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Selection for Postweaning Gain in Rats: I. Correlated Response in Feed Utilization and Body Composition¹

University of Nebraska and U.S. Department of Agriculture, Lincoln 68583-0908

Abstract

Response to selection for up (U) and (D) 3- to 9-wk gain in rats on average daily gain (ADG), average daily feed intake (ADI), gain/feed (G/F), body composition (BC), fasting metabolic rate per unit metabolic size (MR) and partial efficiency of weight gain (ADG/Fg) was evaluated after 34 generations of mass selection. At 3-wk weaning, 120 litters representing F₁ crosses of two replicates within each of the U, D and control (C) selection lines were divided within sexes between bulk-feeder and tube-feeder cage types for recording feed intake until 9 wk of age. Rats from tube-feeder cages representing 16 litters/line were utilized for MR and BC data. Response in ADG was asymmetrical; 16% higher for U line but only 8% lower in D line, compared with C line. Correlated responses were positive and significant in both U and D lines for ADI (6% and –3%) and G/F (5% and –5%). Line differences in MR were not significant but both selected lines were slightly higher than C line in MR at 6 wk of age, and the reverse at 9 wk of age. Over the period of 6 to 9 wk of age, maintenance requirements per unit metabolic size and ADG/Fg were 1 and 5% above for the U and –1 and –4% for D lines, relative to C line. Females of both selected lines were fatter than C line (P<.05) at 9 wk of age, but only D line males were fatter than C line.

¹ Published as Paper No. 7878, Journal Ser., Nebraska Agr. Exp. Sta., Lincoln. Appreciation is expressed to A. B. Chapman and J. J. Rutledge, Univ. of Wisconsin, Madison, for providing the genetic stocks used in this study and to T. Greninger, now at Lincoln Univ., Jefferson City, MO for technical expertise in collecting calorimetric data.

² Present address; Facultad de Zootecnia, Univ. of Chihuahua, Chihuahua, Mexico.

³ Anim. Sci. Dept., Univ. of Nebraska, Lincoln, 68583-0908.

⁴ Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, 225 Baker Hall, Univ. of Nebraska, Lincoln, 68583-0908.

⁵ Agr. Eng. Dept., Univ. of Nebraska, Lincoln, 68583-0726.

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