22. Impact of Postweaning Stair-step Nutritional Regimen in Beef Heifers on Reproductive Development and Productivity as Primiparous Cows

Abstract Replacement beef heifers represent a substantial investment for cattle producers and require appropriate management to achieve puberty, conceive, and calve efficiently. This study evaluated the impact of post-weaning body weight (BW) gain on reproductive development and subsequent productivity in primiparous cows. Sixty Angus crossbred heifers were stratified by age and BW (237 ± 6 d; 226 ± 6 kg) at 28 d post-weaning (d -3) and randomly assigned to one of three supplementation strategies from d 0 to 168: 1) a forage-based diet designed to restrict BW gain (low control; LC), 2) concentrate supplementation to promote accelerated BW gain (high control; HC), or 3) a stair-step regimen consisting of HC for 2 months, followed by LC for 2 months, followed by HC for the final 2 months (SS). Heifers were maintained in a single pasture with ad libitum access to grass hay and individually fed supplement treatments six days weekly. Shrunk BW was recorded on d -3 and 169 to calculate average daily gain (ADG). Blood samples were collected weekly from d 0 to 168 to assess pubertal status via plasma progesterone. From d 169 to 222, heifers were subjected to a fixed-time artificial insemination protocol combined with natural service. As designed, overall ADG from d 0 to 168 was greater (P < 0.01) in HC vs. SS and LC (0.76, 0.68, and 0.48 kg/d, respectively; SEM = 0.02), and greater (P < 0.01) in SS vs. LC heifers. Puberty attainment was delayed (treatment × day; P < 0.01) in LC heifers compared to HC and SS. No treatment differences were detected (P = 0.69) for pregnancy rate. No treatment differences were detected (P ≥ 0.11) for heifer BW or body condition score at calving, or calf birth BW. Calving rate was greater (treatment × day; P < 0.01) in SS and LC heifers compared to HC. Heifers were assessed for milk production via weigh-suckle-weigh at 51.3 ± 1.4 d postpartum followed by milk sample collection and mammary artery hemodynamic assessment via Doppler ultrasonography 24 h later. No treatment differences were detected (P ≥ 0.50) for milk yield or composition. Average resistance index was greater (P = 0.03) in SS vs. LC, and average pulsatility index was greater (P ≤ 0.05) in SS vs. LC and HC, indicating reduced mammary blood flow in SS vs. LC. No treatment differences were detected (P ≥ 0.11) for offspring age or BW at weaning. Collectively, results from this experiment indicate that HC and SS hastened the reproductive development of replacement heifers, without negatively affecting their milk productivity as primiparous cows. Exposure to a stair-step nutritional regimen may be an opportunity to hasten puberty attainment in replacement beef heifers, while reducing feed input.