When the telephone rings at 2 a.m. in the springtime, I'm pretty sure the caller will be a beef client with a first-calf heifer needing calving assistance. That was the case recently, and when all was said and done, the owner expressed surprise that the first calf out of his new herd bull appeared to be quite large.
He'd bought a bull with a 76-lb. birth weight, and the seller claimed he'd be a good bull for calving ease due to his “small head and shoulders.” As I drove home after the call, I shook my head wondering where we'd failed our producers in disseminating sound science about calving ease.
The actual birth weight of a bull isn't a particularly accurate measurement of his progeny's birth weight. For example, all things being equal, a bull born to a heifer will weigh less at birth than a bull born to a mature cow. And a bull born in the summer or fall will weigh less than a bull born in winter or spring. A bull born as a twin will be lighter than a single calf. None of these reasons are genetic, and none influence his progeny's birth weight.
Using a bull's expected progeny difference (EPD) for calving ease and/or birth weight is a much more accurate predictor of progeny birth weight and calving ease. Research shows EPDs are much more accurate in predicting subsequent calving ease compared to using a bull's own birth weight.
Dystocia is a major scourge
Numerous studies have also looked at various measurements of the newborn calf, finding very low to no correlation of these measurements to calving ease in future offspring. So, a bull with a “small head and shoulders” means nothing as far as dystocia prevention is concerned.
In the 1996 National Animal Health Monitoring Service survey, dystocia directly accounted for 31.8% of all calf death loss. We also know a calf needing assistance is more likely to get sick, mostly due to increased time from birth to initial nursing, which inhibits the amount of colostral antibodies absorbed through the calf's intestinal wall.
If you're getting ready to choose a sire for artificial insemination (AI) for your heifers, numerous bulls will sire calves well below average for birth weight while still producing acceptable growth. In fact, I looked at the sire summaries of the top eight beef breeds and found 45 bulls below an Angus equivalent birthweight EPD of 0 with 80% accuracy, and above average weaning and yearling EPDs.
Are all these bulls “guaranteed” to produce 100% unassisted, live births? Of course not, but if more than 15% of your heifers needed assistance this spring, I'm confident using one of these bulls will improve that number.
Don't forget the mama
Many times we forget about the dam's contribution to the dystocia equation — she supplies half the calf's genetics. If you have a heifer with a very high birth weight EPD, her calf could be quite large. I suggest putting high birth weight EPD heifers in the feedlot.
Another frequently mentioned factor is pre-calving nutrition. Statements like “I fed the heifers 5 lbs. of corn/day and it all went to the calf” are frequently expressed.
Sure, if a heifer is ridiculously fat — body condition score (BCS) of 8 or 9 — you could add enough fat internally to create a problem. But giving 5 lbs. of corn/head/day and calving heifers in BCS 6-7 won't increase dystocia. In fact, having her in BCS 6-7 will improve calf vigor, colostrum quality and quantity, and rebreeding rate.
Because decreased gestation length reduces birth weight, some work has looked at targeting gestation length to reduce dystocia. The problem for breeds with an EPD for gestation length is the bulls in the bottom 5% of the breed for gestation length are only 2-4 days less than breed average. This isn't sufficient to be a reliable factor in reducing dystocia rate by itself.
Nearly every study shows calving ease EPD is the most reliable method. If a breed doesn't supply such a number, then consider birth weight EPD, as it's the figure most highly correlated to calving ease.
Visit with your herd health veterinarian about using AI for your heifers this year. If you can't AI, purchase a bull with an Across Breed EPD (Angus-based) of 0 or below.
Young bulls have low EPD accuracies, but utilizing EPDs is far superior to looking only at actual birth weight or any other parameter.
W. Mark Hilton, DVM, is a clinical associate professor of beef production medicine at Purdue University in West Lafayette, IN.
Working with EPDs
Expected progeny differences (EPDs) are still a conundrum to many producers. The key letter in EPDs is the “D.” We use EPDs to look at the expected difference in progeny weights between two bulls.
If Bull A's birth weight (BW) EPD is 0, and Bull B's is 3.0, then on average we expect B's calves to weigh 3 lbs. more at birth than Bull A's. EPDs tell us nothing about breed average EPDs, but you can find them on the breed's Web site. Zero may have been breed average when the EPDs were introduced, but not now.
Another confusing factor is the inability to compare bulls of different breeds using their breed's EPDs. An adjustment factor must be used.
Below is the adjusted BW EPD equivalent for each of the top breeds (adjusted to Angus base), according to 2005 U.S. Meat Animal Research Center data.
Thus, an Angus bull with BW EPD of 0 should sire calves with the same BW as a Gelbvieh at -4.7 for BW, if the cows are similar.