PSXI-12 Effects of pen dimension and group size on growth, physiological responses, and puberty attainment of beef heifers reared in drylots at a high stocking density.

Abstract This experiment evaluated growth, physiological responses, and puberty attainment in beef heifers reared in drylots with a high stocking density, but with different drylot dimension × group size arrangements. A total of 120 heifers (75% Angus ´ 25% Brahman) were used in this experiment. Heifers were ranked by age (267 ± 2 d) and body weight (BW on d -5; 228 ± 2 kg) on d 0, and assigned to a) 1 of 3 drylot pens (10 × 14 m pens; 10 heifers/pen) with a stocking density of 14 m2/heifer (HDENS), b) 1 of 3 drylot pens (10 × 28 m pens; 20 heifers/pen) with a stocking density of 14 m2/heifer (HDENS-XL), or c) 1 of 3 pastures (1-ha pastures; 10 heifers/pasture) with a stocking density of 1,000 m2/heifer (CON) as positive control. Negligible forage was available for CON, whereas heifers from all treatments received the same limit-fed diet (~7 kg/heifer daily, dry matter basis). Shrunk BW was recorded after 16 h of feed and water withdrawal on d -5 and 171 to calculate average daily gain (ADG). Blood samples were collected weekly for progesterone analysis (d 0 to 170). Hair samples from the tail switch were collected every 28 d (d 0 to 170). Data were analyzed with pen or pasture as experimental unit, via ANOVA using the MIXED or GLIMMIX procedures of SAS (SAS Inst. Inc., Cary, NC). No treatment effects were detected (P = 0.35) for heifer ADG during the experiment (0.582, 0.539, and 0.567 kg/d for HDENS, HDENS-XL, and CON, respectively; SEM = 0.020). Hair cortisol concentrations were less (P < 0.05) for CON compared to HDENS and HDENS-XL heifers (12.6, 12.5, and 10.2 pg/mg of hair, respectively; SEM = 0.4) on d 170 (treatment × day interaction; P = 0.05). Heifers from CON experienced hastened puberty attainment as a greater (P < 0.05) proportion of CON heifers were pubertal beginning on d 120 compared with HDENS and HDENS-XL (treatment × day interaction, P = 0.02). Puberty attainment was similar between HDENS and HDENS-XL heifers (P ≥ 0.85). Collectively, results from this experiment demonstrate that heifers reared in drylots at a stocking density of 14 m2/heifer, even when a greater group size and total drylot space is used, still experience delayed puberty attainment compared with heifers reared on pasture at a low stocking density. This work is supported by the USDA National Institute of Food and Agriculture (#2021-67015-34083).