Effects of drylot dimension and group size on growth, physiological, and puberty responses of replacement beef heifers.

This experiment evaluated growth, physiological responses, and puberty attainment in beef heifers reared in different stocking densities. Heifers (n = 240; 75% Angus × 25% Brahman) were ranked by age (270 ± 1 d), body weight (BW on d -3; 232 ± 2 kg) and temperament score on d 0, and assigned to (a) 1 of 6 drylot pens (10 × 14 m pens; 10 heifers/pen) with a stocking density of 14 m2/heifer (HDENS), (b) 1 of 6 drylot pens (10 × 28 m pens; 20 heifers/pen) with a stocking density of 14 m2/heifer (HDENS-XL), or (c) 1 of 6 pastures (1.38-ha pastures; 10 heifers/pasture) with a stocking density of 1,380 m2/heifer (CON). Shrunk BW was recorded (16 h of feed and water withdrawal) on d -3 and 171 to calculate BW gain. Heifers were fitted with an ear tag on d 0 to record behavioral responses. Blood samples were collected weekly for plasma progesterone analysis. Whole blood samples were collected on d 0, 58, 114, and 170 for mRNA isolation. Hair samples from the tail switch were collected on d 0, 30, 58, 85, 114, 142, and 170. Data were analyzed with pen or pasture as experimental unit. No treatment effects were detected (P = 0.67) for BW gain (∼0.635 kg/d). Time spent eating was greater (P < 0.01) in CON compared with HDENS and HDENS-XL. Time spent non-active, active and highly active were less (P < 0.01) in CON compared with HDENS and HDENS-XL. Time spent active was also less (P = 0.04) in HDENS compared with HDENS-XL. Hair cortisol concentrations were greater (P ≤ 0.05) for HDENS and HDENS-XL compared with CON on d 30 and 170, greater (P ≤ 0.04) for HDENS compared with CON and HDENS-XL on d 58, and greater (P = 0.03) for HDENS compared with CON on d 142. Expression of heat shock protein (HSP)-70 mRNA was greater (P < 0.01) for HDENS compared with HDENS-XL on d 114, greater (P ≤ 0.05) for HDENS compared with HDENS-XL and CON on d 170, and also greater (P = 0.03) for HDENS-XL compared with CON on d 170. Expression of HSP-72 mRNA was greater (P < 0.01) for HDENS compared with HDENS-XL and CON on d 170. Puberty rate was greater (P ≤ 0.05) in CON compared with HDENS in weeks 8, 9, 17 and 23, greater (P ≤ 0.05) for CON compared with HDENS and HDENS-XL from weeks 10 to 12, and greater (P ≤ 0.05) in CON and HDENS-XL compared with HDENS from weeks 13 to 16 and weeks 18 to 22. Collectively, increasing drylot dimension and group size attenuated physiological stress responses and improved reproductive development of beef heifers reared at a high stocking density.