Nutritional Influences
on Reproduction: Energy and Protein
Research suggests that only 75% to 85% of all U.S. beef cows deliver a calf each year. It is estimated that three-quarters of those cows that fail to reproduce simply don't become pregnant during the breeding season. The remainder conceive but do not carry their calves to term — usually due to embryonic loss. According to University of Idaho Reproductive Physiologist John Hall, nutrition is the most important driver of reproductive efficiency, and nutritional deficiencies often are the cause of reproductive failures.
"Energy is the primary nutrient for which deficiency can affect reproduction," Hall said, addressing the Applied Reproductive Strategies in Beef Cattle (ARSBC) Conference Aug. 6 in Nashville, Tenn. According to Hall, energy availability and the timing of dietary energy increase or deprivation appear to influence release of hormones associated with the female's reproductive cycle, as well as the animal's response to those hormones.
Restricted energy intake during late gestation can result in a longer postpartum interval — that period of time between calving and resumption of the estrous cycle — and reduce subsequent pregnancy rates. But even if the female does cycle in timely fashion and breed back, energy deprivation can jeopardize embryo survival.
"If energy is deficient, we may be compromising the quality of the oocyte (egg)," Hall explained.
Calling cow body condition a good indicator of whether the diet is supplying adequate energy for cows to rebreed readily, Hall recommended calving cows in body condition score (BCS) 5 or 6. For heifers, he recommended BCS 6 or 7.
Hall said heifers raised on low-energy diets typically are delayed in reaching puberty and have lower pregnancy rates during their first breeding season compared to heifers raised on high-energy diets. Heifers developed to approximately 65% of mature weight by 12 to 13 months of age generally reach puberty by breeding time.
Hall noted recent studies indicating heifers could be developed to 50%-55% of mature weight by breeding time, and thus reduce development costs without affecting pregnancy rates. However, results also showed the percentage of heifers cycling at the beginning of the breeding season was reduced, as were pregnancy rates for heifers bred through artificial insemination (AI).
"So be careful," Hall warned. For a reduced development program to work effectively, producers must provide sufficient nutrition for heifers after breeding for continued growth. Pregnancy rates to AI will be improved if producers use a progestin-based synchronization system to induce puberty in a maximum number of heifers. Hall said it's also beneficial to have a ready market for open heifers.
While body condition at calving has the greatest impact on cow reproduction, Hall said maintaining body condition postcalving is important for cows, too. Even cows that calve while in good condition but experience a period of weight loss after calving can be compromised.
"Dietary energy for cows and heifers can come from a variety of sources. It's calories that matter," said Hall. "It doesn't matter what the sources of calories are, as long as they get into the animal."
Hall mentioned that supplementing fat to increase energy density in the diet and improve reproduction has been studied. Some positive effects result from diets containing 5%-8% fat, but exceeding the 8% level can impair rumen function. Hall said feeding fat during the last 45 days of gestation has been shown to improve pregnancy rates among mature cows.
"The take-home message is this," said Hall, "If you can feed fat at a reasonable cost, feed it during late gestation and there should be some benefit."
Hall said protein deficiency also delays return to estrus after calving. When forages provide less than 7% to 10% crude protein, supplementation of pregnant and early-lactation cows is advised to help improve forage digestibility and enhance energy intake.
"In (the southeastern U.S.), crude protein is often adequate in forages, though it does depend on the type of forage available. That's often not the case west of the Mississippi," said Hall, referring to protein-deficient dormant western range forages.
Because pasture and other forages contain mostly rumen degradable protein, researchers have studied the potential benefits of feeding undegradable intake protein (rumen bypass protein). Benefits to reproduction, said Hall, are inconsistent.
To manage breeding herds for successful reproduction, Hall advised producers to know their cows, their nutritional needs, and the nutrition that available forages provide.
"Test forages, for goodness sake," Hall declared. "Supplements are expensive. Don't waste money."
For additional coverage from the ARSBC meeting in Nashville, including proceedings, PowerPoints, audio and summaries of other speakers, visit http://www.appliedreprostrategies.com/newsroom.html.
For information on how to condition score cows, visit http://www.cowbcs.info.
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