Research and Reviews: Poultry and Swine

Special Circular 164-99

Effect of Crossing a Line Selected for Increased Shank Width with Two Commercial Sire Lines on Performance and Walking Ability of Turkeys

Karl E. Nestor1
John W. Anderson
The Ohio State University
Department of Animal Sciences

1 For more information, contact at: The Ohio State University, Ohio Agricultural Research and Development Center, 1680 Madison Ave., Wooster, OH 44691; 330-263-3757; Fax: 330-263-3949; e-mail: nester.1@osu.edu.

Abstract

Published research indicated that a line (FL) of turkeys selected for increased shank width and exhibiting good walking ability (WA) and improved leg structure was useful in improving WA of a cross with an unimproved commercial sire line while remaining competitive in body weight (BW) and body conformation. The purpose of the present study was to determine if the FL line was useful in improving WA of two improved commercial primary-breeding sire lines and to study the inheritance of BW traits in the pure lines and reciprocal crosses of the pure lines.

Samples of a primary breeding sire line from each of two major commercial turkey breeders was obtained as unpedigreed eggs and designated as Lines A and B. Lines A and B were reciprocally crossed with the FL line that had been selected for increased shank width for 16 generations. Pure lines and reciprocal crosses were produced by artificially mating 10 males to 15 females using pooled semen. The same semen pools were used to produce the pure lines as were used to produce the crosses. Traits measured included BW at eight, 16, and 20 weeks of age, and length, width, and depth of the shank, width of the breasts, and WA at 16 weeks of age.

The A and B lines were larger, had wider breasts, and narrower and shorter shanks than the FL line. Line B was larger than Line A. Shank measurements and WA score did not differ between Lines A and B in either sex. Breasts of Line B females were wider than those of Line A females, but there was no male line difference in breast width of males.

Heterosis in BW was larger in Line A crosses (average of 4.6% with a range of 2.5 to 7.2% for BW at different ages) than in Line B crosses (average of 2.2% with a range of 0.1 to 5.3%). Heterosis in WA scores was negative (crosses had better WA than pure lines) for males [14% for the Line A and FL crosses (P æ 0.001) and 4% for the Line B and FL crosses (P Ñ 0.05)] but was positive and nonsignificant for females of the A and FL and the B and FL crosses (average of 2.6%). With one exception, heterosis was not an important source of variation for shank width or shank depth, and there was a low level of heterosis for shank width. No heterosis was observed for breast width. Reciprocal effects were an important source of variation in BW and shank measurements for females but not for males in both sets of crosses.

The results of the present study indicated the use of the FL line to improve WA of improved primary-breeding commercial sire lines was not a feasible alternative. The slight improvement noted in WA of the male crosses was offset by the reduction in BW and breast width of the crosses relative to the pure commercial sire lines.

Introduction

Mortality resulting from leg problems in turkeys, particularly in males, occurs during the latter part of the growth period after the birds have consumed a large quantity of feed. Because WA of males is negatively associated with genetic increases in BW (Nestor, 1984), leg problems may also reduce potential genetic gains in BW. The heritability of a subjective measurement of WA in a randombred control population was low (0.06; Havenstein et al., 1988), indicating that direct selection for improved WA would not rapidly reduce leg problems.

Selection for an increased amount of breast muscle and increased BW has increased total BW and amount of breast muscle at a faster rate than the increases observed in muscles of the legs (Marsden, 1940; Miller, 1968; Clayton et al., 1978). The relative amount of leg muscles declines with age as the birds get heavier (Harshaw and Rector, 1940). Similarly, the skeleton exhibits a relative decline with age (Clayton et al., 1978).

