Effect of pasture crude protein and fermentable energy supplementation on blood metabolite and progesterone concentrations and on embryo survival in heifers

Seasonal milk production systems rely on heavy inputs of nitrogenous fertil izer, which typically generate pastures with a high crude protein (CP) and low fermentable energy concentration. High intake of CP, particularly in as sociation with low rumen fermentable energy, increases systemic ammonia and urea and has been associated with reduced fertility in cattle. The objecti ve of this study was to examine the relationship between pasture protein in take and fermentable energy supplementation on a range of blood metabolites and on embryo survival and development in cattle. Oestrous synchronized, nulliparous beef heifers (no. = 175) were randomly a ssigned to one of four pasture-based dietary treatments in a 2 X 2 factoria l study carried out over 2 years. Animals were randomly allocated to either high (85 kg nitrogen (N) per ha; HN) or low (0(.)0 kg N per ha; IN) N fert ilized pastures and within pasture treatment were randomly allocated to rec eive either zero or three (+3P) kg dry matter (DM) of molassed sugar-beet p ulp (MSBP) per head per day as follows: (1) HN (no. = 44), (2) HN + 3P, (no . = 43), (3) IN (no. = 44), (4) HN + 3P (no. = 44). Blood samples were coll ected to measure systemic concentrations of ammonia, urea, insulin, glucose and progesterone. Heifers were artificially inseminated (AI) and pregnancy diagnosis was carried out by ultrasonography 30 days after AI. Subgroups o f pregnant animals across treatments were slaughtered 40 days after AT to e stimate conceptus development. The HN pasture had a higher CP (P < 0(.)001) and lower water-soluble carboh ydrate (P < 0(.)01) concentration. Plasma concentrations of ammonia (P < 0( .)05) and urea (P < 0(.)001) were higher in the animals on the HN pastures and were reduced (P < 0(.)05) by MSBP supplementation, but only in animals on the HN pastures. Embryo survival rate across treatments was high overall (71 %) and not related to pasture CP concentration, fermentable energy sup plementation or systemic concentrations of ammonia, urea, glucose or insuli n. There was no relationship between dietary treatment or systemic metaboli tes and any of the estimates of conceptus development. Systemic insulin was not affected by pasture N treatment or MSBP supplementation (P > 0(.)05). Systemic concentrations of glucose were not affected by pasture N treatment (P > 0(.)05) but were increased by MSBP supplementation (P > 0(.)05). Syst emic progesterone was not affected by pasture CP or MSBP supplementation (P > 0(.)05) but at day 7 after AT was positively related (P < 0(.)05) to emb ryo survival. Intake of high CP herbage elevated systemic ammonia and urea but there was no association with embryo survival rate or embryo developmen t in heifers.