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Interrelationships of Milk Yield, Body Weight, and Reproductive Performance1
Interrelationships of Milk Yield, Body Weight, and Reproductive Performance I L. B A D I N G A , R. J. COLLIER, C. J. WILCOX, and W. W. T H A T C H E R Dairy Science Department University of Florida Gainesville 32611
ABSTRACT
Records (2263) from a single north Florida herd for 3 yr were evaluated in a series of analyses. Environment was subtropical. Data set included only cows that had normal milk records and became pregnant. Holsteins and Jerseys averaged 6799 and 4504 kg milk, 587 and 418 kg postpartum b o d y weight, 164 and 141 days open, and 2.3 and 2.1 services per subsequent pregnancy. Pooled within breed, repeatabilities and heritabilities of these performance measures were .37, .15; .53, .25; .20, .06; and .30, 0. Estimable genetic correlations were milk yield and body weight - . 0 9 , milk yield and services per conception .38, and services per conception and body weight .37; range of standard errors was .22 to .39. Substantial genetic antagonism may exist between milk yield and body weight, and efficient reproduction. If results of this research are verified elsewhere, breeders should be aware that selection for increased weight could lead to decreased reproductive efficiency. INTRODUCTION
Progress has been substantial in selection of dairy cattle for high milk production in recent years (17). Of practical importance are effects of genetic change of milk yield on reproductive performance. Several field studies indicated marked genetic antagonisms between high milk yield and fertility (3, 8, 9, 12, 13), but other research showed little association of yield and fertility (1, 18, 19, 23).
Spalding et al. (21) reported that cows with milk yields more than 907 kg above herdmates were 20.5% lower for conception rates of first service than cows 907 kg below herdmates. Earlier Everett et al. (8) found that phenotypic correlations of 120-day milk yield with measures of fertility essentially were zero, but genetic correlations were approximately .50. In 1981 Berger et al. (3) reported genetic correlations for measures of reproductive performance with 305-day yield from .48 to .62. Smith and Legates (20) stated that phenotypic relationships between 305-day yield and days open could be due largely to influences of gestation on yield. These positive relationships (e.g., yield and days open) indicate an antagonistic (i.e., undesirable) relationship. Review by Martin (14) indicated that heritabilities for services per conception and days open ranged from - . 1 5 to .10 and .01 to .09. Repeatabilities for services per conception were from .0 to .11. Low heritabilities for measures of fertility have led many workers (4, 6, 8) to conclude that selection for reproductive traits would be slow. In contrast, numerous studies [e.g., (24)] indicated that heritability for 305day milk yield was approximately .20 to .25. Most investigators agreed that large cows gave more milk than small cows (7, 15, 16). However, relationship between body weight and yield was such that heavier cows possessed little, if any, superiority of production efficiency (5, 15). Our objectives were to evaluate relationships of milk yield and body weight to fertility in lactating Holstein and Jersey cows in a commercial dairy herd in a subtropical environment (30°32'N latitude). M A T E R I A L S AND METHODS
Received June 15, 1984. ~Florida Agricultural Experiment Station Journal Series No. 5593. 1985 J Dairy Sci 68:1828-1831
Detailed breeding records from a purebred dairy herd in MonticeIIo, FL, for j a n u a r y 1, 1975, to December 31, 1977, were matched
1828
PRODUCTION TECHNICAL NOTE
1829
TABLE 1. Means and standard deviations (SD) for response variables. Pooled (2263) t Traits Milk yield, kg Body weight, 2 kg Days open Services per conception
Holstein (1584)*
Jersey (679) l
"X
SD
"X
SD
"X
SD
6110 537 157 2.2
2080 90 121 1.6
6799 587 164 2.3
1914 49 124 1.7
4504 418 141 2.l
1478 36 111 !.4
*Number of records. 2At beginning of lactation.