Nestor et al. (1985) hypothesized that there appears to be a biologically incompatible combination in commercial turkeys of increased BW with relatively less support (leg muscles and bones), and this inherent stress probably magnifies the effect of various causes of leg problems. To test the hypothesis, a selection experiment was initiated to attempt to increase the relative amount of leg bones and leg muscles by selection and to study the effects of such a selection scheme on walking ability. A line (F), selected over 13 generations for increased 16-week body weight and exhibiting an increase of leg problems (Nestor, 1984), served as the base population. Sublines of F were mass selected for increased shank width at the dew claw (FL) or were family selected for increased leg muscles (FM). Selection continued in the F line.

Selection was effective in the FL line in increasing shank width and BW (Nestor et al., 1985) and increasing the relative proportion of leg bones (Nestor et al., 1987, 1988; Emmerson et al., 1991b) without any loss in WA. The relative proportion of breast muscles declined in comparison to the F line (Emmerson et al., 1991b). Results in the FL line tend to substantiate this hypothesis. Although the amount of leg muscles increased slightly in the FM line, the relative proportion of leg muscles did not increase (Emmerson et al., 1991b). Therefore, the results in the FM line could not be used to test the hypothesis.

Ye et al. (1997) crossed the FL line on an unimproved commercial sire line to determine whether WA of the cross could be improved without seriously compromising growth rate and body conformation. At the time of the cross, the FL and commercial sire line had similar BW at 8, 16, and 20 weeks of age . The commercial sire line had poorer WA, shorter and narrower shanks, and wider breast than the FL line. The F1 birds exhibited overdominance in BW at all ages. Heterosis in BW ranged from 3.2 to 7.8%. At 16 weeks of age, the WA scores were reduced (better legs) in the F1 relative to the average of the parental lines. Heterosis of WA scores was -10.5% (P >= 0.05) for males and -23.5% (P <= 0.05) for females. Part of the gains in BW and breast width obtained in the F1 was maintained in a backcross to the sire line. The authors suggested that a line, such as FL, exhibiting improved leg structure and WA and poorer conformation might be used to improve the WA of a cross involving commercial sire lines. The purpose to the present study was to determine whether the FL line could be used to improve WA of two improved commercial sire lines without seriously compromising growth rate and body conformation and to study the inheritance of growth traits in the pure lines and their crosses.

Materials and Methods

Samples of a primary breeding sire line from each of two major turkey breeders, designated A and B, were obtained as unpedigreed eggs. The offspring were grown in confinement with sexes intermingled. Both males and females were provided with a declining protein five-ration system (Naber and Touchburn, 1970) based on the schedule for males. Breast width was measured by a caliper at 6.35 cm of body depth at a point approximately 3.18 cm from the anterior point of the keel. The FL line, mass selected for increased shank width (Nestor et al., 1985), was maintained with 36 parental pairs (Nestor, 1977a) until the 12th generation, after which the line was maintained by mating 36 males to 54 females.

Pure A, B, and FL lines and a reciprocal cross of the A and B lines and the FL line were produced by artificially inseminating 10 males to 15 females in each line or cross using pooled semen. The same semen pools used in matings of the pure line were also used to produce the crosses to minimize the influence of sampling error. The FL line was in the 17th generation of selection at the time of the crosses.

Offspring from three biweekly hatches were used in the study. The different genetic groups were grown intermingled with the sexes housed in different buildings. All birds were provided with a declining protein five-ration system (Naber and Touchburn, 1970) based on the schedule for males. The total number of birds per genetic group ranged from 47 to 65 with an average of 53.5.

Body weights were measured at 8, 16, and 20 weeks of age. At 16 weeks of age, breast width (measured at 6.35 cm of body depth at a point approximately 3.18 cm from the anterior point of the keel), shank width (measured from side to side at the dew claw), shank depth (measured from front to back at the dew claw), and shank length were obtained. Walking ability at 16 weeks of age was estimated by the method of Nestor et al. (1985) in which each bird was given a score of 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 two, three, and four represented intermediate values.

