How Early is Too Early? Are gestation lengths changing in cattle? |

How Early is Too Early? Are gestation lengths changing in cattle?

Ruth Wiechmann
for Tri-State Livestock News
Some producers say the official gestation length of low birthweight "heifer" bulls should be changed to more accurately reflect their calves' earlier births. Photo by Ruth Weichmann

“They were SUPPOSED to start calving February fifteenth, but they started January 26th!”

The frustration was obvious. A set of bred heifers purchased by a neighbor were calving early, not just by a few days, but by three weeks. Was this an intentional ‘sell ‘em so we don’t have to deal with ‘em’ situation, an accidental record keeping oversight, or was it a result of bred in calving ease from the use of low birth weight bulls for several generations?

Are our first calf heifers calving too early? Are the low birth weight Angus bulls being used on first calf heifers for multiple generations affecting the gestation length of the Angus breed overall? How significantly will this affect other breeds, commercial cattle herds, and the beef industry overall? Are we at risk for ‘over selecting’ for calving ease to the detriment of other important traits? Do we need to change the official ‘due date’ for our cows?

We may have more questions than answers.

Vaughn Thorstenson, Selby, South Dakota, raises purebred Angus, Gelbvieh, and Balancer bulls. Over several decades he has seen more of a change in the gestation length of the Angus cattle than in the Gelbvieh. “Gelbvieh were always 286 days and I’m pretty sure they’re still at 286, although this year we did have a few that came almost as early as some of the Angus calves,” he said. “The Angus are probably closer to 280, or 278, and certain sires that are really high calving ease are consistently two weeks early. Not all Angus bulls are that consistent but there are some that are. Usually we have half the Angus heifers calved out before the Gelbvieh start.”

Is this a problem? Some folks are beginning to think so, but Thorstenson isn’t so sure. He feels that the slightly shorter gestation actually pushes performance in his cattle. “If you can get that calf on the ground a week early as opposed to two weeks late he’s got a jump start on life. Nobody keeps their late born calves, because nobody wants to buy a bull with a 110 pound birth weight. That calf is going to be a steer.” This selection process has shortened up the gestation length in the Angus breed, and Thorstenson feels that this is a positive thing both for the Angus breed and the cattle industry so long as breeders are careful to select for performance traits as well as calving ease.

Thorstenson is careful to use both performance genetics and calving ease bulls in his herd to maintain a balance and believes that true prematurity in calves is more of an environmental issue than a genetic one. “If you don’t take good care of your cows during that third trimester then you’re going to see more small, weak, early calves simply because the cow didn’t get the nutrition she needed and was stressed,” he said.

Leo McDonnell of Columbus, Montana, is a breeder of registered Angus cattle and past owner of Midland Bull Test. Like Thorstenson, McDonnell believes balance is key: “Breeding cattle is always about maintaining balance and not going to extremes where we usually find negative consequences to other economically important traits.” He believes some may be breeding themselves into a corner if they’re not careful. He is concerned that more and more calves are arriving so early that they are weak and slow to get going, lacking the body mass to thrive in tough environmental conditions. McDonnell says his first calf heifers are usually ninety percent done calving by their 283 day ‘due date’ and that many of his customers are experiencing the same. He feels that the gestation length for Angus cattle should be adjusted by 7-10 days.

“Ranchers purchasing bred heifers aren’t always expecting the calves to show up a week or two prior to the date they were advertised to start calving,” McDonnell said.

Ethan Andress, DVM, says that he and his colleagues at West River Veterinary Service in Hettinger, North Dakota, are having the same conversation. They are involved with many producers in the area from conception to calving and beyond, and they are concerned about the patterns they’re seeing. They haven’t put together any hard numbers but the pattern they’ve observed over the last few years is more calves coming earlier and earlier. They are seeing more undersized calves and more calves that may appear normal but are weaker at birth. Their clients run predominately Angus cattle and include both purebred herds and commercial herds that AI yearling heifers with low birth weight Angus bulls. These bulls are popular because they have been proven to produce low birth weight calves, but part of the low birth weight is actually a shorter gestation length.

“We need to be asking ourselves the questions: is it safe? Is it practical? Is it profitable?” Andress said.

Producers are choosing low birth weight bulls for these same reasons: they want the safest delivery possible for first calf heifers. They want their cattle to be as trouble-free as possible, especially during the long hours of calving season; they all need that bottom line to be in the black at the end of the year. Getting calves on the ground alive is paramount, but do we need to be concerned that we’re getting too small? Too early?

Andress and his colleagues believe they are seeing a strong trend in that direction.

“Some of our AI programs are virtually done by their due date. Eighty-five to ninety percent of the heifers are calved out before they are ‘due.’ We are pushing the bell curve way up front,” Andress said.

In a few very specific cases they have seen calves come as early as twenty days prior to their due date. Within the clinic, they have already adjusted their estimated due date down to 280 days from 285 days when ultrasounding, and they are talking about moving it further. “We may need to move it to 275 or even 270,” Andress said.

What are the numbers from the Angus breed overall? The American Angus association doesn’t have all the data gathered yet. Kelli Retallick, American Angus Association, is hopeful that breeder participation in their new Maternal Plus herd reporting program will give conclusive information. “As the program grows, we will be able to run some of those numbers to get better answers,” she said.

George Perry, South Dakota State University Extension Beef Reproductive Management Specialist says we’re asking a loaded question and we simply do not have the data to answer it for a wide spectrum.

“So many things affect gestation length. Are there genetics involved? Definitely. But there are so many factors to consider. It’s easy to say a calf is premature and forget that stress from the calf is actually what signals parturition to begin.”

The weather during February and March of 2019 has certainly increased the stress levels pregnant cows have had to tolerate. Extreme cold and repeated storms and the type and quality of feed the cows are getting will also affect gestation length and a newborn calf’s resilience level.

Perry emphasized that there are significant differences between breeds. He agrees that there is a trend, especially in Angus cattle, toward shorter gestation length that corresponds to low birth weight but says we just don’t know if we actually have a problem when it comes right down to it because we don’t have the research giving us hard numbers that say we’re calving too early to a point of being dangerous.

He said we may just be noticing this trend toward shorter gestation more because we’re keeping better records today than we did a couple of generations ago. “Grandpa would turn out a bull, then maybe pregnancy test fifty or sixty days after he took the bull out. Then we were lucky to have a sort between first and second cycle. Now we’re synchronizing our cattle, AI’ing, and we have an ultrasound that says a calf is a certain gestational age.”

Producers need to remember that a ‘due date’ is just a number. “I’ve always said that two weeks early to two weeks late is ‘normal,’ Perry said. “270 days is only one day off from 14 days ‘early’ if you’re figuring a 285 day gestation length.” 270 days is within that timeframe if you’re figuring a 283 day average gestation.

Do producers need to be prepared for calves to come early? Perry said yes. Definitely. Do we need to change ‘the book?’ He’s not so sure about that. “We need to remember that these calving books are formulated for all breeds all across the country,” he said. “I grew up in south Texas, and when you add a little Brahman influence, 285 days is short!”