K. E. Nestor 1,
J. W. Anderson,
and R. A. Patterson
The Ohio State University Department of Animal Sciences
1For more information, contact at: The Ohio State University,
Ohio Agricultural Research and Development Center, 125 Gerlaugh Hall, 1680 Madison Ave.,
Wooster, OH 44691; 330-263-3757; Fax: 330-263-3949; e-mail: nestor.1@osu.edu.
A line (F) of turkeys was selected over 30 generations for increased 16-week body weight. The base population for the F line was a randombred control population (RBC2) that was maintained along with the F line without conscious selection. In order to study the effects of selection on genetic parameters, the effect of selection on selected and correlated traits in the F line was studied, whenever possible, at 10-generation intervals (1 to 10, 11 to 20, and 21 to 30) and over all generations (1 to 30). Values of the F line were expressed as deviations from the RBC2 line in order to remove yearly environmental variations.
Selection was effective in increasing 16-week body weight in the F line. Selection differentials based on the mean of selected parents minus mean of entire population (intended) and intended selection differentials weighted for number of offspring produced (actual) did not consistently differ, indicating that natural selection was not opposing artificial selection. The realized heritability (h2; ratio of genetic variation to total variation in a trait) of 16-week body weight in the F line based on the linear regression of selection response on accumulated actual selection differentials declined with selection, and the decline appeared to be slightly different for males than females. For both sexes combined, the realized h2 was 0.309 ± 0.022 (standard error), 0.268 ± 0.033, 0.242 ± 0.026, and 0.254 ± 0.007, respectively, for Generations 1 to 10, 11 to 20, 21 to 30, and 1 to 30.
Genetic increases in 16-week body weight in the F line were positively associated with body weight at other ages (8, 20, and 24 weeks of age and at 50% egg production), days from stimulatory lighting to production of the first egg, and egg weight and negatively associated with egg production, intensity of lay (maximum and average clutch length and rate of lay), and hatch of fertile eggs. There was no significant relationship of 16-week body weight and total days lost from broodiness or fertility. Genetic changes in some correlated traits were not consistent in all generation intervals studied, indicating that the genetic correlation between the selected trait (16-week body weight) and the correlated trait changed with selection.
Heritability estimates (h2) for body weight of turkeys during the growing period are generally large. Based on earlier literature, Nestor et al . (1967) reported that the unweighted averages of published h2 estimates of body weight in selected populations were 0.40, 0.42, 0.43, and 0.36, respectively, for birds in the age groups 0 to 8, 9 to 16, 17 to 24, and greater than 24 weeks of age. Based on data from eight commercial flocks, Arthur and Abplanalp (1975) found that the h2 of 18-week BW was 0.42. Using randombred control populations, h2 estimates of body weight were high, ranging from 0.40 to 0.68 (McCartney, 1961; Nestor et al ., 1967; Havenstein et al ., 1988).
Results of earlier selection studies also suggested that h2 of body weight was high. Abplanalp et al . (1963) found that the realized h2 of 8- and 24-week body weight was 0.43 and 0.62, respectively. McCartney et al . (1968) observed that the realized h2 of 8- and 24-week body weight was 0.44 and 0.39, respectively. Based on four generations of selection, Mukherjee and Friars (1970) reported that the realized h2 of 12-week body weight ranged from 0.37 to 0.57 in different base populations. Johnson and Gowe (1962) observed that the growth curve of the turkey could be changed by selecting for increased body weight at different ages.
Little or no association between egg production and body weight was observed in earlier selection studies during the first few generations of selection for either increased egg production (Kosin and Becker, 1959; Shoffner and Leighton, 1962) or increased body weight (Ogasawara et al ., 1963; Mukherjee and Friars, 1970). Cook et al . (1962), Clayton (1971), and Arthur and Abplanalp (1975) estimate the genetic correlation between body weight and egg production was -0.1.
The purpose of the present study was to analyze direct and correlated responses to long-term selection for increased 16-week body weight of turkeys over 30 generations of selection and to estimate changes in genetic parameters. To do this, changes occurring at 10 generation intervals were analyzed.
