Angus breeders have led the charge in breeding curve-bending bulls who moderate birth weight while increasing growth. There are now a large number of Angus bulls that are negative for birth weight (BW) EPD, over 60 lbs for weaning weight (WW) EPD, and over 100 lbs for yearling weight (YW) EPD. This has led us to question , “Is this the type of bull we should use on heifers?”
Using a bull with a higher WW EPD should result in calves with higher actual weaning weights. For example, if calves out of a bull who has a +30 for WW average 500 lbs, calves out of a bull with +60 for WW should weigh 530 lbs (60-30=30 lbs extra WW). This led us to question what effect the higher WW would have on the cow’s body weight and body condition, especially in first calf heifers. Using individual cow and calf weights at weaning, we examined how increasing calf WW affects cow weight on first calf heifers.
From 2002 to 2011 we recorded weaning weights for 167 bull calves and 168 heifer calves off first calf heifers. The bull calves averaged 520 lbs and the heifers averaged 489 lbs.The dams of the bull calves averaged 1004 lbs while the dams of the heifer calves averaged 1023 lbs. In this case, comparing bull calf to heifer calf weight is similar to comparing a sire with a +60 lb weaning weight EPD to a sire with a +30 lb weaning weight EPD. There is roughly 30 lbs difference in genetic potential for growth.
Increasing weaning weights by 31 lbs resulted in 19 lbs loss in cow body weight, or roughly 3 lbs of increased weaning weight resulted in 2 lbs of lost body weight in the two year old. Depending on your management and feed situation this may or may not be an acceptable trade off. However,this indicates that using high growth bulls on first calf heifers does put added strain on the heifer which may compromise her ability to get bred back as a two or three year old. Simply put, higher weaning weights do not come without a cost to the cow.
Most low birth weight, high growth sires will also increase mature cow weight. Both weaning weight and yearling weight are highly correlated to mature cow size. This is another consideration if you keep replacements out of your first calf heifers. Simple math tells us if a heifer needs to reach 60% of her mature body weight by the beginning of breeding season she would need to weigh 780 lbs if her potential mature body weight is 1300 lbs. She would only need to weigh 720 lbs if her potential mature body weight is 1200 lbs. Those extra pounds needed are probably easier to attain on heifers out of mature cows than heifers out of two year olds. Therefore, replacement heifer calves out of two year olds and high growth sires may be more difficult to get bred than replacements out of moderate growth sires especially if feed may be limited.