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Twenty-seven crosses derived from three random-mating, unselected strains of mice (SWO, J, PGH) with 16 matings/pureline or cross were cohabited continuously and monogamously for 365 d to evaluate long-term reproductive performance of the parents. Data from 3,249 litters born were analyzed to study the effects of parity (number of litters) and type of cross (pureline, two-way, backcross, three-way) on the following traits: 1) continuity of reproduction (COR), 2) litter size at birth (LSB), 3) age at first litter (AFL), 4) fraction alive at birth-the ratio of number born alive to LSB (FAB), 5) birth to birth interval (BBI) and 6) reproductive rate-the ratio of number born alive to birth to birth interval (RRT). Reproductive rate was also studied in a cumulative manner rather than on a per parity basis by regressing cumulative number born alive on cumulative birth to birth interval. Parity effects were important (P less than .01) for all traits measured. Differences among cross-types were highly significant for COR, LSB and RRT. Three-way crosses were more productive than backcrosses, and both were superior to two-way and pureline matings. Uniformity was greater among three-way crosses for all traits except COR. Cross-types differed importantly in the pattern of change in performance over parities for COR, LSB, FAB, RRT and cumulative number of live young (P less than .01). Number of parities were most important for variation in longevity of reproduction (COR) and somewhat for litter size (LSB). Mean and cumulative reproductive rates were influenced more by longevity and litter size than by interval between births.

Twenty-seven crosses of mice derived from three random-mating, unselected strains (SWO, J. PGH) with 16 matings/pureline or cross were cohabited continuously and monogamously for a 365-d reproduction period to evaluate lifetime reproductive performance of the parents. Data from 3,249 litters born (2,792 surviving to weaning) were analyzed to study the effects of parity (number of litters) and type of cross (pureline, two-way, backcross, three-way) on certain traits measured at weaning (21 d). The traits evaluated were 1) weaning to weaning interval (WWI), 2) litter size at weaning (LSW), 3) fraction alive at weaning (FAW)-the ratio of LSW to number born alive, 4) total litter weight at weaning (LWT), 5) pup weight at weaning (PWT), 6) weaning weight per pup observed alive at birth (WPP), 7) parental feed consumption per day (FPD) and 8) parental costs (PCT). Cost per live weanling animal was estimated as the ratio of cumulative parental costs to cumulative number weaned. Cost per unit of weaning weight was defined as the regression of cumulative parental costs on cumulative litter weight. Parity effects were important (P less than .01) for all traits measured. Type of cross effects were noted for LSW, LWT, PWT, WPP, FPD (P less than .01) and PCT (P less than .05). A parity X cross-type interaction was expressed for WWI, LSW, PWT, WPP, FPD (P less than .01) and FAW (P less than .05). Three-way crosses were more productive than backcrosses, two-way crosses and pureline matings for all traits studied. Three-way crosses displayed the smallest cost per animal weaned and smallest cost per unit of weaning weight. The influence of parity on traits measured at weaning was less than for traits measured at birth. Weight per pup born alive (WPP) was found to be a better indicator of reproduction and lactation ability than LWT because WPP accounts for preweaning mortality.

INTRODUCTION THE value of using part of the annual egg production for the early selection of superior sire and dam lines in fowl has been widely accepted. An early report of Lerner and Cruden (1948) showed that because of the high genetic correlation between part year production and total annual production selection for the latter based on the former would not diminish genetic progress as compared to selection based on complete records. In fact, the decreased generation interval due to the use of part records could actually increase genetic progress. The object of the present analysis was to substantiate, if possible, and to amplify these results from observations taken in a random bred flock. The suitability of random bred data for the establishment of genetic parameters has been discussed elsewhere (King, Carson and Doolittle, 1959). MATERIALS The monthly trapnest records of the Cornell Random Bred Control flock of White Leghorns…

Spot size in descendants from the Goodale white-spotted stock of mice responded to selection for increased spot size. The realized heritability estimate was 0.52. However, no correlated response of reproduction to spot size selection was found in the present study, nor was there any correlated response among body weight variables.

SUMMARYMice were selected for high and low body weight at 5 and at 10 weeks of age. Selection was performed (1) separately for each trait, and (2) for various combinations of the two traits, using (a) independent culling levels and (b) restricted indices. Two-way selection for each trait separately gave large responses and correlated responses. Selection by independent culling levels intended to increase 5-week weight while restricting change in 10-week weight gave no demonstrable response; selection by culling levels intended to decrease 5-week weight while restricting change in 10-week weight resulted in decreases in body weights at both ages. Index selection, intended to change weight at one age while holding that at the other age constant, was generally successful. Observed responses did not conform very well with predicted responses for either index or culling levels selection. The significance of these observations in regard to the problem of selection involving restriction of traits is discussed.

The effects on body weight of mixing litters to attain constant density of mice per cage, as opposed to housing litters in separate cages, was studied. Mixing litters resulted in a decreased weight gain between 21 and 42 da of age and a decreased adult body weight at 63 da of age compared to housing litters in separate cages, whether the separately housed litters were allowed to vary in density or not. Mixed litter housing also increased the variance of the body weight measures among males. Housing litters separately, even if it entails variable density of housing, appears to be the preferred method for studies involving inheritance of body weight.

Body composition and skeletal muscle weights, fiber diameters, fiber numbers, and RNA and DNA concentrations were studied in mice selected for large body size (GLW) compared to an unselected control (C) line. At 60 days of age, body weight of GLW mice was 70 to 75% greater than C mice, but % body water, fat or dry fat-free residue did not differ between the lines. In both GLW and C mice, greater muscle weight in males compared to females was due to larger diameter fibers and not to significant differences in fiber number between sexes, although in most cases, males had slightly higher mean fiber number. Fiber diameters were similar in the two lines and the greater muscle weights of GLW mice were due to more fibers than in the muscles of C mice. RNA and DNA concentrations were slightly, but not significantly, higher in GLW skeletal muscles.

Mice from three outbred stocks (PGH, J, and SWO) were fed either pelleted feed from overhead hoppers or ground feed from jars, from weaning at 21 days old to slaughter at 42, 49 or 56 days old. Live weights, weights of carcasses before and after extraction of carcass moisture and fat, and feed consumption were recorded. The PGH stock gained more in dry lean tissue and deposited less fat, and had a greater feed efficiency, compared to either of the other stocks. Males gained more than females, in both dry lean tissue and fat; females had a higher proportion of lean tissue and a greater feed efficiency. Increases in live weight between 49 and 56 days of age were mainly increases in fat content, especially in the offal portion of the body. Jar feeding restricted feed intake and reduced live weight gains. The reduction, however, was principally in fat deposition; feed efficiency was also greater for jar fed mice. It was found that the amount of lean tissue could be accurately predicted from carcass weight and carcass moisture measurements. It may be possible to estimate lean tissue accurately without employing the expensive and time consuming process of either extraction.