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GENETICS |
Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691
2 Corresponding author: velleman.1{at}osu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: turkey • body weight • egg production • reproduction • genetic parameter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Short-term selection studies suggested that the realized h2 of BW was also high. Abplanalp et al. (1963) found that the realized h2 of 8- and 24-wk BW was 0.43 and 0.62, respectively. McCartney et al. (1968) observed that the realized h2 of 8- and 24-wk BW was 0.44 and 0.39, respectively. Based on 4 generations of selection, Mukherjee and Friars (1970) reported that the realized h2 of 12-wk BW ranged from 0.37 to 0.57 in different base populations. Johnson and Gowe (1962) observed that the values of the parameters of the growth curve of the turkey could be changed by selecting for increased BW at different ages. In the only long-term selection study in turkeys, Nestor et al. (2000) found that the realized h2 of 16-wk BW declined with selection, being 0.31, 0.27, and 0.24, respectively, in generations 1 to 10, 11 to 20, and 21 to 30. The decline in realized h2 appeared to be slightly different for males than females.
Little or no association between egg production and BW was observed in earlier studies during the first few generations of selection for either increased egg production (Kosin and Becker, 1959; Shoffner and Leighton, 1962) or increased BW (Ogasawara et al., 1963; Mukherjee and Friars, 1970). Cook et al. (1962), Clayton (1971), and Arthur and Abplanalp (1975) estimated that the genetic correlation between BW and egg production was –0.1. In a long-term selection study for increased 16-wk BW, Nestor et al. (2000) found that, in general, egg production declined with genetic increases in BW, but the changes in egg production were not consistent across generations.
The purpose of the present study was to analyze direct and correlated responses to long-term selection for increased 16-wk BW of turkeys over 40 generations of selection and to estimate changes in genetic parameters by analyzing the changes occurring at 10-generation intervals. Results in early generations of selection have been summarized previously (Nestor, 1977c, 1984; Nestor et al., 1996c, 2000).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Management changes have occurred during the long-term selection study. The RBC2 and F lines were maintained in flocks hatched in April and May during generations 1 through 11 and 15 through 27. Offspring were grown in confinement with sexes combined until 8 wk of age and then range-reared until 20 (generations 9 through 27) or 24 wk of age (generations 1 through 8). In generations 12 through 14, offspring were hatched in May and June and grown entirely in confinement. To estimate the effect of the difference in environment, during generation 28, one-half of the offspring from the first hatch was reared entirely in confinement, whereas the other half was reared in confinement until 8 wk of age and then range-reared until 20 wk of age. Based on data from the first hatch in generation 28, Noble et al. (1996) reported that there was no interaction between the RBC2 and F lines and environment on growth traits, suggesting that the environmental change did not affect the selection study. Offspring from other hatches in generation 28 and in all hatches in generations 29 and later were reared in confinement, sexes separate, from hatching through 20 wk of age.
Offspring from the RBC2 and F lines were fed a declining protein, 6-ration system (Naber and Touchburn, 1970) during the growing period based on the schedule for males. Some minor improvements were made in the rations during the course of the selection study. However, in all generations, both lines were fed the same rations. In all generations, the rations met or exceeded National Research Council standards.
Selected males of the RBC2 and F lines were housed in a pole shelter at 20 or 24 wk of age in generations 1 through 27 and in a windowless house at 20 wk of age in generations 28 through 40. Beginning at stimulatory lighting (14 h/d), the breeder males were fed a ration containing 15.3% CP, 0.93% calcium, 0.62% phosphorus, and 2,963 kcal/kg of ME.
Selected females of the RBC2 and F lines were housed in a windowless breeder house and exposed to simulated declining daylight conditions until 8 wk before stimulatory lighting. At this time, light was restricted to 6 h per day. The hens were given stimulatory lighting of 14 h light per day at an intensity of 51 lx when they were approximately 39 wk of age. The hens were fed a ration containing 15.3% CP, 2.25% calcium, 0.64% available phosphorus, and 2,751 kcal/kg of ME beginning 1 wk before stimulatory lighting. Egg production records were obtained by trapnesting the hens 4 or more times daily. Broody hens were treated to decrease broodiness by first identifying the hens and then changing the environment (Nestor and Renner, 1966).
