronizing egg development, heat and ovulation.

What does it mean to the average beef producer?
Most importantly, knowing that cows need to have either a rise in progesterone or a short cycle before a fertile heat will occur means that cows need to have enough time between calving and the start of the breeding season for these normal processes to occur. So, calving seasons need to be short enough to allow cows time to recuperate after calving. This means 60 to 90 day calving seasons.

Also, administering progesterone in the form of estrous synchronization products (MGA or CIDRs) will "jump start" non-cycling cows and reduce the incidence of short cycles. The MGA (0.5 mg/ cow/day) can be used in natural service herds if it is fed for only 7 to 10 days. This will jump start cows without synchronizing them too tightly for the bull to handle. Use of MGA or CIDRs in late calving cows is a good way to decrease the length of the breeding/calving season.

Next month: Pre-calving a critical time

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