with DHI (Dairy Herd I m p r o v e m e n t ) records for milk yield. After screening, resulting data set consisted of 2263 reproductive and lactation records. Measures of p o s t p a r t u m reproductive performance were days open and services per conception. Body weight (tape measure on day of parturition), days in milk, sire of cow, and m o n t h , year, and age of cow at parturition were recorded. R o u t i n e inseminations were initiated at 45 days postpartum. Details of herd m a n a g e m e n t and climate are presented (2). Records were included in the study only if parturition was followed b y a diagnosed pregnancy. Data were analyzed b y m e t h o d of least squares analyses of variance b y c o m p u t e r programs of Harvey (11). General linear mixed model was : Yijklm = /2 + B i + Sij + Cij k
were b y intraclass correlation; daughters b y the same sire were considered to be half-sisters. Models from which estimates of variance and covariance were obtained are demonstrated in (11). RESULTS A N D DISCUSSION
Means and standard deviations for measures of performance are in Table 1. As expected, Holsteins produced more milk and were heavier at initiation of lactation than Jerseys. However, the latter had fewer days open and tended to require fewer services per conception than Holsteins. This agreed with other reports of fertility of Jerseys and Holsteins in warm climates (10, 22). Differences in fertility between breeds likely reflected differences associated with milk yield and ratios of surface to b o d y weight. High milk yield is associated with increased metabolic heat production, which m a y affect thermal
+ F 1 + eijkl m where Yijklm is the response for each dep e n d e n t variable, /a is the overall mean, Bi is breed effect (fixed), Sij is sire effect nested in breed (random), Cij k is cow effect nested in sire and breed (random), F 1 is a set of other fixed effects (i.e., days in milk, year and m o n t h of parturition, and age of cow), and eijkl m is remainder, representing variability a m o n g records of the same cow with usual assumptions for residuals. Age and days in milk were c o n t i n u o u s i n d e p e n d e n t variables to the third degree o f polynomial regression. Milk yield was included as a fixed i n d e p e n d e n t variable in several analyses. Estimates of repeatability and heritability
TABLE 2. Repeatabilities (t) and heritabilities (h 2) for response variables.
Traits
t
h2
Milk yield Body weight Services per conception Days open
.37 .53 .20 .30
.15 .25 .06 02
SE (h2) 1 .08 .08 .07 ...
t Approximate standard errors. 2 Negative estimate. Journal of Dairy Science VoL 68, No. 7, 1985
1830
BADINGA ET AL.
TABLE 3. Phenotypic and genetic ~ correlations.
Milk yield Services per conception Days open Body weight
Milk yield
Services per conception
Days open
Body weight
138 (.39) ~" NE 3 -.09 (.22)
.12 ... NE .37 (.34)
.21 .05 ... NE
<.01 .01 --.02 ...
Genetic correlations below diagonal. 2Approximate standard errors in parentheses. s Not estimable because of negative sire variances.
balance of high-producing cows. In contrast, tendency of Jerseys to maintain fairly good reproductive performance in subtropical environments more likely is due to lower heat production and greater ratio of surface to b o d y weight (25). Repeatabilities of milk yield, b o d y weight, and measures of fertility are in Table 2. Estimates for days open and services per conception (.30 and .20) were higher than those reported by Martin (14). Heritabilities for yield, weight, and fertility also are in Table 2. Estimates for milk yield (.15) and services per conception (.06) were similar to other reports (3, 14, 24). Phenotypic (rp) and genetic (rA) correlations are in Table 3. Correlations between milk yield and services per conception were positive (rp = .12; rA = . 38). These indicated that highproducing cows required more services per conception than tow-producing cows. As indicated by (3, 8), substantial genetic antagonism may exist between high yield and fertility in dairy cattle. Phenotypic correlation between milk yield and days open was .21. Phenotypic correlation between body weight at initiation of lactation and subsequent services per conception was essentially zero (rp = .01), whereas the genetic correlation was positive (rA = .37). Differences in services per conception associated with b o d y weight may be attributable partially to differences in thermoregulatory responses. Heavier cows have reduced ratios of surface to b o d y weight and, therefore, were more vulnerable to high enJournal of Dairy Science Vol. 68, No. 7, 1985
vironmental temperatures than smaller cows with higher rates of heat dissipation. Thus, care should be taken in selection for increased body weight to avoid possible associated decreases of fertility if results reported here are confirmed elsewhere. A host of other factors also may account for this association. We hypothesize that heavier cows may have a greater incidence of other problems (i.e., ketosis, milk fever, dystocia, fatty infiltration of the oviduct, etc.) that may compromise fertility. Phenotypic and genetic correlations between milk yield and body weight in this herd were low (Table 3). As suggested by McDaniel and Legates (15), milk yield can be improved without materially increasing body weight of dairy cattle. Because b o d y weight is heritable, selection for increased weight should result in larger cows with little or no correlated increase in milk yield. Because larger cows cost more to maintain, this clearly would not be a profitable selection scheme. Substantial genetic antagonism may exist between high milk yield and fertility as in this and other research relationship was positive between yield and services per conception. Body weight genetic correlation with services per conception likewise was positive. ACKNOWLEDGMENTS
The authors wish to acknowledge assistance of D. W. Webb and J. Moya, University of Florida, and to thank personnel of Bassett Bros. Dairy, Monticello, FL, for providing records and other financial and technical assistance.
