Effects of stocking density on growth, physiological, and puberty responses of replacement beef heifers reared in drylots.

This experiment evaluated growth, physiological responses, and puberty attainment in heifers reared in drylots with different stocking densities. A total of 240 heifers (75% Angus × 25% Brahman) were used. Heifers were ranked by age (270 ± 1 d), body weight (BW on day -3; 232 ± 2 kg) and temperament score on day 0, and assigned to drylot pens with: 1) 14 m2/heifer (HDENS), 2) 28 m2/heifer (MDENS), 3) 42 m2/heifer (LDENS), or to pastures with 1380 m2/heifer (CON). Each pen or pasture (n = 6/treatment) housed 10 heifers. Negligible forage was available for CON, and all treatments received the same limit-fed diet (∼9 kg/heifer daily, dry matter basis). Shrunk BW was recorded on day -3 and 171 to calculate BW gain. Heifers were fitted with an ear tag on day 0 to record behavioral responses. Blood samples were collected weekly for plasma progesterone analysis. Whole blood samples were collected on day 0, 58, 114, and 170 for mRNA isolation. Hair samples from the tail switch were collected on day 0, 30, 58, 85, 114, 142, and 170. Data were analyzed with pen or pasture as experimental unit. No treatment effects were detected (P = 0.92) for BW gain (∼0.647 kg/d). Heifers from CON spent more time eating (P < 0.01) and less time in other activity categories (active, highly active, and non-active; P ≤ 0.05) compared with heifers reared in drylots. Hair cortisol concentrations were greater (P ≤ 0.02) for HDENS compared with CON and MDENS on day 30, greater (P ≤ 0.04) for HDENS compared with all other treatments on days 58 and 142, and less (P ≤ 0.05) for CON compared with all drylot treatments on day 170 (treatment × day; P = 0.08). Expression of heat shock protein (HSP)-70 mRNA was greater (P ≤ 0.02) for HDENS compared with all other treatments on day 170 (treatment × day; P = 0.04). Expression of HSP-72 mRNA was greater (P < 0.01) for LDENS compared with all other treatments on day 114, and greater (P < 0.01) in HDENS compared with all other treatments on day 170 (treatment × day; P < 0.01). A greater (P ≤ 0.05) proportion of CON were pubertal by week 22 of the experiment compared with all other treatments and compared with MDENS and HDENS on week 23 (treatment × day; P < 0.01). In summary, rearing heifers in drylots with high stocking density (14 m2/heifer) increased chronic stress and delayed puberty compared with pasture heifers, whereas reducing drylot stocking density (28 or 42 m2/heifer) attenuated stress responses but did not benefit heifer reproductive development.