The base population was a randombred control (RBC2) established in 1966 from pooled reciprocal crosses of two commercial large-bodied strains (Nestor et al ., 1969). The RBC2 line was maintained using a mating system (Nestor, 1977) utilizing 36 parental pairs. A subline (F) of the RBC2 line was initiated by mass selection for increased 16-week body weight. The F line was maintained with 36 parental pairs from Generations 1 through 21. In Generation 22 and later, the F line was reproduced with 36 males and 72 females (each male being mated with two females).
Offspring from the RBC2 and F lines were fed a declining protein six-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.
Body weights were recorded at 8, 16, and 20 (Generations 9 through 30) and 24 (Generations 1 through 8) weeks of age. At 16 weeks 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 from 1 to 5, with 1 indicating that the bird had legs without any lateral deviation and had no difficulty walking, and 5 representing birds whose legs exhibited extreme lateral deviation or had extreme difficulty walking, or both. Ratings of 2, 3, and 4 represented intermediate values between these extremes.
Selected males of the RBC2 and F lines were housed in a curtain-sided pole shelter at 20 or 24 weeks of age in Generations 1 through 27 and in a windowless house at 20 weeks of age in Generations 28 through 30. Beginning at stimulatory lighting (14 hours of light per day), the breeder males were fed a ration containing 15.3% crude protein, 0.93% calcium, 0.62% phosphorus, and 2,963 kcal/kg metabolizable energy.
Selected females were housed in a windowless breeder house and exposed to simulated declining daylight conditions until eight weeks prior to stimulatory lighting. At this time, light was restricted to six hours per day. The hens were given stimulatory lighting of 14 hours light per day at an intensity of 51 lx when they were approximately 39 weeks of age. The hens were fed a ration containing 17.6% crude protein, 2.25% calcium, 0.64% available phosphorus, and 2,751 kcal/kg metabolizable energy beginning one week prior to stimulatory lighting.
Egg production, in general, was recorded for a 180-day egg production period beginning with the first egg laid. The 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 five or more consecutive days) according to the methods presented by Nestor (1972).
Each hen was artificially inseminated twice on the week the first egg was laid and biweekly thereafter for the RBC2 line and for Generations 1 through 16 of the F line. After Generation 16, hens of the F line were inseminated weekly. Volume of semen inseminated per hen varied but was almost always greater than the minimum amount (0.025 cc) generally recommended for maximum fertility. Fertility and hatch of fertile eggs were recorded for a 12-week hatching period beginning when the hens first attained an egg production level of approximately 50%. Body weights of the females were also obtained at this time and used as a measure of adult body weight. Egg weight was obtained by group weighing all eggs laid by each hen every two weeks throughout the 12-week hatching period.
The average increase in inbreeding per generation was calculated 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 traits over generations were estimated by the linear and quadratic regression coefficients of line means on generations. The significance of the regression coefficients was evaluated by t tests. Values of the F line were expressed as a deviation from the RBC2 line. Whenever possible, the selection period was divided into 10 generation intervals to evaluate changes in genetic parameters associated with selection. A separate analysis was also done on the entire 30 generations. In order to evaluate the effect of selection over the entire period, a one-way ANOVA of traits was used to estimate the effect of line in the 30th generation of selection.
Intended and actual selection differentials of 16-week 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.
Realized h2 was estimated by 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 by generations. The standard errors of the regression coefficient served as an approximation of the standard errors of the h2 estimates.
The total increase in inbreeding over the 30 generations was 12.4 and 27.1%, respectively, in the RBC2 and F lines. The respective average increase per generation was 0.41 and 0.90%.
The total changes in growth and reproduction traits observed in the F line are given in Tables 1 and 2, respectively. After 30 generations of selection, body weight of the F line approached twice that of its base population, the RBC2 line. This increase in body weight of the F line was associated with an increase in shank width and length and poorer walking ability (higher walking ability scores). Genetic increases in 16-week body weight in the F line had no influence on mortality to eight weeks of age but was associated with decreased total number of eggs laid primarily due to decreased intensity of lay as measured by clutch length and rate of lay. There was no significant change in broodiness in the F line. Days to first egg from stimulatory lighting and egg weight increased in the F line. Hatch of fertile eggs decreased, but there was no change in fertility in the F line.
Changes in growth traits in the F line as measured by linear regression coefficients of deviation from the RBC2 line on generations of selection are presented in Table 3. All of the changes in growth traits in the F line were linear because no significant quadratic regression coefficients were observed. Gains in 16-week body weight were greater in Generations 1 to 10 than in Generations 11 to 20. The largest gains in 16-week body weight were observed in Generations 21 to 30 when the number of breeders in the F line was increased. Changes in shank measurements were positive and linear over the period in which these traits were recorded.