Traits Measured
Body weights were recorded at 8, 16, and 20 (generations 9 through 40) or 24 wk of age (generations 1 through 8) and when the hens first achieved approximately 50% production (females only). Mortality was recorded to 8 wk of age. At 16 wk of age in some generations, shank width and length (Nestor et al., 1985) were measured and the birds were subjectively rated for walking ability. Each bird was given a rating of 1 to 5, where 1 = the bird had legs without any lateral deviation and had no difficulty walking and 5 = birds whose legs exhibited extreme lateral deviations or had extreme difficulty walking, or both. Ratings of 2, 3, and 4 represented intermediate values between these extremes.
Egg production, except in 1 generation, was recorded for a 180-d production period beginning when the first egg was laid. In generation 15, the production period was shortened to 120 d as part of a successful program of eradication of a Mycoplasma meleagridis infection from both lines (Saif and Nestor, 1983). Egg records were analyzed to obtain days required from stimulatory lighting to production of the first egg and measurements of intensity of lay (rate of lay and maximum and average clutch length) and broodiness (total days lost from periods of nonproduction of 5 or more consecutive days including those at the end of the production period) according to the methods presented by Nestor (1972). Rate of lay was obtained by the following formula: number of eggs laid/(180 d - total days broody).
Data Analysis
The average increase in inbreeding coefficient per generation was estimated from one-half the reciprocal of the effective population size (Falconer, 1964). The effective population size was based on variation in family size.
Changes in the selected and correlated traits over generations were estimated by the linear and quadratic regression coefficients of line means on generations with the values of the F line expressed as deviation from the RBC2 line. The significance of the regression coefficients was evaluated by t-tests. Whenever possible, the selection period was divided into intervals of 10 generations to evaluate possible changes in genetic parameters with selection. A separate analysis was also completed for the entire 40 generations. In addition, to evaluate the effect of selection over the entire period, a 1-way ANOVA of traits was used to estimate the effect of line in the 10th, 20th, 30th, and 40th generations of selection in the F line.
Intended and actual selection differentials of 16-wk BW in the F line were obtained. The intended selection differential was defined as the average of the selected individuals minus the population mean, and the actual selection differential was the intended selection differential weighted for the number of offspring produced that contributed to the next generation. Realized h2 were estimated by the following: 1) the linear regression of selection responses in the F line, corrected for environmental fluctuations by expressing the values as deviations from the RBC2 line, on accumulated actual selection differentials; and 2) dividing the total corrected selection response by the total accumulated actual selection differential. For the estimates based on regression, the standard errors of the regression coefficient served as an approximation of the standard errors of the estimate.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Changes in the RBC2 Line
The RBC2 line exhibited positive linear trends in 8-wk BW of males and females (P 
0.01) and in average clutch length (P 0.05) during generations 31 through 40 (Table 1
). Changes in BW at 16 and 20 wk of age, mortality to 8 wk of age, egg production, days from beginning of stimulatory lighting to production of the first egg, maximum and average clutch length, rate of lay, and egg weight did not change significantly in the RBC2 line from generations 31 through 40. Over the 40-generation period, male and female BW at 8 and 20 wk of age had a significant negative linear trend (P 0.01), but there was a significant (P 0.05) positive quadratic regression coefficient in all cases. Rate of lay increased by 0.1% per generation (P 0.001), and egg weight decreased by 0.12 g per generation in the RBC2 line over the entire period of the study. The quadratic regression coefficient was significantly positive for maximum clutch length over the 40 generations. No significant linear or quadratic changes in the RBC2 line were observed for 16-wk BW of males and females, mortality to 8 wk of age, egg production, days from stimulatory lighting to first egg, and average clutch length based on data from 40 generations.
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The total changes in growth and reproduction traits observed in the F line are given in Tables 2 and 3, respectively. During the first 30 generations of selection in the F line, there were large gains in BW at 8, 16, and 20 wk of age in males and females, adult BW in females, and shank length and depth at 16 wk of age in both sexes (Table 2). Associated with the large increases in BW and associated leg traits was a decrease in walking ability in both males and females as indicated by increases in walking ability score. Mortality to 8 wk of age did not change significantly due to selecting for increased 16-wk BW in the F line during the first 30 generations of selection. The F line exhibited substantial total gains in BW at all ages and shank length and shank width at 16 wk of age for males and females from 31 to 40 generations of selection. Changes in walking ability scores (positive in males and negative in females) and mortality to 8 wk of age did not change greatly from generations 31 to 40. Based on gains per generation, the gains in BW at 8 and 16 wk of age and shank width at 16 wk of age were similar in generations 1 to 30 and 31 to 40. Gains in BW at older ages (20 wk of age in males and females and adult weight of females) and shank length at 16 wk of age were decreased in the later generations.
