Effects of Protein Supplementation with Low to Medium Quality Forage on Intake and Circulating Amino Acids in Beef Heifers in Early Pregnancy.

A 2-yr study was conducted to investigate the effects of protein supplementation with low to medium quality forage diets on yearling heifer performance and plasma AA during early pregnancy. Heifers received timed AI (TAI; d 0) and treatments were fed for 37 d (-7 to +30 from TAI). Angus, Hereford, and Angus-cross heifers (402 ± 42 kg initial BW; n=79) were stratified by initial BW and BCS and randomly assigned to 1 of 4 protein supplementation treatments formulated to provide similar CP intake; 1) no supplementation (CON); 2) low-fat dried corn distillers grain (LDG, 7.4% fat; 0.8% BW); and 3) high-fat dried corn distillers grains (HDG, 9.5% fat; 0.8% BW); and 4) cottonseed meal (CSM, 2.2% fat; 0.4% BW). The basal diet consisted of low to medium quality chopped grass hay (8% CP; 55% NDF in yr 1 and 70% NDF in yr 2) fed ad libitum twice daily. Supplements were fed once daily prior to hay feeding and individual intakes were recorded. Body weight and BCS was determined at start on (d -9 and -8) and at the end (d 29 and 30). Blood samples were collected 4 h after supplementation d 0, 5, 9, 14, 21, 26, and 30 to determine plasma urea nitrogen (PUN) and AA. Initial BW, initial BCS, and change in BCS did not differ (P ≥ 0.80) among treatments; however, overall DMI, and ADG were greater (P ≤ 0.0001) for supplemented heifers than CON heifers. Total CP intake was greater (P ≤ 0.0001) for HDG and LDG compared to CON and CSM was intermediate. Plasma VAL, LEU, TYR, ILE, TRP, TAUR, total ketogenic AA, and total AA were greater (P ≤ 0.05) while GLY was less (P ≤ 0.0001) with protein supplementation versus CON. Concentrations of ALA and LYS were less (P ≤ 0.0001) from HDG and LDG fed heifers compared to CON and CSM; however, LDG had greater (P = 0.01) LYS concentrations than HDG. Heifers fed CSM had greater (P ≤ 0.05) ARG and ASP. Concentrations of THR were greater (P = 0.0004) for LDG and CSM. Protein supplementation increased (P ≤ 0.0001) PUN in supplemented heifers compared to CON. Protein supplementation during early pregnancy in beef heifers resulted in greater protein intake and increased circulating PUN and total AA.