Calculating calving distribution is one way to evaluate the previous year’s cow herd reproductive performance. This assessment calculates the number of cows calving in 21-day periods during the calving season (the length of a cow’s estrous cycle). There are two different methods used to determine the starting date. The first is to add 283 (average gestation length) to the first breeding date or bull turnout date, and the second is to assign the starting date as the day when the third mature cow calves. In herds where cow age can be identified along with calving date, calving distribution can be calculated for young cows separately from older cows, which may provide information about breed-up performance that might not otherwise be easily observed. While cows are grouped by age for the purposes of this article, other groupings like breed-of-sire may also be useful. Table 1 illustrates an example calving distribution assessment by cow age from the Beef Improvement Federation Guidelines publication.
What is a good benchmark number for calving distribution? One example comes from the North Dakota Beef Cattle Improvement Association Cow Herd Appraisal Performance Software (CHAPS) program. The 2018 CHAPS benchmark data for percent of cows calved by 21 days was 60%, with 87% of cows calved by 42 days.
From Table 1, do you see a group of cows you might be more concerned with compared to another? Perhaps the 3-year-olds? Check out the graph of this data in Figure 1 for a visual perspective.
In this format, the 3-year-old cows really jump out. All other age groups have the largest percentage of cows calving during the first 21 days, but the majority of 3-year olds calved during the second 21 days. Many beef cattle producers find that getting first-calf heifers to breed back is a challenge. Some strategies to improve young cow reproductive performance include implementing proper heifer development and pre– and post-calving nutrition programs. Some producers start the yearling breeding season 2-3 weeks ahead of the mature cows in an effort to give the heifers more time to recover before breeding season. On the other hand, some producers implement a shortened (around 30-day) breeding season for yearling heifers in an effort to put selection pressure on reproduction. In this scenario, pregnancy rates will be lower than in a longer breeding season, so more potential replacement heifers may need to be retained to ensure an appropriate replacement rate for the cow herd.
Keeping young cows separate from older cows before and after calving (if conditions allow) might also be a good young cow reproductive management strategy. Since young cows are still growing, their nutrient demands are higher than mature cows. Managing them separately allows for more targeted feeding to meet nutrient requirements. When managed together, feeding to meet mature cow requirements will result in a nutrient shortage for the young cows. Feeding to meet young cow requirements will result in overfeeding the mature cows, which could be a fairly expensive proposition.
Does calving distribution have an impact on the bottom line? Absolutely. Age at weaning obviously has a huge impact on weaning weight, so more calves born early in the calving season means a heavier set of calves to offer your buyers. Furthermore, heifers who calve early with their first calf have been shown to have greater lifetime pregnancy rates, leading to increased longevity in the herd. In one study, heifers who calved in the first 21 days of calving weaned heavier calves than their later-calving herdmates, which amounted to the production of nearly one extra calf during their lifetime.
As you plan for your next breeding season, an evaluation of calving distribution might give you some good insight on how last year’s management environment impacted cow herd reproductive performance.
Example calving distribution assessment. Percent cows calving in each calving period by cow age.