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). During generations 31 to 40, egg production increased by a total of 5.1 eggs per hen in the F line. This increase was probably not significant, because a difference of 10 eggs or more per hen is usually required for significance with the number of birds used and the variation in egg numbers. The loss in egg production of the F line during generations 1 through 30 was associated with an increase in days required from stimulatory lighting to production of first egg, and intensity of lay as measured by average and maximum clutch length and rate of lay. Further reductions of maximum and average clutch lengths apparently occurred in the F line during generations 31 through 40, but the changes in rate of lay during this period were positive. Egg weight increased in the F line during generations 1 through 30, but the increase was minor during generations 31 through 40.
Based on the deviation from the RBC2 line, the linear regression coefficients in the F line of BW at 8 and 16 wk of age in males and females on generations of selection were positive and significant for generations 1 to 10, 11 to 20, 21 to 30, and 31 to 40 (Table 4). Likewise, the regression coefficients in the F line of BW at 20 wk of age in males and females on generations were positive and significant for generations 11 to 20, 21 to 30, and 31 to 40. For adult BW in females in the F line, the regression coefficients were positive and significant for generations 1 to 10, 11 to 20, and 21 to 30, but there was no significant linear change for generations 31 to 40 even though a relatively large change (0.67 kg) in adult BW (Table 2) was observed. Over the entire 40-generation period in the F line, there were significant positive linear regression coefficients for BW at 8, 16, and 20 wk of age in males and females and adult BW in females. A significant positive quadratic regression coefficient was observed for BW in the F line at 16 and 20 wk of age, suggesting that the rate of gain was not as great in later generations of selection.
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). Male shank width of the F line had a significant positive quadratic regression coefficient during this period. A negative quadratic regression coefficient was observed for walking ability scores of females of the F line. Mortality to 8 wk of age did not exhibit any significant change in the F line during generations 1 to 10, 11 to 20, 21 to 30, 31 to 40, or 1 to 40.
Significant negative linear regression coefficients were observed for egg production on generations of selection in the F line when expressed as a deviation from the RBC2 line for generations 1 to 10, 21 to 30, and generations 1 to 40 (Table 5). For generations 1 to 40, there was a significant positive quadratic regression coefficient in the change in egg production of the F line over generations. For days from stimulatory lighting to production of the first egg in the F line, a significant positive linear regression coefficient was observed only for generations 1 to 10 and 1 to 40, and a significant quadratic regression coefficient was observed in generations 1 to 40. The linear regression coefficients of measures of intensity of lay (maximum and average clutch length and rate of lay) of the F line on generations of selection were consistently negative, but not always significant, for the various generation intervals. There was a significant positive quadratic regression coefficient for average clutch length and rate of lay of the F line in generations 1 to 40. Total days lost from broodiness in the F line did not exhibit any significant change in any generation interval. Most of the increase in egg weight in the F line occurred during generations 1 to 10 as indicated by a significant positive linear regression coefficient of egg weight on generations of selection during this period, with no significant change occurring in generations 11 to 20, 21 to 30, and 31 to 40. Over the entire period, the linear regression coefficient of egg weight on generations of selection was positive and significant, but there was a significant positive quadratic component in the change.
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The number of birds available at 16 wk of age and percentage of offspring selected to reproduce the F line are shown in Table 6. For both sexes combined, the percentage selected was 24.4 for generations 1 to 40. Selection pressure, as measured by the percentage selected, was greatest in generations 31 to 40, lowest in generations 11 to 20, and similar in generations 1 to 10 and 21 to 30. The number of females used to reproduce the F line was increased in generation 22 from 36 to 72.
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). Realized h2 of 16-wk BW in the F line were greater in males than in females and declined with selection, as estimated by the linear regression of response on the accumulated selection differential. For both sexes combined, the decline in realized h2 averaged about 4.8 percentage points for each 10 generations of selection, and the decline was largest (7.6 percentage points) in generations 31 to 40 from generations 21 to 30. The average decline in h2 estimates for each 10-generation interval was 4.5 and 5.0 percentage points in males and females, respectively. The realized h2 estimates based on estimates at certain points in time (last generation of each interval of selection) were, in general, greater than those based on regression for generations 1 to 10, 11 to 20, 21 to 30, and 31 to 40, but estimates by both methods were very similar for the entire selection period. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Significant trends over generations were observed for several traits of the RBC2 line for various periods of measurement. The direction of the changes was not always consistent over the course of the selection study. Studies by Nestor (1977b) and Noble et al. (1995) using the RBC2 line and other RBC populations suggested that the changes in the control lines were due to environmental changes.
