Graduate Thesis Or Dissertation
 

Impacts of Stocking Density on Development and Puberty Attainment of Replacement Beef Heifers

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  • The following experiment presented herein compared growth, physical activity, stress-related responses, and puberty attainment in heifers reared on high- (HIDENS) or low-stocking densities (LOWDENS). 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; n = 3), or b) 1 of 3 pastures (25-ha pastures; 10 heifers/pasture), resulting in a stocking density of 25,000 m²/heifer (LOWDENS; n = 3). Pastures utilized herein were harvested for hay prior to the beginning of this experiment, and negligible forage was available for grazing to LOWDENS heifers throughout the experimental period (d 0 to 182). Heifers received the same limit-fed diet during the experiment, which averaged (DM basis) 4.0 kg/heifer daily of alfalfa-grass hay and 3.0 kg/heifer daily of a corn-based concentrate. Heifer shrunk BW was recorded after 16 h of feed and water withdrawal on d -3 and d 183 for ADG calculation. On d 0, heifers were fitted with a pedometer fixed behind their right shoulder. Each week during the experiment, pedometer results were recorded and blood samples were collected for puberty evaluation via plasma progesterone. Plasma samples collected on d 0, 28, 56, 84, 112, 140, 161, and 182 were also analyzed for concentrations of cortisol and IGF-I. On d 28, 102, and 175, blood samples were also collected for RNA isolation and analysis of heat shock protein (HSP) 70 and HSP72 mRNA expression. On d 0, 49, 98, 147, and 182, hair samples were collected from the tail switch for analysis of hair cortisol concentrations. No treatment effects were detected (P = 0.66) for heifer BW or ADG. Heifers from LOWDENS had more (P < 0.01) steps/week compared with HIDENS. Heifers from LOWDENS had greater (P = 0.05) mRNA expression of HSP72, and tended (P = 0.10) to have greater mRNA expression of HSP70 compared with HIDENS. Plasma concentrations of cortisol and IGF-I were often greater (P ≤ 0.05) in LOWDENS vs. HIDENS heifers (treatment × day interaction; P < 0.01). Hair cortisol concentrations were greater (P < 0.01) for HIDENS vs. LOWDENS heifers beginning on d 98 (treatment × day interaction; P < 0.01). Heifers from HIDENS experienced delayed puberty attainment and had a lower (P < 0.01) proportion of pubertal heifers on d 182 compared with LOWDENS (treatment × day interaction; P < 0.01). In summary, HIDENS negatively impacted heifer stress-related and physiological responses, and delayed puberty attainment compared with LOWDENS.
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