Impacts of stocking density on growth and puberty attainment of replacement beef heifers

Abstract Text: Sixty Angus x Hereford heifers were ranked by age and BW (210 ± 2 d and 220 ± 2 kg, respectively) on d 0, and assigned to: A) 1 of 3 drylot pens (10 × 14 m pens; 10 heifers/pen) resulting in a stocking density of 14 m²/heifer (HIDENS), or B) 1 of 3 pastures (1-ha pastures; 10 heifers/pasture), resulting in a stocking density of 1,000 m²/heifer (LOWDENS). Prior to the beginning of the experiment, pastures were harvested for hay leaving no forage available for grazing for LOWDENS heifers. All heifers received the same diet consisting of (as-fed basis) 5 kg alfalfa hay and 3.5 kg of corn per heifer/d. On d 0, heifers were fitted with a pedometer (HJ-321; Omron Healthcare, Inc., Bannockburn, IL). Each week for the duration of the experiment (d 0 to d 161), pedometer results were recorded, heifer BW was measured, and blood samples were collected. Heifer shrunk BW (after 16 h of water and feed deprivation) was also collected on d -5 and 162 of the experiment. Puberty onset was determined according to plasma progesterone concentration. Heifers were considered pubertal when plasma progesterone concentration was greater than 1.0 ng/mL for 2 consecutive weeks. A treatment x day interaction was detected (P < 0.01) for BW, considering HIDENS heifers were on average 10 ± 4 kg heavier than LOWDENS heifers beginning on day 28. This difference in body weight can be attributed to increased physical activity of LOWDENS heifers, as they exhibited more (P < 0.01) steps compared to HIDENS heifers. However, ADG using shrunk BW values did not differ (P = 0.49) among treatments. Treatment x day interactions were detected for plasma cortisol (P < 0.01) and IGF-1 (P < 0.01), given that concentration of these hormones were greater for LOWDENS compared to HIDENS heifers on d 84 (P < 0.01) and d 140 (P ≤ 0.04). A treatment x day interaction was also detected (P < 0.01) for puberty attainment, considering a greater proportion of LOWDENS heifers reached puberty compared with HIDENS cohorts during the experiment (54.6 vs. 3.4% of heifers pubertal by d 161, respectively;  P <0.01, SEM = 5.3). In conclusion, heifers reared in a low-stocking density exhibited hastened puberty attainment, despite the observed decrease in heifer BW attributed to increased physical activity, compared with heifers reared in high stocking density.

Keywords: Beef heifers, growth, puberty, stocking density