PRODUCTION TECHNICAL NOTE
REFERENCES 1 Adkinson, R. W., C. J. Wilcox, and W. W. Thatcher. 1977. Effects o f sire of fetus upon subsequent production and days open of the dam. J. Dairy Sci. 60:1964. 2 Badinga, L., R. J. Collier, W. W. Thatcher, and C. J. Wilcox. 1985. Effects of climatic and management factors on conception rate o f dairy cattle in a subtropical environment. J. Dairy Sci. 68: 78. 3 Berger, P. J., R. D. Shanks, A. E. Freeman, and R. C. Laben. 1981. Genetic aspects o f milk yield and reproductive performance. J. Dairy Sci. 64: 114. 4 Carman, G. M. 1955. Interrelationships of milk production and breeding efficiency in dairy cows. J. Anita. Sci. 14:752. 5 Clark, R. D., and R. W. Touchberry. 1962. Effect of body weight and age at calving on milk production in Holstein cattle. J. Dairy Sci. 45:1500. 6 Dunbar, R. S., Jr., and C. R. Henderson. 1953. Heritability of fertility in dairy cattle. J. Dairy Sci. 36:1063. 7 Erb, R. E., and U. S. Ashworth. 1961. Relationships between age, body weight, and yield of dairy cows. J. Dairy Sci. 44:515. 8 Everett, R. W., D. V. Armstrong, and L. J. Boyd. 1966. Genetic relationship between production and breeding efficiency. J. Dairy Sci. 49:879. 9 Fonseca, F. A., J. H. Britt, B. T. McDaniel, J. C. Wilk, and A. H. Rakes. 1983. Reproductive traits of Holsteins and Jerseys. Effects of age, milk yield, and clinical abnormalities on involution of cervix and uterus, ovulation, estrous cycles, detection of estrus, conception rate, and days open. J. Dairy Sci. 66:1128. 10 Gwazdauskas, F. C., C. J. Wilcox, and W. W. Thatcher. 1975. Environmental and managemental factors affecting conception rate in a subtropical climate. J. Dairy Sci. 58:88. 11 Harvey, W. R. 1977. Instructions for use of least squares and maximum likelihood: general purpose program. Mimeo., Ohio State Univ., Columbus. 12 Inskeep, E. K., W. J. Tyler, and L. E. Casida. 1961. Hereditary variation in conception rate of HolsteinFriesian cattle. J. Dairy S ci. 44:1857. 13 Maijala, K. 1976. Possibilities of improving fertility
14 15
16
17
18
19
20
21
22
23 24
25
1831
in cattle by selection. World Rev. Anita. Prod. 11: 69. Martin, T. G. 1978. Reproduction and calving traits. Ch. 2.4 in Large dairy herd management. Univ. Presses Florida, Gainesville. McDaniel, B. T., and J. E. Legates. 1965. Associations between body weight predicted from heart girth and production. J. Dairy Sci. 48:947. Miller, R. H., and L. D. McGilliard. 1959. Relations between weight at first calving and milk production during the first lactation. J. Dairy Sci. 42: 1932. Norman, H. D., and R. L. Powell. 1978, Genetic and economic change attained and possible. Ch. 3.3 in Large dairy herd management. Univ. Presses Florida, Gainesville. Shanks, R. D., A. E. Freeman, and P. J. Berger. 1979. Relationship of reproductive factors with interval and rate of conception. J. Dairy Sci. 62: 74. Shanks, R. D., A. E. Freeman, P. L. Berger, and D. H. Kelley, 1978. Effect of selection for milk production on reproductive and general health of the dairy cow, J. Dairy Sci. 61:1765. Smith, J. W., and J. E. Legates. 1962. Relation of days open and days dry to lactation milk and fat yields. J. Dairy Sci. 45:1192. Spalding, R. W., R. W. Everett, and R. H. Foote. 1975. Fertility in New York artificially inseminated Holstein herds in dairy herd improvement. J. Dairy Sci. 58:718. Stott, G. H. 1961. Female and breed associated with seasonal variation in dairy cattle. J. Dairy Sci. 44:1698. Whitmore, H. L., W. J. Tyler, and L. E. Casida. 1974. Effects of early postpartum breeding in dairy cattle. J. Anim. Sci. 38:339. Wilcox, C. J. 1978. Genetic considerations of economic importance: milk yield, composition and quality. Ch. 2.1 in Large dairy herd management. Univ. Presses Florida, Gainesville. Worstell, D. M., and S. Brody. 1953. Environmental physiology. XX. Comparative physiological reactions of European and Indian cattle to changing temperatures. Missouri Agric. Exp. Stn. Bull. 515:1.
