Calving season: The first 21 days

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Several studies have quantified the different advantages that calves born during the first 21 days of the calving season have over those that are born later. Each day a calf is on the ground, it has the opportunity to gain weight, and the relationship between birth date and actual weaning weight is obvious: Older calves are typically heavier at weaning compared with younger calves. However, these effects extend well beyond the time of weaning.

In calves placed in feedlots, the greater feedlot entry weights (a function of greater weaning weights) are followed by heavier final carcass weights, improvements in carcass quality grade and the proportion of carcasses qualifying for premium beef programs for calves born during the first 21 days of the calving season compared with those born later. In addition, a greater proportion of replacement heifers born during the first 21 days of the calving season were cycling at the start of their first breeding season, and this subsequently led to greater overall pregnancy rates compared with heifers born later in the calving season.

We also begin to see impacts of early calving on the cows themselves. The pattern of late-calving cows becoming perpetually late calving and subsequently not becoming pregnant is familiar to all of us. Early calving cows are more likely to become pregnant early in the next breeding season and a recent report (Kill et al., 2012) began to quantify the impacts of replacement heifers calving within the first 21 days of the calving season on longevity in the cow herd.

The average time early calving heifers remained in the herd was 5.1 years compared with only 3.9 years for heifers that calves after the first 21 days of the calving season in a group of 2,195 South Dakota producer-owned cattle. In a group of 16,549 cattle managed at the U.S. Department of Agriculture’s Meat Animal Research Center, heifers calving in the first 21 days, second 21 days and later had an average longevity of 8.2, 7.6 and 7.2 years, respectively. In both cases, data were confined to cattle culled for nonpregnancy, and other types of culls (conformation, temperament, etc.) were removed for the analysis. Taken together, this work showed that early calving heifers had at least a one-calf lifetime advantage compared with late-calving heifers.

This one-calf lifetime advantage also was complemented by extra weaning weight at the end of the breeding season that accumulated to the weight of an additional calf during the lifetime of the cow. Thus heifers that calved during the first 21 days of the calving season had the equivalent of a two-calf lifetime advantage over those heifers that calved after the first 21 days of the calving season. The moral of this story should be to focus on keeping heifers that become pregnant during the first 21 days of the breeding season.

With this in mind, producers may want to consider their heifer development and management strategies and related costs a few different ways. We are all very cognizant of the costs associated with developing heifers through their first breeding season. Producers may not wish to retain any more heifers on breeding pastures than they wish to keep for themselves to control costs.

However, an alternative method of stocking replacement heifer breeding pastures would be to stock enough heifers so that the number of replacements needed would be met solely by those heifers becoming pregnant during the first 21 days of the breeding season. To achieve this stocking rate, the number of heifers on breeding pastures would need to increase according to the proportion we anticipate becoming pregnant early. Data from the North Dakota Beef Cattle Improvement Association’s Cow Herd Appraisal and Performance Software (CHAPS) revealed that the proportion of females becoming pregnant during the first 21 days of the breeding season ranged from 58 to 64 percent during the past 10 years. Therefore, we would conservatively estimate that 60 percent of the heifers would become pregnant during the first 21 days.

To calculate the stocking rates of breeding pastures in this scenario, we would divide the number of replacements we are targeting to retain by 60 percent (0.60). For example, if a producer wishes to retain and calve out 50 replacement heifers, 84 heifers would be stocked onto breeding pastures (50 ÷ 0.60 = 84 total heifers). The divisor used is herd-specific, and producers knowing the proportion of heifers becoming pregnant early in their herds should anticipate accordingly. Perhaps the benchmark of 65 percent of the cows becoming pregnant within the first 21 days is achieved regularly and only 77 heifers would need to be kept.

The number of heifers mentioned above is obviously a much larger number of heifers than normally would be run on many operations. Several items are critical to the success of developing a successful system of retaining only those females pregnant within the first 21 days of the breeding season:

• Enough high-quality heifers and bull power to stock breeding pastures at suggested rates - If sufficient numbers of high-quality replacements are not available but a second-tier group of heifers is available, then producers are faced with another question: What is better for the long-term profitability of the herd - a better-quality heifer that likely will not last in the herd, or a slightly lower-quality heifer that likely will last in the herd? This can be answered only by the herd manager.

• Enough winter feed supply and grazing pasture, or money to secure each, to develop extra heifers - Aside from that exception of producers who normally retain nonreplacement heifers as yearling stocker cattle, grazing plans and stored feed supplies would need to be adjusted to facilitate the greater number of breeding heifers maintained.

• A method of identifying heifers that are pregnant within the first 21 days of the breeding season - Accuracy and timing of pregnancy diagnosis are critical when building a system that relies on knowing when conception occurred. The earlier pregnancy determination can be conducted relative to breeding, the more accurate it will be. In addition, the timing of pregnancy determination is critical to ensure that all pregnant heifers are detected and appropriately classified into groups according to estimated conception dates (for more details, see the August 2011 article in The Ranch Hand titled “Consider Early Pregnancy Checking”).

• A solid marketing plan for nonpregnant heifers and for heifers that became pregnant after the 21-day breeding target - Remember that we started with a high-quality group of replacements, and because of the diversity in the beef industry, the heifers that became pregnant outside of one producer’s target may be exactly what another producer is looking for. If natural-service bulls are used, then a market for a group of bred heifers needs to be secured. Additionally, a favorable market for nonpregnant heifers should be identified. Quite likely, the open-heifer markets will be complemented by the timing of pregnancy determination (see previous item) because nonpregnant heifers identified early could be sold as grass calves in the late-summer yearling markets.

An additional production system utilized by some beef operations is to breed each heifer a single time via artificial insemination and not run any cleanup bulls. Pregnant heifers are kept and open heifers are sold as stockers at the end of summer or retained through the feedlot phase. In either case, both systems identify the heifers that become pregnant early in the breeding season, and both systems take advantage of the additional longevity and accumulated weaning weight that accompany these early calving heifers.

Given the lifelong benefits of heifers calving early in the calving season, producers may want to implement a system that focuses on retaining only these heifers. Before making this decision, several items need to be considered and a thorough plan developed. In addition, producers should evaluate nutrition and management decisions that offer heifers the greatest likelihood of early pregnancy.

However a question remains: Is early calving per se what leads to the benefits highlighted in the above paragraphs or is it something inherent in heifers that naturally calve early that drives the observed advantages? Whether heifers that become pregnant early only as a result of additional management experience the same benefits of longevity and calf performance as those heifers that become pregnant early without intervention is unknown at this time. Either way, I hope that you have a high proportion of heifers calving within the first 21 days of the calving season that go on to wean large, healthy calves and continue to stay in the herd for many years to come!

Source: Carl Dahlen, NDSU Extension Beef Cattle Specialist


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