Blood serum concentration of IGF-I was analyzed to determine its relationship with individual postweaning feed efficiency (gain/feed) of 36 crossbred steer calves fed at three levels of feed intake (n = 12 at each level). Diets consisted of a corn silage-based growing diet for 84 d followed by a 91% concentrate finishing diet for 56 d. Dietary intake levels were at 80, 90, or 100% of ad libitum. Diets were formulated to ensure equal daily intake of protein, vitamins, and minerals across intake treatment levels. Intake was measured daily; ADG, DMI, and feed efficiency were calculated at 28-d intervals, through d 140. Individual weights and serum samples were collected at the beginning of the study and at 28-d intervals thereafter. The IGF-I concentrations were determined with a RIA. Data were analyzed as a multivariate split-plot in time. Imposed dietary intake restrictions did not affect serum IGF-I concentration (P = .90) or individual feed efficiency (P = .36), even though the least squares means for IGF-I concentration tended to decrease and the feed efficiency means tended to increase under the restricted intake levels. Serum IGF-I concentration, ADG, and feed efficiency were affected (P < .001) by collection date. Residual correlations between IGF-I concentrations at adjacent 28-d sampling times averaged .72. Diet intake level x sampling time interactions existed for ADG (P = .02) and feed efficiency (P < .001). Positive residual correlations of .28 (P < .001) and .16 (P = .07) existed between IGF-I and ADG and between IGF-I and feed efficiency, respectively. Regression analysis indicated that a 1 ng/mL increase in serum IGF-I concentration was associated with a .00135 kg/d increase in ADG (P < .001) and a .0001 kg gain/kg feed increase in feed efficiency (P = .04). These results support the hypothesis that serum IGF-I plays a role in growth and in efficiency of feed utilization in beef cattle.
Four selection strategies aimed at maximizing egg production in laying hens were compared with respect to expected annual genetic gain (GA). The selection strategies were: 1) (S1P) Traditional single-stage selection based on a single-house production system using partial records for both the individual and its ancestors, 2) Single-stage selection based on a single-house production system using full records for both the individual and its ancestors (S1F), 3) Single-stage selection based on a two-house production system using partial records for the individual and full records for its ancestors (S2P), and 4) Multistage selection based on a two-house production system using partial records for the individual and all available ancestral records (M2P). Strategy M2P resulted in the shortest generation interval (0.538 yr) and was the most efficient (deltaGA4 = 3.620 eggs per year), whereas strategy S1F generated the longest generation interval (2 yr) and was the least efficient (deltaGA2 = 1.334 eggs per year). Strategies S1P and S2P resulted in generation intervals of 1 yr, and were intermediate in efficiency (deltaGA1 = 2.232 eggs per year, deltaGA3 = 2.593 eggs per year). It was concluded that a two-house production system utilizing multistage selection was the most effective selection methodology. Further, selection based on M2P is expected to improve persistency of lay, whereas selection on S1P will not.
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