Changes per generation in reproduction traits in the F line are presented in Table 4. The changes in egg production were not consistent over the entire selection period, and there was a significant negative quadratic regression coefficient for the entire period. The largest decrease in egg production occurred in the first 10 generations of selection in the F line. No significant change in egg production was observed in Generations 11 to 20, but there was a loss of one egg per hen per generation during Generations 21 to 30. The increase in days from stimulatory lighting to production of the first egg occurred only in the first 10 generations of selection in the F line. The largest decreases in intensity of lay (maximum clutch length, average clutch length, and rate of lay) and increases in egg weight in the F line occurred during the first 10 generations of selection. There was no significant change in total days broody in the F line. The linear regression coefficient of hatch of fertile eggs on generations was negative for all periods of measurement but significant only for Generations 11 to 20 and 1 to 30. No linear changes were noted for fertility in the F line, but the quadratic regression coefficient was positive and significant for the entire period.
The number of birds available at 16 weeks of age and percentage of offspring selected to reproduce the F line are shown in Table 5. For both sexes combined, the percentage of offspring selected in the F line was similar in Generations 1 to 10 and 21 to 30 even though population size of the F line was increased in the latter period. Selection intensity, as measured by percentage selected, was not as great (percentage selected was greater) in Generations 11 to 20.
Intended and actual selection differentials did not consistently differ in the F line (Table 5). Realized h2 of 16-week body weight in the F line as estimated by the linear regression of response on accumulated actual selection differentials was higher in males than in females and declined with selection. For both sexes combined, the decline was about three or four percentage points for each 10 generations of selection. The decline in realized h2 of 16-week body weight did not appear to be the same in the two sexes. For males, the h2 declined about two percentage points in Generations 11 to 20 relative to that in Generations 1 to 10 and further declined by about six percentage points in Generations 21 to 30. A decline of about six percentage points in h2 occurred in females from Generations 1 to 10 to Generations 11 to 20 with no further change in Generations 21 to 30.
Long-term selection studies for increased body weight have been conducted at Virginia using chickens (Dunnington and Siegel, 1996), at Georgia (Anthony et al ., 1996; Marks, 1996) and in Ohio (Anthony et al ., 1996) using Japanese Quail, and in Ohio (Nestor et al ., 1996c) 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 in Georgia (Marks, 1996) and Ohio (Nestor et al ., 1996a), and in the present study with turkeys. However, the realized h2 was still greater than 0.20 at the 30th generation of selection in the F line indicating that a plateau in response is not likely in the near future of this line.
Measurements of correlated response were made only periodically in the chicken selection study in Virginia and in the Japanese quail selection study in Georgia. In the Japanese quail selection study (Nestor et al ., 1996b) in Ohio and in the present study using turkeys, many correlated traits were measured each generation so that changes with selection in the correlated traits could be measured. The results of the present study indicate that the genetic relationship, particularly in magnitude, between 16-week body weight and many correlated traits changed with selection. The changes in number of eggs produced per hen was negative and significant in the F line only for Generations 1 to 10 and 21 to 30. For intensity of lay traits (maximum and average clutch length and rate of lay), the changes were larger in early generations of selection. The days required from stimulatory lighting to production of the first egg increased only in Generations 1 to 10 of the F line. A significant decline in hatch of fertile eggs in the F line was observed only in Generations 11 to 20. The results of the present study indicate that the magnitude of genetic correlations, as well as the h2, may change with selection.
