Energy & Protein Influences on Reproduction
JOPLIN, Mo. (Sept. 1, 2011) — Reproductive failure among cows and heifers, either because females aren’t cycling or because of embryonic mortality, is often related to their nutritional status. Addressing the Applied Reproductive Strategies in Beef Cattle (ARSBC) conference, in Joplin, University of Idaho Extension Beef Specialist and researcher John Hall said the relationship between nutrition and reproduction is likely an adaptive mechanism, which prevents reproduction during times of limited nutrient availability. During such times, maintenance of the cow’s or heifer’s own body takes precedence.
Hall reviewed the roles of energy and protein nutrition as related to reproduction. He called energy the nutrient of first concern, noting its role in regulating release of hormones. In addition to overall energy availability, the timing of energy increase or deprivation is important in determining pregnancy rates.
“Energy reserves act as a buffer. That’s why body condition at calving time is important,” said Hall, reminding producers that cows and heifers should exhibit body condition scores (BCS) of 5 to 6, and 6 to 7, respectively.
With regard to replacement heifer development, Hall noted the long-standing recommendation that heifers be grown to 65% of mature weight by breeding time. Reasons include nutrition’s influence on age of puberty as well as the goal of building energy reserves. Hall said increased feed costs have caused researchers as well as producers to question the traditional target. Studies have shown that developing heifers to 50%-55% of mature weight can reduce development costs without affecting pregnancy rates.
However, evidence suggests the percentage of heifers cycling at the beginning of the breeding season and artificial insemination (AI) pregnancy rates may be reduced. In order for a program targeting lower breeding weights to work satisfactorily, Hall recommended that producers have an effective market for open heifers, provide sufficient post-breeding nutrition to keep heifers growing and incorporate a progestin-based estrus synchronization program to induce puberty in a maximum number of heifers.
“Producers need to know their cattle. If heifers typically are slow to start cycling, they won’t want to stray far from the 65% target,” Hall warned.
Since first-calf heifers still undergo considerable growth while raising a calf, Hall advised careful attention to nutrition, to assure optimum breedback. The reproductive performance of thin cows may also be improved when they are provided supplemental energy. Hall did warn that while fat is a source of energy, exceeding dietary levels of 5%-8% fat can impair rumen function.
According to Hall, protein deficiency may also delay return to estrus, but protein supplementation of pregnant or early-lactating cows grazing protein deficient forages can decrease the postpartum interval and increase pregnancy rates. Rather than the commonly used crude protein measurement, he recommended balancing diets for metabolizable protein, which is an estimate of protein that reaches and is absorbed by the small intestine. Hall also warned that excessive levels of dietary protein may be detrimental to pregnancy rates, but is most problematic when diets are also energy-deficient.
“The connections and signals between nutrition and reproduction are extremely complicated,” Hall stated, adding that nutritional management should focus on maintaining cattle in proper nutritional status or achieving that status by critical reproductive events. “The easy-to-use guides are to manage for proper body condition score at calving and maintain that through breeding.”
Hall spoke during Thursday's ARSBC session focused on the nutritional influences on reproduction. Visit the Newsroom at www.appliedreprostrategies.com to view the PowerPoint slides and proceedings paper submitted by Hall to accompany his presentation. Audio of the presentation will be available soon.
Comprehensive coverage of the symposium is available online at www.appliedreprostrategies.com. Compiled by Angus Productions Inc. (API), the site is made possible through sponsorship by the Beef Reproductive Task Force, SEK Genetics, and liveauctions.tv. Coverage includes summaries of the speaker presentations, PowerPoints, proceedings and audio.
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