Abstract: 306

PREGNANCY RATES, PROGESTERONE AND BLOOD UREA NITROGEN CONCENTRATIONS IN BEEF HEIFERS FED EXCESS DEGRADABLE INTAKE PROTEIN BEFORE BREEDING.

James Berardinelli^{1 *}, Peter Burfening¹ , Ron Adair¹
Montana State University, Bozeman, Montana ¹

The objective of this study was to determine if feeding a diet high in degradable intake protein during a synchronized estrous cycle alters progesterone (P₄) or blood urea nitrogen (BUN), or pregnancy rates in beef heifers. Heifers were individually fed either 100 (C; n = 20) or 200% (HP; n = 19) of the NRC protein recommendation for maintenance during a synchronized estrous cycle and for five d after the next estrus. Each heifer was bred by artificial insemination (AI) using a single sire. Jugular blood samples were taken daily during the synchronized cycle and on d 0 to 5 and 11 of the next cycle. Ultrasonic examinations of the uterus for the presence of an embryo or fetus were performed at 30, 45, 60 and 125 d after AI. Samples were assayed for BUN and P₄. Synchronized estrous cycle lengths or proportions of heifers showing estrus did not differ between (p > .10) HP- and C-fed heifers. BUN concentrations were higher (p < .05) in HP-fed heifers than in C-fed heifers during the synchronized cycle and the first five d of the next cycle. BUN concentrations did not differ (p > .10) between HP- and C-fed heifers on d 11; 5 d after terminating the HP diet. P₄ concentrations during the synchronized cycle and first five d of the next cycle did not differ (p > .10) between HP- and C-fed heifers. Pregnancy rates at 30 d after AI did not differ (p > .10) between HP- (71%) and C-fed (74%) heifers. However, pregnancy rates at 45 and 60 d after AI were lower (p < .05) for HP-fed heifer (59%) than for C-fed heifers (74%). By 125 d, after exposing heifers to bulls for 30 d, pregnancy rates did not differ (p > .10) between HP- (89%) and C-fed (100%) heifers. Feeding heifers excess digestible intake protein during an estrous cycle and the first 5 d after breeding increased embryonic mortality between 30 and 45 d after fertilization. The mechanism for this effect is not known.

    This abstract is being presented on Monday, August 2 at 8:00 AM to 10:15 AM at CUB 2nd Floor Ballroom.