Journal of Dairy Science Vol. 68, No. 7, 1985
ABSTRACT
Records (2263) from a single north Florida herd for 3 yr were evaluated in a series of analyses. Environment was subtropical. Data set included only cows that had normal milk records and became pregnant. Holsteins and Jerseys averaged 6799 and 4504 kg milk, 587 and 418 kg postpartum b o d y weight, 164 and 141 days open, and 2.3 and 2.1 services per subsequent pregnancy. Pooled within breed, repeatabilities and heritabilities of these performance measures were .37, .15; .53, .25; .20, .06; and .30, 0. Estimable genetic correlations were milk yield and body weight - . 0 9 , milk yield and services per conception .38, and services per conception and body weight .37; range of standard errors was .22 to .39. Substantial genetic antagonism may exist between milk yield and body weight, and efficient reproduction. If results of this research are verified elsewhere, breeders should be aware that selection for increased weight could lead to decreased reproductive efficiency. INTRODUCTION
Progress has been substantial in selection of dairy cattle for high milk production in recent years (17). Of practical importance are effects of genetic change of milk yield on reproductive performance. Several field studies indicated marked genetic antagonisms between high milk yield and fertility (3, 8, 9, 12, 13), but other research showed little association of yield and fertility (1, 18, 19, 23).
Spalding et al. (21) reported that cows with milk yields more than 907 kg above herdmates were 20.5% lower for conception rates of first service than cows 907 kg below herdmates. Earlier Everett et al. (8) found that phenotypic correlations of 120-day milk yield with measures of fertility essentially were zero, but genetic correlations were approximately .50. In 1981 Berger et al. (3) reported genetic correlations for measures of reproductive performance with 305-day yield from .48 to .62. Smith and Legates (20) stated that phenotypic relationships between 305-day yield and days open could be due largely to influences of gestation on yield. These positive relationships (e.g., yield and days open) indicate an antagonistic (i.e., undesirable) relationship. Review by Martin (14) indicated that heritabilities for services per conception and days open ranged from - . 1 5 to .10 and .01 to .09. Repeatabilities for services per conception were from .0 to .11. Low heritabilities for measures of fertility have led many workers (4, 6, 8) to conclude that selection for reproductive traits would be slow. In contrast, numerous studies [e.g., (24)] indicated that heritability for 305day milk yield was approximately .20 to .25. Most investigators agreed that large cows gave more milk than small cows (7, 15, 16). However, relationship between body weight and yield was such that heavier cows possessed little, if any, superiority of production efficiency (5, 15). Our objectives were to evaluate relationships of milk yield and body weight to fertility in lactating Holstein and Jersey cows in a commercial dairy herd in a subtropical environment (30°32'N latitude). M A T E R I A L S AND METHODS
Received June 15, 1984. ~Florida Agricultural Experiment Station Journal Series No. 5593. 1985 J Dairy Sci 68:1828-1831
Detailed breeding records from a purebred dairy herd in MonticeIIo, FL, for j a n u a r y 1, 1975, to December 31, 1977, were matched
1828
PRODUCTION TECHNICAL NOTE
1829
TABLE 1. Means and standard deviations (SD) for response variables. Pooled (2263) t Traits Milk yield, kg Body weight, 2 kg Days open Services per conception
Holstein (1584)*
Jersey (679) l
"X
SD
"X
SD
"X
SD
6110 537 157 2.2
2080 90 121 1.6
6799 587 164 2.3
1914 49 124 1.7
4504 418 141 2.l
1478 36 111 !.4
*Number of records. 2At beginning of lactation.
