375 The effect of supplemental protein from weaning until first breeding on heifer development and uterine environment characteristics

Replacement heifer development is one of the most critical components of productive beef operations. By identifying the ideal uterine environment in pre-breeding heifers, fertility and reproductive efficiency can be maximized. Our hypothesis was that different levels of protein supplementation would affect the rate of development and uterine environment of beef heifers. To test the effects of dietary supplementation on these outcomes, a randomized complete block design with repeated measures was implemented. Commercial Angus heifers (n = 60) were blocked by BW into 4 weight classes. Within each weight class, three pens of 5 heifers each were randomly assigned to one of three treatment groups of supplemental protein [10% (control), 20% (P20), and 40% (P40)]. All heifers were allowed access to ad libitum grass hay. Bodyweight and BCS were collected every 2 weeks to monitor heifer development. Uterine fluid samples were collected monthly by flushing with sterile saline to evaluate components of the uterine environment. Relative uterine pH was calculated by subtracting saline pH from the uterine flush pH. Separate mixed model ANOVAs and mean separation via PROC GLIMMIX (SAS 9.4, Cary, NC) were used to determine if protein supplementation treatments, time and their interactions influenced BW, BCS, or uterine environment. Bodyweight of the control group was significantly lower (P < 0.05) than P20 or P40 groups from December to March. There was an interaction between treatment and time for relative uterine pH (P = 0.002). In November, the P40 group exhibited less change in pH compared to the control group (0.33±0.1 vs. 0.74±0.1; P < 0.05). Whereas in December the P40 (0.23±0.1) group showed more basic uterine environment compared to the P20 (-0.22±0.1) or control (-0.13±0.1) groups (P < 0.05). In conclusion, higher protein supplementation does improve heifer growth and development, as well as altering the pH of the uterine fluid from weaning to first breeding