Evaluating post-partum interval and calving rates

Rick Rasby and T.L. Meyer, Nebraska Extension

February 1, 2024

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Let’s talk about ways to positively impact post-partum interval (PPI) and pregnancy rates. Consider the following criteria for an efficient beef cow herd:

  • Calving interval (number of days between calves) is 365 days or less.
  • Females raise a calf to weaning.
  • Cow mature weight and milk production fit the resources and need little to no extra feed or supplements.

If the calving interval is 365 days or less, a cow or heifer must be rebred and be pregnant by about 80 to 85 days after calving (gestation is 280 to 285 days). To make this happen, understand the factors that impact the post-partum interval length. The post-partum interval (PPI) is the time from calving until the cow resumes a fertile estrous cycle. After calving, cows will enter post-partum anestrus. Anestrus means cows are not exhibiting normal estrous cycles and cannot become pregnant. Under “ideal” conditions, the anestrus lasts 40 to 60 days in beef females. During this time, uterine involution (the uterus returns to normal size) occurs and she prepares herself for the next pregnancy. If conditions are not ideal, PPI can last 100 days or more.

What factors impact the post-partum interval?

  • Cow age – 2 and 3-year-old females have longer PPI.
  • Body condition at calving – thin cows, BCS 4 and less have longer PPI.
  • Calving difficulty – dystocia lengthens PPI.

Because first-calf heifers have a longer PPI, they should be bred to calve their first calf ahead of the mature cow herd. The focus is to breed heifers to calve ahead of the cows and to breed heifers to calving ease bulls. Calves in first-calf heifers will be in gestation fewer days when bred to calving ease bulls. Remember, even with ideal management for reproductive performance, first-calf heifers will have a longer PPI, so they need extra time post-calving to get ready reproductively to become pregnant with their second calf.

To manage PPI, feed cows to calve in BCS of 5, and first-calf-females to calve in BCS of 6. Depending on feeds available, first-calvers will likely require some supplementation to get to BCS 6 prior to calving. For the mature (3-years-old and older) cows, if they can be in BCS 5 before calving with little to no supplementation that would indicate milk production and mature weight fit the feed/forage resources. If replacement heifers are selected from within the herd and mature cows must continually be fed extra feeds and supplements to get them to ideal body condition before calving, then reevaluate your breeding program. It may indicate mature cow weight is increasing, milk production is increasing, or both are increasing at the same time.

There is a belief that reducing dietary energy during late pregnancy will decrease fetal size and result in improved calving ease. Underfeeding energy does not decrease calving difficulty. Underfeeding beef females will increase the length of the PPI, especially underfeeding young beef females.

Some research shows reproductive performance of cows calving in a 4, 5, or 6 BCS is not different (Mulliniks et al., (2012) J. Anim. Sci. 90:2811-2817). This may suggest cows adapt to their production environment with acceptable reproductive performance of cows at a lower body condition, but it may take years of selection to get there. Spring-calving cows that calve in a BCS of 4 can have high reproductive performance if the weather cooperates from calving to the start of the breeding season and energy is not limited post-calving. There is also data that females that calve in low body condition (BCS 4 or less) have calves with a lower amount of immunoglobulin in the blood stream 24 hours after birth and are slower to stand and suckle compared to calves born to females in BCS 5 to 6 at calving. The question is how risk adverse of a producer are you? Data also say late spring/early summer calving cows can calve in lower body condition and have good reproductive performance. Regarding body condition, it is hard to play catch up, especially after calving, if there is a need to increase body condition. Consider always calving first-calf females in a BCS of 6.

Studies have consistently shown that, by far, the greatest factor impacting calving difficulty in beef females is the birth weight of the calf. Next in line is sex of the calf. Male calves have heavier birth weights compared to female calves because the gestation length of male calves is longer. Also, age of dam impacts calving difficulty. First-calf females have a greater incidence of dystocia. Pelvic area, gestation length, and cow weight had much, much less influence on calving difficulty. For first-calf females, breed them to bulls where the focus is on reducing calving difficulty (Birth Weight, Calving Ease Direct, and Calving Ease Maternal EPDs).

A question becomes – what is an acceptable reproductive rate? This number differs based on the operation. Based on some old Standardized Performance Analysis and CHAPS data, if percent calves weaned per females exposed to the bull during the breeding season is in the 87 to 92% range, the cow enterprise has a good chance to be economical.

The greatest costs for the cow-calf enterprise are feed costs. Again, any time cows can reproduce at an acceptable rate with little to no extra feeds or supplements means that mature weight and milk production fit the forage/feed resources of the operation. If growth is the selection criteria and heifers are retained, cow mature weight will increase. It is probably worth mentioning, milk EPD is pounds of a calf’s weaning weight attributed to milk and mothering ability of the dam. Increasing weaning weight by increasing milk also increases the cow’s nutrient needs, and not only during lactation. Higher-milking cows also have greater visceral organ weight. Even when not lactating these organs increase the cow’s nutrient needs.

Focus on management strategies to economically improve reproductive performance of the herd. This will have greater impact on the financial status of the cow-calf enterprise than most other management strategies. In addition, the management strategies discussed will have a positive impact on weaning weight.

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