with DHI (Dairy Herd I m p r o v e m e n t ) records for milk yield. After screening, resulting data set consisted of 2263 reproductive and lactation records. Measures of p o s t p a r t u m reproductive performance were days open and services per conception. Body weight (tape measure on day of parturition), days in milk, sire of cow, and m o n t h , year, and age of cow at parturition were recorded. R o u t i n e inseminations were initiated at 45 days postpartum. Details of herd m a n a g e m e n t and climate are presented (2). Records were included in the study only if parturition was followed b y a diagnosed pregnancy. Data were analyzed b y m e t h o d of least squares analyses of variance b y c o m p u t e r programs of Harvey (11). General linear mixed model was : Yijklm = /2 + B i + Sij + Cij k
were b y intraclass correlation; daughters b y the same sire were considered to be half-sisters. Models from which estimates of variance and covariance were obtained are demonstrated in (11). RESULTS A N D DISCUSSION
Means and standard deviations for measures of performance are in Table 1. As expected, Holsteins produced more milk and were heavier at initiation of lactation than Jerseys. However, the latter had fewer days open and tended to require fewer services per conception than Holsteins. This agreed with other reports of fertility of Jerseys and Holsteins in warm climates (10, 22). Differences in fertility between breeds likely reflected differences associated with milk yield and ratios of surface to b o d y weight. High milk yield is associated with increased metabolic heat production, which m a y affect thermal
+ F 1 + eijkl m where Yijklm is the response for each dep e n d e n t variable, /a is the overall mean, Bi is breed effect (fixed), Sij is sire effect nested in breed (random), Cij k is cow effect nested in sire and breed (random), F 1 is a set of other fixed effects (i.e., days in milk, year and m o n t h of parturition, and age of cow), and eijkl m is remainder, representing variability a m o n g records of the same cow with usual assumptions for residuals. Age and days in milk were c o n t i n u o u s i n d e p e n d e n t variables to the third degree o f polynomial regression. Milk yield was included as a fixed i n d e p e n d e n t variable in several analyses. Estimates of repeatability and heritability
TABLE 2. Repeatabilities (t) and heritabilities (h 2) for response variables.
Traits
t
h2
Milk yield Body weight Services per conception Days open
.37 .53 .20 .30
.15 .25 .06 02
SE (h2) 1 .08 .08 .07 ...
t Approximate standard errors. 2 Negative estimate. Journal of Dairy Science VoL 68, No. 7, 1985
1830
BADINGA ET AL.
TABLE 3. Phenotypic and genetic ~ correlations.
Milk yield Services per conception Days open Body weight
Milk yield
Services per conception
Days open
Body weight
138 (.39) ~" NE 3 -.09 (.22)
.12 ... NE .37 (.34)
.21 .05 ... NE
<.01 .01 --.02 ...
Genetic correlations below diagonal. 2Approximate standard errors in parentheses. s Not estimable because of negative sire variances.
balance of high-producing cows. In contrast, tendency of Jerseys to maintain fairly good reproductive performance in subtropical environments more likely is due to lower heat production and greater ratio of surface to b o d y weight (25). Repeatabilities of milk yield, b o d y weight, and measures of fertility are in Table 2. Estimates for days open and services per conception (.30 and .20) were higher than those reported by Martin (14). Heritabilities for yield, weight, and fertility also are in Table 2. Estimates for milk yield (.15) and services per conception (.06) were similar to other reports (3, 14, 24). Phenotypic (rp) and genetic (rA) correlations are in Table 3. Correlations between milk yield and services per conception were positive (rp = .12; rA = . 38). These indicated that highproducing cows required more services per conception than tow-producing cows. As indicated by (3, 8), substantial genetic antagonism may exist between high yield and fertility in dairy cattle. Phenotypic correlation between milk yield and days open was .21. Phenotypic correlation between body weight at initiation of lactation and subsequent services per conception was essentially zero (rp = .01), whereas the genetic correlation was positive (rA = .37). Differences in services per conception associated with b o d y weight may be attributable partially to differences in thermoregulatory responses. Heavier cows have reduced ratios of surface to b o d y weight and, therefore, were more vulnerable to high enJournal of Dairy Science Vol. 68, No. 7, 1985
vironmental temperatures than smaller cows with higher rates of heat dissipation. Thus, care should be taken in selection for increased body weight to avoid possible associated decreases of fertility if results reported here are confirmed elsewhere. A host of other factors also may account for this association. We hypothesize that heavier cows may have a greater incidence of other problems (i.e., ketosis, milk fever, dystocia, fatty infiltration of the oviduct, etc.) that may compromise fertility. Phenotypic and genetic correlations between milk yield and body weight in this herd were low (Table 3). As suggested by McDaniel and Legates (15), milk yield can be improved without materially increasing body weight of dairy cattle. Because b o d y weight is heritable, selection for increased weight should result in larger cows with little or no correlated increase in milk yield. Because larger cows cost more to maintain, this clearly would not be a profitable selection scheme. Substantial genetic antagonism may exist between high milk yield and fertility as in this and other research relationship was positive between yield and services per conception. Body weight genetic correlation with services per conception likewise was positive. ACKNOWLEDGMENTS
The authors wish to acknowledge assistance of D. W. Webb and J. Moya, University of Florida, and to thank personnel of Bassett Bros. Dairy, Monticello, FL, for providing records and other financial and technical assistance.
