Reproductive efficiency is critical to cow/calf profits. No other factor has a bigger impact on the rancher's pocketbook. Beef cows should ideally calve every 365 days throughout their productive lifetimes. However, that doesn't always happen. Calving intervals are an important measure of sustained reproductive success, or the lack thereof. To help producers understand key factors that influence reproduction in their herds, CowTek, Inc., a cattle technology company based in Valencia, CA, recently sponsored a large-scale analysis of calving intervals in beef cows 2 to 4 years of age.

CowTek partnered with PRISM Group of Dodge City, Kansas, to conduct the research, which utilized data from seven major beef breed associations (Angus, Charolais, Gelbvieh, Hereford, Limousin, Red Angus and Simmental). The study included more than 400,000 young cows. "We're committed to enhancing the profitability of U.S. cow/calf producers," says Gary Marsh, President of CowTek. "And we understand that strategic data and information can lead to improved management practices and a better bottom line. That's why we became involved in the calving interval analysis."

Sex of Calf Affects Dam's Re-Breeding
The research produced several important findings, according to Quentin Conant, President of PRISM Group. "In 1995, a National Cattlemen's Beef Association study showed that young cows raising steer calves experience shorter calving intervals than their contemporaries with intact male calves at side," Conant says. "Our analysis confirms that this phenomenon is for real. Early castration of bull calves reduces breed back time for their 2-year-old dams by 6 to 7 days. Six-day shorter calving intervals were also observed in 3-year-old dams raising steer calves versus those that raised bull calves."

It may surprise some producers that sex of calf has an influence on how quickly a cow re-breeds, but that is exactly what CowTek's research documented. Young cows raising bull calves take a day or two longer to re-breed compared to similar-age cows nursing heifer calves, probably because bull calves suckle their dams more aggressively. "Most cattlemen would have predicted that outcome, but our finding with the steer calves represents something not widely known," states Conant. "Females weaning steer calves exhibit significantly shorter calving intervals compared to those with either bull or heifer calves at side, likely due to an interruption in normal suckling behavior brought about by castration."

High Performance Cows Exhibit Longer Calving Intervals
In another part of the study, Milk and Yearling Weight EPDs were analyzed against calving intervals. The intent was to quantify genetic effects on a young cow's ability to re-breed quickly after calving. Longer intervals between calves were consistently observed in cows with high genetic ability for milk and growth, compared to same-age/same breed females with lower milk and growth genetics. "This portion of the analysis demonstrates that high performance genetics can sometimes be at odds with reproductive efficiency," says Tom Brink, beef industry analyst, who assisted CowTek and PRISM Group with their research. "Furthermore, the antagonism was apparent in all seven breeds we evaluated."

Young cows ranking in the top 40 percent for Milk EPD and top 40 percent for Yearling Weight EPD in each participating breed were grouped together and compared to those in the bottom 40 percent for both traits. High milk/high growth females averaged 15 pounds higher for Milk EPD, and their Yearling Weight EPDs averaged 27 pounds higher. This high-performance group exhibited 5-day longer calving intervals between their first and second calves (P<0.01), and 3-day longer calving intervals from their second-to-third calf (P<0.01). All seven breeds exhibited the same directional difference, suggesting that genetic and nutritional factors behind this effect are similar across breeds.

Calving Heifers Early Beneficial
First-calf heifers often experience elongated calving intervals, which is why some producers calve these young females 2 to 4 weeks earlier than the rest of their cowherd. Calving heifers early is a good defensive strategy, according to Conant. "We compared heifers calving with their first calf at 22-24 months of age versus those calving at 24-26 months," he explains. "Heifers that calved younger exhibited a higher likelihood of producing their second calf by the time they reached 38 months of age. This means they're more likely to breed back soon enough to stay in the herd." For the seven breeds combined, 93 percent of all heifers that calved initially at 22-24 months had calved again by 38 months of age. A less impressive 79 percent of those calving first at 24-26 months produced a second calf on or before reaching 38 months (14 percent difference is significant, P<0.01). ©