Long-term selection studies for increased BW have been conducted at Virginia Tech using chickens (Dunnington and Siegel, 1996), at the University of Georgia (Anthony et al., 1996; Marks, 1996), and at The Ohio State University (Nestor et al., 1996a,b) using Japanese quail and at The Ohio State University (Nestor et al., 1996c, 2000) using turkeys. Direct responses to selection have been measured in all of the selection studies. In general, realized h2 estimates declined with selection in chickens (Liu et al., 1994), in Japanese quail at the University of Georgia (Marks, 1996) and The Ohio State University (Nestor et al., 1996a), and in turkeys (Nestor et al., 2000; present study). In the present study, the realized h2 of 16-wk BW based on the linear regression of response on accumulated actual selection differentials was 0.202, 0.130, and 0.166, respectively, for males, females, and sexes combined during generations 31 to 40, which indicated that a plateau in response is not likely in the near future of this line. Estimates of realized h2 of 16-wk BW in the F line over the entire 40 generations of selection were very similar when based on linear regression or response divided by total accumulated actual selection differentials at the 40th generation. When the selection period was divided into 10-generation intervals, h2 estimates based on a single point measurement of response divided by the accumulated actual selection differentials in the final generation were generally greater than those obtained by regression of response on accumulated actual selection differentials (Nestor et al., 2000; present study).
During reproduction, natural selection was apparently not opposing artificial selection, because the intended selection differential (deviation of selected birds from the population mean) and actual selection differential (intended selection differential weighted by the number of offspring produced) did not differ (Nestor et al., 2000; present study). This result was unexpected, because genetic increases in BW in the F line were associated with a decrease in egg production (Nestor et al., 2000; present study) and with a decrease in hatch of fertile eggs (Nestor et al., 2000). The F line was reproduced early in the reproduction period when egg production, fertility, and hatch of fertile eggs were at a maximum. Likely, if the reproduction period was extended over a longer period of time, the intended and actual selection differentials would be different.
Measurements of correlated responses were made routinely for some traits and only periodically for other traits in the chicken selection study at Virginia Tech and in the Japanese quail selection study at the University of Georgia. In the Japanese quail selection study at The Ohio State University (Nestor et al., 1996b) and in the turkey study at The Ohio State University (Nestor et al., 2000; present study), many correlated traits were measured most generations so that changes with selection in the correlated traits could be measured. The results of the study of Nestor et al. (2000) and the present study indicated that the genetic relationship, particularly in magnitude, between 16-wk BW and many correlated traits changed with selection. Based on the linear regression coefficients on generations of selection, the changes in number of eggs laid were negative and significant for only generations 1 to 10 and 21 to 30. For intensity of lay traits (maximum and average clutch lengths and rate of lay), the changes were larger in generations 1 to 10 than in other generation intervals. Of the intensity of lay measures, only average clutch length changed significantly (–0.030, P 0.01) during generations 31 to 40. Days from stimulatory lighting to production of the first egg and egg weight increased in the F line only during generations 1 to 10. The results of the present study and those of Nestor et al. (2000) suggested that the magnitude of genetic correlations, as well as the h2, may change with selection.
Nestor (1971) postulated that the change in the genetic relationship between egg production and BW during the growing period in turkeys selected for increased egg production was due to the relationship between intensity of lay, egg production, and BW. In early generations of a turkey line selected for increased egg production, large increases in egg production were not associated with decreases in BW. During this time, the increases in egg production were primarily associated with decreases in broodiness. In later generations of the egg line, when increases in intensity of lay had a greater influence on the increases in egg production of the egg line than broodiness, BW declined. The decline in egg production observed in the F line in the present study and that of Nestor et al. (2000) was primarily due to reduction in intensity of lay, and there was no significant change in broodiness. The results of Nestor et al. (2000) and the present study support the hypothesis of Nestor (1971) that the negative association of BW and egg production is mediated through intensity of lay.
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FOOTNOTES |
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Received for publication April 1, 2008. Accepted for publication June 1, 2008.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Anthony, N. B., K. E. Nestor, and H. L. Marks. 1996. Short-term selection for four-week body weight in Japanese quail. Poult. Sci. 75:1192–1197.[Web of Science][Medline]
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Havenstein, G. B., K. E. Nestor, V. D. Toelle, and W. L. Bacon. 1988. Estimates of genetic parameters in turkeys. 1. Body weight and skeletal characteristics. Poult. Sci. 67:1378–1387.[Web of Science][Medline]
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