Statistical Analysis

The data were analyzed within sex using the General Linear Model Procedures of SAS (SAS Institute, 1988) with genetic group (pure lines and crosses) and hatch as sources of variation. Orthogonal contrasts (SAS Institute, 1988) were used to estimate additive genetic effects (contrast of A vs. B, A vs. FL, and B vs. FL), heterotic effect (contrast of averages of the parental lines with the average of the reciprocal crosses), and sex-linked or maternal effects (contrast of reciprocal crosses). The data were analyzed in one analysis, but for simplicity the results of the A and FL and B and FL comparisons will be presented separately.

Results

Hatch effects were significant (P <= 0.05) for most traits measured in both sexes. Only WA score for males and eight-week BW of females did not differ significantly among hatches.

Additive Genetic Effects

Comparison of Lines A and B

Body weights of males and females at eight, 16, and 20 weeks of age were larger in Line B than in Line A (Tables 1 through 4). Shank measurements and WA scores did not differ between Lines A and B in either sex. Breast width of females was greater for Line B than A (15.32 vs 14.02 cm) but there was no line difference in males.

Comparison of Lines A and FL

Body weights were larger and breasts were wider in Line A than Line FL for both males (Table 1) and females (Table 2). Shank width and depth were larger in Line FL than in Line A, in both sexes. The length of the shank was longer in FL males than in A males, but there was no significant line difference in shank length of females. Walking ability scores did not differ between lines for either sex.

Table 2. Effect of Reciprocally Crossing Commercial Sire Line A and a Line (FL) Selected for Increased Shank Width on Performance of Females.
Variable	Parental Lines			Crosses			Additive Genetic Effect1	Reciprocal Effect2	Percentage Heterosis3	SEM
	A	FL	x¯	A x FL	FL x A	x¯
Body weight, kg
8 week	3.59	3.02	3.30	3.59	3.41	3.50	***	NS	6.0**	0.061
16 week	10.44	8.27	9.30	9.78	8.96	9.37	***	*	0.1	0.139
20 week	12.47	9.51	10.99	11.81	10.89	11.35	***	**	3.3*	0.158
Walking Ability	2.67	2.52	2.60	2.56	2.54	2.55	NS	NS	-1.9	0.064
Score4
Shank
Length, cm	17.33	17.76	17.54	18.14	17.14	17.64	NS	*	0.6	0.105
Width, mm	16.01	19.31	17.66	18.09	16.24	17.16	***	**	-2.8	0.195
Depth, mm	21.38	22.42	21.90	22.69	21.48	22.08	***	***	0.8	0.083
Breast
Width, cm	13.91	10.89	12.40	12.19	11.59	11.89	***	NS	-4.1	0.130
1 Measured by contrast of parental lines. 2 Measured by contrast of reciprocal crosses. 3 Percentage heterosis = (average of reciprocal cross/average of parental lines) x 100. Significance was established by contrasts. 4 Birds were subjectively rated from 1 to 5 indicating no, slight, moderate, severe, and extreme difficulty walking, respectively. * P <= 0.05. ** P <= 0.01. *** P <= 0.001.

Comparison of Lines B and FL

Body weights were larger and breasts were wider for both sexes in Line B than in Line FL (Tables 3 and 4). Shank width and depth were larger in the FL line than in the B line in both sexes. The shanks were longer in FL males than in B males, but there was no line difference in females. The FL females had improved WA (lower WA scores) relative to B females, but there were no line difference for males.

Reciprocal Effects

No differences between reciprocal crosses of the A and FL lines were observed in males for any trait (Table 1), but reciprocal effects were significant for 16- and 20-week BW and shank measurements for cross females. In the crosses of the B and FL lines, reciprocal effects were significant for eight-week BW of males (Table 3) and eight-week, 16-week, and 20-week BW and shank width and depth of females (Table 4).

