The effects of ruminally undegradable protein, propionic acid, and monensin on puberty and pregnancy in beef heifers.

Two hundred ten spring-born English crossbred beef heifers were used in two experiments in consecutive years using a randomized complete block design. The objectives were to determine the effects of increasing dietary intake of ruminally undegradable protein or propionic acid on the occurrence of puberty and to elucidate the mechanism by which monensin decreases age of puberty in beef heifers. Diets were fed for 120 d beginning in mid-November and consisted of mature grass hay and barley straw with .45 kg.animal-1 x d-1 supplement (CON) formulated to obtain .4 kg.animal-1 x d-1 of BW gain; .5 kg of supplement similar to CON and containing an additional 250 g of ruminally undegradable intake protein supplied from blood and corn gluten meal (UIP); .45 kg of supplement similar to CON composition and containing 400 g.animal-1 x d-1 of a 50% water and 50% propionic acid mixture (PROP); or .45 kg of supplement similar to CON supplying 200 mg.animal-1 x d-1 of monensin (MON). The hay:straw ratio in each diet was adjusted biweekly, to maintain similar BW gains for heifers fed each dietary treatment. As planned, BW gains were similar (P > .1) for all heifers. The heifers fed MON were 21 d younger (P < .05) at first estrus than heifers fed UIP; CON and PROP were intermediate and similar to heifers fed MON and UIP. The heifers fed UIP were 7 to 8 kg heavier (P < .05) at puberty than CON- or PROP-treated heifers. The heifers fed PROP were 6 kg heavier (P < .1) at puberty than heifers fed MON. Heifers that received UIP required less (P < .05) TDN than did heifers that received CON or PROP to achieve similar gain. Pregnancy rate was similar for all treatment groups. Feeding UIP in excess of NRC CP requirements may improve energy utilization of heifers fed mature roughages but may delay the onset of puberty compared with heifers fed monensin. This study demonstrated that nutrient composition of the diet may influence age of puberty independent of growth rate.