Comparison of Spayed Heifers to Intact Heifers Fed Melengestrol Acetate on Feedlot Performance and Carcass Characteristics

The experiment evaluated the effects of spaying heifers at feedlot arrival using the Willis drop-spay technique on feedlot performance and carcass characteristics compared with intact heifers fed Melengestrol acetate (MGA). Crossbred heifers (n = 666; Initial BW = 316 ± 29 kg) were assigned to eight pens in a randomized complete block design with four replicates per treatment (approximately 85 per pen). Treatments consisted of (1) heifers spayed upon feedlot arrival or (2) remained intact and fed MGA for the duration of the 212-day feeding period. Heifers were blocked into two groups by feedyard arrival date, held off feed for 24 h before initial processing (as a requirement for the spay protocol), and were gate-sorted into treatment groups by alternating sets of three heifers. The spay procedure was administered at time of initial processing. Since intact heifers averaged 4 kg heavier than spayed heifers at initial processing, initial BW was included as a covariate when significant (P ≤ 0.05). When adjusted for initial BW, intact heifers had a 7.7 kg heavier final BW and 4.9 kg HCW advantage compared with spayed heifers (P = 0.05). Without the covariate, intact heifers tended to have heavier carcass-adjusted final BW and HCW (P > 0.08). Intact heifers fed MGA demonstrated a 3.0 % improvement in ADG (P = 0.02) and tended to have greater DMI (P = 0.09), resulting in a similar feed conversion efficiency between treatments (P = 0.82). Ribeye area and marbling score did not differ (P ≥ 0.13). However, intact heifers exhibited greater subcutaneous fat thickness (0.20 cm) relative to spayed heifers (P = 0.02) and higher USDA Yield Grade (P = 0.05), with intact heifers averaging 3.15 and spayed heifers averaging 2.94. These data indicate that spaying heifers at feedlot entry does not confer performance or carcass advantages compared with intact heifers receiving MGA. The increased fat deposition and higher numerical YG in intact heifers suggest earlier attainment of compositional maturity relative to spayed counterparts. Therefore, extending days on feed for spayed heifers may be warranted to achieve a similar fat endpoint and degree of finish as intact heifers fed MGA.