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Title: INFLUENCE OF MILK PRODUCTION POTENTIAL ON FORAGE DRY MATTER INTAKE OF MULTIPAROUS AND PRIMIPAROUS BRANGUS FEMALES

Author
item JOHNSON, C. - OSU
item LALMAN, D. - OSU
item BROWN, MICHAEL
item Appeddu, Lisa
item BUCHANAN, D. - OSU
item WETTEMAN, R. - OSU

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2003
Publication Date: 7/1/2003
Citation: JOHNSON, C.R., LALMAN, D.L., BROWN, M.A., APPEDDU, L.A., BUCHANAN, D.S., WETTEMAN, R.P. INFLUENCE OF MILK PRODUCTION POTENTIAL ON FORAGE DRY MATTER INTAKE OF MULTIPAROUS AND PRIMIPAROUS BRANGUS FEMALES. JOURNAL OF ANIMAL SCIENCE. 2003. 81:1837-1846.

Interpretive Summary: In order for cow-calf producers to optimize efficiency in their production systems, it is imperative for cows to calve at 12-mo intervals. The first-calf beef heifer presents challenges to achieving this goal, as they typically have longer postpartum intervals and lower pregnancy rates upon rebreeding compared with cows that have previously calved. Reduced reproductive performance in cows and heifers can result from inadequate nutrient intake before or after calving as well as lactational stresses after calving. Selection for increased milk production based upon sire EPD effectively increases milk yield. However this increase may be at the cost of body nutrient reserves. Lactating cows consume more forage DM than gestating cows. Furthermore, as milk yield increases, so does forage DMI. The relationship between the predicted differences in milk yield, stage of production, and forage DMI has not been established. Research at the USDA-ARS Grazinglands Research Laboratory in cooperation with Oklahoma State University evaluated the influence of cow parity and predicted difference in milk production on forage DMI in late gestation, early lactation, and late lactation. Results of these experiments suggest that selecting cows for increased milk production within a breed does not alter forage intake during late gestation. However, forage intake is sensitive to genetic potential for milk production during lactation; even when low quality forage is the primary feed resource. In fact, when modeling dry matter intake of lactating beef cows, the adjustment for milk yield may need to be increased compared to the one currently being used by the National Research Council. Dry matter intake is similar between first-calf heifers and older cows when intake is expressed relative to body weight. Therefore, separate prediction equations should not be necessary for different parity classes. Finally, models to estimate forage intake of beef cows should incorporate more sensitive adjustments to account for the dramatic increase in intake that occurs after parturition. An adjustment for stage of lactation, beyond milk yield, does not seem to be necessary.

Technical Abstract: Brangus cows (n = 29) were used to evaluate the effects of parity (multiparous vs primiparous) and potential genetic merit for milk production (high vs low) on forage intake during late gestation, early lactation, and late lactation. Cows were selected for milk production based upon their sire's EPD for milk production (MEPD). Cows had ad libitum access to (130% of previous 2-d average intake) low quality hay (5.3% CP and 76% NDF) and cottonseed meal was used to meet protein requirements. All females were adapted to diets at least 7 d and individual intake data were collected for 9 d. During the lactation trials, actual milk production was determined using a portable milking machine, following a 12-h separation from calves. During late gestation, multiparous cows consumed 24% more (P = 0.01) forage DM (kg/d) than primiparous cows. However, parity class did not influence forage intake when intake was expressed relative to BW. Furthermore, MEPD did not influence forage intake during late gestation. During early lactation, multiparous cows produced 66% more (P < 0.001) milk than primiparous cows, and high MEPD tended (P = 0.1) to produce more milk than low MEPD. Multiparous cows consumed 19% more (P < 0.0001) forage DM than primiparous cows when expressed on an absolute basis, but not when expressed on a BW basis. High MEPD cows consumed 8% more (P < 0.05) forage DM than low MEPD. During late lactation, multiparous cows produced 84% more milk than primiparous cows, although MEPD did not influence (P = 0.4) milk yield. Also, multiparous cows consumed 17% more (P < 0.01) forage DM per day than primiparous cows, but when intake was expressed relative to BW, neither parity nor MEPD influenced forage DMI during late lactation. Milk yield (kg/d) and BW (kg) explained significant proportions of the variation in forage DMI during early and late lactation. Each kg increase in milk yield was associated with 0.33 kg and 0.37 kg increase in forage DMI for early and late lactation, respectively. Results suggest that multi- and primiparous cows consume similar amounts of low-quality forage DM, expressed per unit of BW, during late gestation and lactation. Selecting beef cows for increased genetic merit for milk production increases forage DMI during early lactation.