PSIV-4 The effects of source and level of rumen undegradable protein on circulating lipid metabolites in beef heifers.

Abstract Increased non-esterified fatty acids (NEFA) concentrations in serum and follicular fluid can hinder embryonic development in cattle. Supplementing additional ruminal undegradable protein (RUP) increases insulin concentrations, therefore it is probable NEFA and cholesterol concentrations are reduced. Pregnancy rates were increased in grazing heifers supplemented animal sourced 50% RUP compared to heifers supplemented 36% animal sourced RUP. No differences in puberty attainment and ovarian parameters were observed in heifers offered a plant sourced 36% and 50% RUP. The effects of source and level of RUP on serum lipid metabolites and steroid hormones in heifers developed in a dry-lot remain unclear. The objective of this study was to evaluate the influence of different sources and levels of RUP on NEFA, beta-hydroxybutyrate (BHB), cholesterol, and progesterone (P4) concentrations in heifers developed in a dry-lot. Heifers (n = 20) were randomly assigned to one of four treatments:1) plant source 36% RUP (36P, n = 5), 2) plant source 50% RUP (50P, n = 5), 3) animal source 36% RUP (36A, n = 5), and 4) animal source 50% RUP (50A, n = 5). Heifers were individually supplemented using a Calan gate feeding system for 77 days. Blood samples were collected every two weeks during the supplementation period to measure circulating NEFA, cholesterol, BHB, and P4 concentrations. Serum NEFA, cholesterol, BHB, and P4 concentrations were analyzed as repeated measures using PROC MIXED of SAS (v9.4). Level, source, and level × source did not influence serum NEFA, cholesterol, and P4 concentrations (P ≥ 0.12). Heifers supplemented the 36 level tended to have greater BHB concentrations than heifers supplemented the 50 level (P = 0.09). Source and source × level did not affect BHB concentrations (P ≥ 0.72). Source × day tended to affect NEFA concentrations (P = 0.06). Heifers fed animal sourced protein tended to have elevated NEFA concentrations on d 36 compared to plant sourced heifers (P = 0.09), however by d 50, plant sourced heifers tended to have greater NEFA concentrations than animal sourced heifers (P = 0.09). Level × day tended to influence cholesterol concentrations (P = 0.09) with level 50 heifers tending to have increased cholesterol concentrations on d 50 compared to level 36 heifers (P = 0.09). By day 64, serum cholesterol concentrations were similar between levels (P = 0.15). Level × day impacted P4 concentrations (P = 0.02) with 50 level heifers tending to have greater P4 concentrations on d 64 (P = 0.07) compared to 36 level heifers however by d 77 P4 concentrations were similar between levels (P = 0.32). In conclusion, source × level of the RUP diet did not markedly influence serum metabolites and progesterone and therefore would not likely alter reproductive function in beef heifers.