| Table 1. Effect of Selecting Turkeys Over Thirty generations for Increased 16-Week Body Weight on Body Weight at Various Ages, Shank Measurements, Walking Ability, and Mortality. | |||
|---|---|---|---|
| Line1 | |||
| Variable | RBC2 | F | F-RBC2 |
| Male body weight, kg | |||
| 8 wk | 2.60 | 4.59 | 1.99*** |
| 16 wk | 7.66 | 13.84 | 5.82*** |
| 20 wk | 9.80 | 17.86 | 8.06*** |
| Female body weight, kg | |||
| 8 wk | 2.07 | 4.01 | 1.94*** |
| 16 wk | 5.79 | 10.63 | 4.84*** |
| 20 wk | 6.44 | 12.29 | 5.85*** |
| Adult2 | 9.48 | 17.37 | 7.89*** |
| Shank length, cm | |||
| Males | 19.95 | 22.12 | 3.27*** |
| Females | 16.54 | 18.10 | 1.56*** |
| Shank width, mm | |||
| Males | 13.44 | 16.71 | 3.27*** |
| Females | 11.63 | 15.48 | 3.85*** |
| Walking ability score3 | |||
| Males | 1.54 | 3.04 | 1.50*** |
| Females | 1.38 | 2.89 | 1.51*** |
| Mortality, 0 to 8 wk, % | 7.0 | 11.1 | 4.1 |
| 1 RBC2 = randombred control line; F = subline of RBC2 selected for increased 16-week BW. 2 Body weight when hens first achieved 50% egg production. 3 Birds were subjectively rated at 16 weeks of age from 1 to 5, with 1 representing birds whose legs did not have any defects and had no difficulty walking, and 5 indicating birds whose legs exhibited extreme lateral deviation or had great difficulty walking. Ratings of 2, 3, and 4 represented intermediate values. *** P=< 0.001. |
|||
| Table 2. Effect of Selecting Turkeys Over Thirty Generations for Increased 16-week Body Weight on Reproduction Traits | |||
|---|---|---|---|
| Line1 | |||
| Variable | RBC2 | F | F-RBC2 |
| Egg production2, no./hen | 92.6 | 68.7 | -23.9*** |
| Days to first egg3 | 20.1 | 24.4 | 4.3*** |
| Clutch length2, d | |||
| Maximum | 6.8 | 3.4 | -3.4*** |
| Average | 2.20 | 1.46 | -0.74*** |
| Total days of broody2,4 | 28.1 | 35.2 | 7.1 |
| Rate of lay2,5, % | 60.8 | 47.0 | -13.8*** |
| Egg weight6, g | 89 | 98 | 9*** |
| Fertility6, % | 89 | 87 | -2 |
| Hatch of fertile eggs6, % | 85 | 75 | -10*** |
| 1 RBC2 = randombred control line; F = subline of RBC2 selected for increased 16-week BW. 2 Based on an 180-day production period. 3 Days from beginning of stimulatory lighting (14 hours/day) to production of first egg. 4 Total number of days lost during pauses in egg production of five or more consecutive days. 5 Rate of lay = number of eggs laid. (180d-total days broody). 6 Based on the first 12-weeks of egg production. ***P=< 0.001. | |||
| Table 3. Linear Regression Coefficients of Growth Traits on
Generations of Selection in the F Line After Adjustment for Changes in the Environment. 1 | ||||
|---|---|---|---|---|
| Generations of Selection | ||||
| Variable | 1 to 10 | 11 to 20 | 21 to 30 | 0 to 30 |
| Male body weights, kg | ||||
| 8 wk | 0.049*** | 0.054*** | 0.072** | 0.072*** |
| 16 wk | 0.193*** | 0.115*** | 0.258*** | 0.185*** |
| 20 wk | 0.193*** | 0.347*** | 0.535***,2 | |
| Female body weights, kg | ||||
| 8 wk | 0.049*** | 0.050*** | 0.094*** | 0.066*** |
| 16 wk | 0.160*** | 0.115*** | 0.213*** | 0.152*** |
| 20 wk | 0.158*** | 0.245*** | 0.185***,2 | |
| Adult3 | 0.226*** | 0.260*** | 0.332*** | 0.225*** |
| Shank length, cm | ||||
| Males | 0.048***,4 | |||
| Females | 0.046***,4 | |||
| Shank width, mm | ||||
| Males | 0.121***,4 | |||
| Females | 0.140***,4 | |||
| Walking ability score5 | ||||
| Males | 0.045***,4 | |||
| Females | 0.116***,6 | |||
| Mortality, 0 to 8 wk, % | -0.250 | -0.100 | 0.439 | -0.043 |