Table 4. Effect of Reciprocally Crossing Commercial Sire Line B and a Line (FL) Selected for Increased Shank Width on Performance of Females.
Variable	Parental Lines			Crosses			Additive Genetic Effect1	Reciprocal Effect2	Percentage Heterosis3
	B	FL	x¯	B x FL	FL x B	x¯
Body weight, kg
8 week	4.12	2.99	3.56	3.78	3.47	3.62	***	**	1.8
16 week	11.81	8.08	9.94	10.30	9.60	9.95	***	***	0.1
20 week	13.74	9.38	11.56	12.05	11.27	11.66	***	**	0.8
Walking Ability	2.88	2.46	2.67	2.63	2.86	2.74	*	NS	2.6
Score5
Shank
Length, cm	17.24	17.63	17.44	17.51	17.59	17.55	NS	NS	0.6
Width, mm	17.12	19.04	18.08	18.61	17.12	17.86	***	**	-1.2
Depth, mm	21.86	22.38	22.12	22.40	21.60	22.00	**	**	-0.5
Breast
Width, cm	15.32	10.70	13.01	12.76	12.51	12.63	***	NS	-2.9
1 Male listed first in cross. 2 Measured by contrast of parental lines. 3 Measured by contrast of reciprocal crosses. 4 Percentage heterosis = (average of reciprocal cross/average of parental lines) x 100. Significance was established by contrasts. 5 Birds were subjectively rated from 1 to 5 indicating no, slight, moderate, severe, and extreme difficulty walking, respectively. * P <= 0.05. ** P <= 0.01. *** P <= 0.001.

Heterotic Effects

Heterosis in BW ranged from 2.5 to 4.6% in males (Table 1) and from 4.5 to 7.2% (Table 2) in females in the crosses of Lines A and FL. In males, the heterosis was significant only for 20-week BW. Heterosis was not an important source of variation for shank measurements in the A and FL line crosses being significant only for male shank length. No significant heterosis was observed for breast width in either sex in the offspring of the A and FL cross. Large negative heterosis was observed for WA scores of males (Table 1) but not females (Table 2).

Heterosis for BW was not as large in the crosses of Lines B and FL (Tables 3 and 4) as in the crosses of Lines A and FL (Tables 1 and 2). Only the heterosis for eight-week BW of males was significant in the crosses of Lines B and FL. Heterosis was negative for shank width (significant only for males) in the crosses of Lines B and FL. No significant heterosis was noted for breast width, shank depth, and walking ability scores in crosses of the B and FL lines. Heterosis for shank length was significant for males but not females in this cross.

Discussion

Additive genetic variation, as indicated by differences among lines, was large for BW at all ages measured, and shank width and depth and breast width at 16 weeks of age. Such variation was expected since it has been shown that large gains in BW (McCartney et al., 1968; Nestor, 1977b, 1984; Nestor et al., 1996), breast width (Nestor et al., 1969), and shank width (Nestor et al., 1985) can be made by selection.

Nonadditive genetic variation in BW of turkeys is usually not an important source of genetic variation (Kondra and Shoffner, 1955; Clark, 1961; McCartney and Chamberlin, 1961; Nestor, 1971), but heterosis or nonadditive genetic variation has been observed in some crosses in which the parents differed greatly in body conformation (Asmundson, 1945, 1948; Emmerson et al., 1991a; Ye et al., 1997). There were large line differences in breast width between the FL and A and FL and B lines.

Heterosis in breast width was not significant in crosses of the FL line with the A and B lines. Similar results were obtained by Asmundson (1948) in a cross of bronze strains differing in breast width. Using diallel crosses among six strains, McCartney and Chamberlin (1961) found the nonadditive genetic variation accounted for 8% of the total genetic variation in breast width of males, but there was no nonadditive genetic variation in breast width of females.

Heterosis in shank width in the present experiment was a significant source of variation in only one of four possible comparisons. Emmerson et al. (1991a) did not observe significant heterosis in shank width of the F1 of a cross between an experimental F line selected for increased growth rate and a commercial sire line. Significant heterosis was observed in the F2 of the proceeding cross for shank width. No significant heterosis was observed by Ye et al. (1997) in a cross of the FL line and an unimproved commercial sire line. No significant heterosis was observed for shank depth in the present experiment or in the study of Ye et al. (1997).

