Making great cows: Strategic replacement-female selection
By Bob Weaber
KSU Extension Cow-Calf Specialist
The continued drought conditions that persist across much of Kansas may require that many cow-calf producers forego retention of replacement females this year in order to conserve remaining forage resources for mature cow's winter supplementation. However, producers retaining heifers this fall should consider several key strategies to select the best replacement candidates from their weaning pens.
The first step in any selection decision is knowing what it is that you desire. In animal breeding terms, that's your breeding objective. In the case of replacement heifers, your breeding objective should include a listing of the traits and attributes that you believe make a heifer a good candidate for selection as a replacement female for your herd. The traits included in this list should be focused on maternal traits that will aid a cow in being reproductively successful for a long period of time under your management. Traits often included include fertility, longevity, calving ease, milk, docility, mature weight, growth to weaning or yearling endpoints. Other attributes may include coat color, polledness, breed or breed combinations to generate maternal heterosis. Recognize that many commercial heifers won't have EPDs for any of these traits, so they will largely be influences by the sire selection that occurs in a herd over time.
In many cases, limiting environmental conditions will dictate that moderate or optimal levels of growth and milk or lactation potential be selected for rather than maximization of these traits. American Angus Association provides an effective tool for evaluating nutrient availability and selecting optimal ranges of Milk EPDs for sires of replacement heifers. (See http://www.angus.org/Performance/OptimalMilk/OptimalMilkMain.aspx.) Cows with high growth and mature weights and lactation potential may outstrip nutrient availability on native range and require substantial supplementation. Use of selection indexes that heavily weight terminal traits are strongly discouraged for use as selection tools for sires of replacement heifers.
Below are some additional factors that help identify replacement females that have a leg up:
Crossbred--Heifers that represent optimal combinations of breeds known for superior maternal performance generally are a better alternative to straightbred heifers of equal quality. Maternal heterosis, or the heterosis the heifer will exhibit as a cow, has been shown in numerous studies to be very beneficial to commercial cow-calf production. About two-thirds of the economic benefit of crossbreeding comes from having crossbred cows; one-third from having crossbred calves. A bulk of the maternal heterosis benefit is driven by the improved maternal calving ease, fertility and longevity of crossbred females. First cross (F1) crossbred cows typically last about 1.5 years longer in herd and have a 23 to 30 percent improvement in weaning weight per cow exposed thus improving production efficiency dramatically.
Produced by proven sire--Replacement female selection should start with selection of sires. Sires should be selected to produce heifers that meet the replacement female breeding objective outlined above. Use of fixed time AI to proven sires with high accuracy EPDs for maternal traits makes for an effective breeding/selection system. Select sires that optimize traits of cows so they fit your production environment.
Calved by a proven dam--Replacement heifers that are born to cows that have been reproductively successful under your management for a long period of time are natural candidates as replacements. It's like these cows are among the more fertile cows in your herd and are of appropriate mature size and lactation potential for your environment.
Born early in calving season--Heifers born early in the calving season will be older at the initiation of their first breeding season that calves born later. They have a better chance of having reached puberty by start of breeding season and have a higher likelihood of breeding early in the season. These heifers are also likely from dams that conceived early in the breeding season and 'fit' in your management system and environment.
From middle group of Adjusted 205-day weaning weights--If your cows are bigger than you would like to fit your environment, consider selecting replacement heifers from the middle part of the weaning weight distribution. Keeping the biggest, fleshiest heifers from your herd over time contributes to increases in mature cow weights and increased nutrient demand as cows. You should use age-of-dam adjusted 205 d. weaning weights to classify your heifers' potential for growth. The adjustment procedures remove bias due to age of calf and age of dam at weaning. Heifer of similar genetic potential born at opposite ends of a 90 calving season can have a difference in weaning weights of more than 200 pounds so correcting for age is very important.
A handy worksheet to compute adjusted performance measure for beef cattle is available from Kansas State University at http://www.asi.ksu.edu/doc5906.ashx.