| 1 The F line was selected for increased 16-week BW. In order to remove environmental variation, values for the randombred control were subtracted from those of the F line prior to regression analysis. 2 Based on Generations 9 through 30. 3 Body weight when hens first achieved approximately 50% egg production. 4 Based on Generations 14 through 30. 5 Birds were subjectively rated from 1 to 5, with 1 representing birds whose legs did not have any defects and had no difficulty walking, and 5 indicating birds whose legs exhibited extreme lateral deviatioin or great difficulty walking. Ratings of 2,3 and 4 represented intermediate values. 6 Based on Generations 21 through 30. ***P=< 0.001. | ||||
| Table 4. Linear Regression Coeffecients of Reproduction Traits on Generations of Selection in the F Line After Adjustment for Changes in the Environment.1 | ||||
|---|---|---|---|---|
| Generations of Selection | ||||
| Variable | 1 to 10 | 11 to 20 | 21 to 30 | 1 to 30 |
| Egg production2, no./hen | -2.961** | -0.113 | -1.061* | -0.2313 |
| Days to first egg4 | 0.143** | 0.251 | 0.032 | 0.1153 |
| Clutch length2, d | ||||
| Maximum | -0.364** | -0.092 | -0.157* | -0.103*** |
| Average | -0.068** | -0.050* | -0.021 | -0.026*** |
| Total days of broody2,5 | 2.405 | -0.261 | 0.699 | -0.418 |
| Rate of lay2,6, % | -0.011*** | -0.010* | -0.003 | -0.004** |
| Egg weight7, g | 0.224* | -0.194 | 0.181 | 0.311*** |
| Fertility7, % | -1.194 | 0.384 | 0.667 | -0.0118 |
| Hatch of fertile eggs7, % | -0.133 | -0.827* | -0.067 | -0.276*** |
| 1 The F line was selected for increased 16-week BW. In order to remove environmental variation, values for the randombred control were subtracted from those of the F line prior to regression analysis. 2 Based on an 180-day production period. 3 Significant (P=< 0.05) negative quadratic regression coeffecient. 4 Days from beginning of stimulatory lighting (14 hours/day) to production of first egg. 5 Total number of days lost during pauses in egg production of five or more consecutive days. 6 Rate of lay = number of eggs laid. (180d-total days broody). 7 Based on a 12-week period. 8 Significant (P=< 0.05) positive quadratic regression coeffecient. * P=< 0.05. ** P=< 0.01. ***P=< 0.001. | ||||
| Table 5. Selection Intensity, Selection Differentials, and Realized Heritablities in the F Line .1 | ||||
|---|---|---|---|---|
| Generations of Selection | ||||
| Variable | 1 to 10 | 11 to 20 | 21 to 30 | 1 to 30 |
| Number of birds2 | ||||
| RBC2 line | 376 | 364 | 324 | 355 |
| F line | 305 | 275 | 384 | 322 |
| Percentage selected | ||||
| Males | 24.8 | 32.4 | 18.0 | 25.1 |
| Females | 23.2 | 29.8 | 31.1 | 28.0 |
| Both sexes | 24.0 | 31.1 | 24.6 | 26.5 |
| Selection Differentials, kg3 | ||||
| Males | ||||
| I | 0.6269 | 0.6201 | 1.3054 | 0.8508 |
| A | 0.6178 | 0.6128 | 1.3090 | 0.8465 |
| I-A | -0.0091 | -0.0073 | 0.0036 | -0.0043 |
| Females | ||||
| I | 0.5372 | 0.4409 | 0.6087 | 0.5289 |
| A | 0.5398 | 0.4373 | 0.6141 | 0.5304 |
| I-A | 0.0026 | -0.0036 | 0.0054 | 0.0015 |
| Realized heritablity | ||||
| Linear regression4 | ||||
| Males | 0.337±0.030 | 0.320±0.041 | 0.320±0.026 | 0.285±0.006 |
| Females | 0.280±0.021 | 0.217±0.036 | 0.218±0.028 | 0.234±0.006 |
| Both sexes | 0.309±0.022 | 0.309±0.022 | 0.268±0.033 | 0.254±0.007 |
| Final generation | ||||
| Males | 0.392 | 0.341 | 0.306 | 0.306 |
| Females | 0.321 | 0.255 | 0.241 | 0.241 |
| Both sexes | 0.356 | 0.298 | 0.274 | 0.274 |
| 1 The F line was developed from a randombred control population (RBC2) by selecting for increased 16-week body weight. 2 Based on birds alive at 16 weeks of age. 3 I = intended selection differential ( mean of selected birds minus mean of all birds); A = intended differential weighted bty the number of offspring hatched. 4 Heritablity ± standard error | ||||
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