How many replacement heifers should be kept?
Cow/calf producers in SD culled at a 14% rate in 2011. Typically it is advised that 150% of the heifers needed are kept for development. This will allow for some leeway to cull heifers that do not perform well and or do not become pregnant. For example: If you have a mature cow herd of 100 and cull at an annual rate of 14% + 50% additional heifers would need to be kept, for a total of 21 heifers to entry the replacement heifer development program.
How important are genetics to the success of a replacement heifer?
Genetics play a major role in the successful entry of a replacement heifer into the cow herd. Puberty is the major limiting factor in replacement heifer development and is moderately heritable (age at puberty r2= 0.20-0.60). In addition, it is highly correlated to overall fertility. Therefore by keeping good production records and utilizing them to select your replacements, you can capitalize on the productive genetics within your cow herd. The genetics of the bull should also be taken into consideration. One of the best predictors of the bull’s daughter’s reproductive performance is their yearling scrotal circumference. This is a moderately heritable trait that is a good predictor age at puberty and lifetime pregnancy rates.
Should the larger heifers be culled from the herd?
It is important to have a mature cow that fits her environment. In some areas a 1400 pound cow can be very productive, but in some areas a 950 mature cow is more effective. With that in mind, one should take into account frame size and body condition of heifers at selection, and the mature size of cows that are most efficient in their environment. However, the age of the heifer should be considered. Typically the larger, heavier heifers are older calves, from a mother who had higher milk yields. Therefore, the selection of the large and older heifers should not be ruled out as you may be selecting the heifers from the most productive dams within your herd.
How does age of the heifers contribute to reproductive success?
Cows that calve earlier in the calving season tend to continue to do so in subsequent years, remaining in the annual production cycle, and producing greater total pounds of calf over their lifetime. This trait can be passed on to their daughters. Therefore, by selecting the older heifers, you are selecting heifers from dams that calved earlier in the calving season, which would typically be the most productive group of females in your herd. In addition, age is a factor that limits the attainment of puberty. If heifers are old at the beginning of the development phase, this will help ensure puberty is attained prior to the breeding phase.