EFFECT OF FORAGE-CONCENTRATE RATIO ON DIGESTION AND REPRODUCTION IN PRIMIPAROUS BEEF HEIFERS

We evaluated the effects of high- (HF) and moderate- (MF) forage diets on digestive and reproductive characteristics in beef heifers. Thirty primiparous beef heifers were allotted by weight and backfat thicknes s to receive either 80:20 (HF) or 50:50 (MF) forage:concentrate ratio diets from parturition to at least 90 d postpartum: Alfalfa hay and wh eat straw were the forage sources and barley was the concentrate sourc e. Equal daily amounts of ME were provided to all heifers by restricti ng intake of the MF diet. Digestibility of DM was greater (P < .001) f or MF compared with HF diets, whereas NDF digestibility was not differ ent. Dry matter and NDF digested daily was lower (P < .001) for MF tha n for HF diets. Ruminal fluid pH was lower (P < .05) for MF diets; how ever, the acetate:propionate ratio was not different: Serum insulin co ncentrations were greater for MF diets for all hours (P < .001) and we eks (P < .05) of sampling. Changes in weight, backfat thickness, and b ody condition score at 90 d postpartum were not different between trea tments. Calf gain to 30 d, however, was greater (P < .10) for the MF t han for the HF treatment (25.5 vs 20.7 kg). Maximum size of the ovulat ory follicle was greater (P < .10) for cows receiving the HF diet than for cows receiving the MF diet. However, other aspects of ovarian fol licular growth and wave dynamics and the intervals from parturition to first and second ovulation, first estrus, first service, and concepti on were not different between treatments. Shifts in energy supply from forage to concentrate had minimal effect on digestion and reproductio n in first-calf beef heifers in this study.