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Research: how heifer development impacts pregnancy rates

by Wyoming Livestock Roundup

“For about the past four years, we’ve been trying to see if what we do to a heifer from weaning to breeding has an impact on pregnancy rates,” states South Dakota State University Beef Reproduction Specialist George Perry of research into heifer development methods.
“When we started, one area of research on which we focused was trying to develop heifers in a range environment. We had started early-weaning our heifers, and one problem with early-weaning heifers is they cost a lot more in a feedlot. We had a range station in the northern part of South Dakota, so we early-weaned a group and put them in the feedlot for about 45 days to get over weaning and on feed. Then they were taken back out to pasture and developed on the grass we saved through early-weaning,” says Perry of the first experiment.
“They were also supplemented while on the grass. You can’t just turn them out on range and forget about them. We also took a group and developed them as we normally would in the feedlot, shooting for one pound of gain a day. Then, in May, at early green-up, everything was turned out in the same pasture,” explains Perry.
All heifers were bred in June, when they were also weighed. The heifers developed on grass had maintained a .7-pound average daily gain (ADG), while those developed in the feedlot had a .2-pound ADG.
“These were all shrunk weights, and during their first week on grass the feedlot-developed heifers lost over three pounds a day, while the range-developed heifers gained over two pounds a day. It took the next month for those feedlot-developed heifers to reach the same ADG as the range-developed heifers,” says Perry.
After calculating that data, Perry looked for other studies and found one where heifers turned on grass went from 120 percent of maintenance to 40 percent of maintenance, resulting in 56 pounds lost over two weeks, and 60 percent of the heifers stopped cycling during that time.
“We asked what those kind of numbers would do to conception rates. So, the next year, instead of turning them out at green-up, we turned them out at AI. Ninety-four percent of our feedlot-developed heifers were cycling prior to breeding, versus 84 percent of range-developed heifers.
“What’s interesting is that pregnancy rates went in the opposite direction. There is data that shows the more heifers you have cycling prior to breeding, the better your pregnancy rates are. Here we were actually seeing the lower cycling (range-developed) heifers, have the better conception rate, at 57 percent, versus 44 percent in the feedlot-developed heifers,” notes Perry.
He followed up on those results by asking if a change in diet caused the differences in conception rates.
“We had between 200 and 300 heifers in all of these studies, and we took 300 heifers, from two producers, that were developed in the same feedlot, stuck with one synchronization program, and bred them all same way, then moved them to pasture. We separated each group into two herds at that point, and supplemented one herd in each group with five pounds of distillers grain per head, per day, to try to get some protein and energy to them,” says Perry.
All heifers weighed about the same going onto pasture, and weights were taken when CIDRs were put in and while preg-checking 45 days later. Herd one gained 17 pounds if they weren’t supplemented, and 15 pounds if they were, over the entire 45-day period.
“There is something going wrong when heifers only gain 15 pounds in 45 days,” notes Perry.
“In herd two, the group that was just on pasture alone, lost one pound a day, and the group that was supplemented gained one pound a day,” continues Perry. “In that herd, the group that wasn’t supplemented had a 26 percent conception rate.
“In both of these herds we went out and looked at the pasture, and there was plenty of grass. The nutritionist I work with said that these heifers, if they were eating two percent of their body weight, should have gained over a pound a day.”
“So, that led to the question of why are these heifer that are going to pasture alone losing weight?” notes Perry.
To answer this, he took over 300 forage-developed heifers, synchronized and AI-ed them and then split them into three groups. One-third went to the feedlot, where they experienced a complete environmental change from grass, one-third went back to pasture, and one-third went to pasture and were supplemented.
“In this location we weren’t fortunate enough to have access to scales, so we did everything by body condition score (BCS),” explains Perry. “The group that went back to pasture maintained their BCS, the group on pasture that was supplemented and the group in the feedlot both gained BCS. Those heifers were all gaining great, and their pregnancy rates were basically the same across all three groups, so it wasn’t the environment.”
“We wanted to know why these heifers weren’t adjusting when we moved them to grass. There are some instances when producers literally need to have their heifers in a feedlot, so we wanted to figure out how to overcome this,” notes Perry.
To help answer that, he took two bunches of feedlot-developed heifers and moved half of each bunch to pasture 30 days prior to the breeding season and left the other half in the feedlot. Everything was brought in, synchronized and AI-ed the same way.
“We came back and preg-checked herd one after 30 days. Those moved to pasture averaged 17 pounds of gain in those 30 days and had a pregnancy rate of 50 percent. The group left in the feedlot gained just over six pounds during the same time period, and had 46 percent breed up.
“In herd two we pregged at 70 days, and looked at the differences. We saw those on pasture gain 105 pounds, versus those in the feedlot gaining two pounds over those 70 days. If you’re backgrounding cattle, two pounds in 70 days isn’t very good,” says Perry, adding that conception rates were 59 percent in heifers turned out to pasture, versus 50 percent in those developed in the feedlot.
“Pregnancy rates were four and nine percent higher in those that were moved to grass earlier. If heifers lost weight, in any of these studies, from AI to preg test, conception rates were very low. We found that keeping heifers on a gaining plain of nutrition during that time after breeding is very important. That embryo doesn’t even signal to the mother for two weeks, and she can’t do anything until then to help it survive,” says Perry of his conclusions.
Perry presented his information during the Cattlemen’s College, held in conjunction with the NCBA’s Annual Convention and Trade Show, in Denver, Colo. on Feb 3. Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at

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