Significant heterosis was observed for shank length of male offspring from the crosses of FL with the A and B lines in the present study. No heterosis was evident for shank length of females in these crosses. McCartney and Chamberlin (1961) found that heterosis for shank length was significant for both sexes in diallel crosses of six turkey strains. With sexes combined, Emmerson et al. (1991a) observed significant heterosis in shank length in the F1 and F2 of a cross between the F line and a commercial sire line. Ye et al. (1997) reported significant heterosis in shank length of both sexes in a cross of the FL line and an unimproved sire line.

Negative heterosis (i.e., value of crosses was less than the average of the parental lines) was observed in WA scores for males of the FL and A line (13.7%; P <= 0.001) and FL and B line (4%; P >= 0.05) crosses. Heterosis for WA scores of females from the two crosses was positive (average was 2.65%) and not significant. Negative heterosis for WA scores was observed by Emmerson et al. (1991a) and Ye et al. (1997) in offspring from crosses of pure lines differing greatly in the amount of leg bones. In the studies of Emmerson et al. (1991a) and Ye et al. (1997), the pure line and cross offspring were grown in confinement to eight weeks of age and range reared to 20 weeks of age. Noble et al. (1996a) reported that WA scores of the FL line were higher (poorer legs) when grown in confinement than when grown on range, whereas the WA scores did not differ between environments for three other lines. Noble et al. (1996b) presented little evidence that there was a difference in the amount of walking activity in the two environments, although the data could not be statistically compared because of the presence of interactions. However, prior to weighing, the birds were driven farther to catching locations on range than in confinement, and this may increase WA scores under range-rearing conditions. Perhaps heterosis in WA, in crosses of lines differing greatly in leg structure, would be greater in a more stressful environment.

Differences between reciprocal crosses can be due to maternal effects or sex-linked inheritance. Reciprocal effects were not an important source of variation in male offspring from either cross in the present experiment, being significant only for eight-week BW of offspring from the cross of Lines B and FL. Numerous significant reciprocal effects were noted for BW and leg measurements in females of both crosses. The average egg weights of the A, B, and FL lines were 90, 94, and 98 grams, respectively, in the generation that the eggs were obtained from the commercial breeders (unpublished data). This large line difference in egg weight may have influenced the performance of the reciprocal crosses. Bray (1965) reported that egg weight can influence body weight up to 24 weeks of age. However, if reciprocal effects were due to maternal effects, differences between reciprocal crosses should have been obtained in both sexes. Sex linked inheritance may be the more plausible explanation for the reciprocal differences observed. Reciprocal effects in growth traits have been previously reported by McCartney and Chamberlin (1961), Friars et al. (1963), and Emmerson et al. (1991a).

Based on the results of a cross of the FL line with an unimproved commercial sire line and a backcross of the F1 to the commercial sire line, Ye et al. (1997) suggested that a line, such as FL, exhibiting improved leg structure might be useful in improving the WA of commercial sire lines. In the present study, WA appeared to be improved slightly in males from crosses of the FL with two improved commercial sire lines, but no such effect was observed in females from the crosses. The slight improvement in WA of male offspring of the crosses would not overcome the loss in body weight and breast width observed in the cross offspring relative to the pure commercial sire lines and, therefore, use of the FL line to improve WA of improved commercial sire lines is not a feasible alternative.

Acknowledgments

The authors thank Hybrid Turkeys, Inc., 650 Riverbend Drive, Suite C, Kitchner, Ontario, Canada N2K 3S2, and Nicholas Turkey Breeding Farms, Post Office Box Y, Sonoma, CA 95476, for graciously providing the hatching eggs from their sire lines. Financial support for this project was also provided by British United Turkeys of America, Lewisburg, WV 14901, and Hybrid Turkeys.

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