PRODUCTION TECHNICAL NOTE
REFERENCES 1 Adkinson, R. W., C. J. Wilcox, and W. W. Thatcher. 1977. Effects o f sire of fetus upon subsequent production and days open of the dam. J. Dairy Sci. 60:1964. 2 Badinga, L., R. J. Collier, W. W. Thatcher, and C. J. Wilcox. 1985. Effects of climatic and management factors on conception rate o f dairy cattle in a subtropical environment. J. Dairy Sci. 68: 78. 3 Berger, P. J., R. D. Shanks, A. E. Freeman, and R. C. Laben. 1981. Genetic aspects o f milk yield and reproductive performance. J. Dairy Sci. 64: 114. 4 Carman, G. M. 1955. Interrelationships of milk production and breeding efficiency in dairy cows. J. Anita. Sci. 14:752. 5 Clark, R. D., and R. W. Touchberry. 1962. Effect of body weight and age at calving on milk production in Holstein cattle. J. Dairy Sci. 45:1500. 6 Dunbar, R. S., Jr., and C. R. Henderson. 1953. Heritability of fertility in dairy cattle. J. Dairy Sci. 36:1063. 7 Erb, R. E., and U. S. Ashworth. 1961. Relationships between age, body weight, and yield of dairy cows. J. Dairy Sci. 44:515. 8 Everett, R. W., D. V. Armstrong, and L. J. Boyd. 1966. Genetic relationship between production and breeding efficiency. J. Dairy Sci. 49:879. 9 Fonseca, F. A., J. H. Britt, B. T. McDaniel, J. C. Wilk, and A. H. Rakes. 1983. Reproductive traits of Holsteins and Jerseys. Effects of age, milk yield, and clinical abnormalities on involution of cervix and uterus, ovulation, estrous cycles, detection of estrus, conception rate, and days open. J. Dairy Sci. 66:1128. 10 Gwazdauskas, F. C., C. J. Wilcox, and W. W. Thatcher. 1975. Environmental and managemental factors affecting conception rate in a subtropical climate. J. Dairy Sci. 58:88. 11 Harvey, W. R. 1977. Instructions for use of least squares and maximum likelihood: general purpose program. Mimeo., Ohio State Univ., Columbus. 12 Inskeep, E. K., W. J. Tyler, and L. E. Casida. 1961. Hereditary variation in conception rate of HolsteinFriesian cattle. J. Dairy S ci. 44:1857. 13 Maijala, K. 1976. Possibilities of improving fertility
14 15
16
17
18
19
20
21
22
23 24
25
1831
in cattle by selection. World Rev. Anita. Prod. 11: 69. Martin, T. G. 1978. Reproduction and calving traits. Ch. 2.4 in Large dairy herd management. Univ. Presses Florida, Gainesville. McDaniel, B. T., and J. E. Legates. 1965. Associations between body weight predicted from heart girth and production. J. Dairy Sci. 48:947. Miller, R. H., and L. D. McGilliard. 1959. Relations between weight at first calving and milk production during the first lactation. J. Dairy Sci. 42: 1932. Norman, H. D., and R. L. Powell. 1978, Genetic and economic change attained and possible. Ch. 3.3 in Large dairy herd management. Univ. Presses Florida, Gainesville. Shanks, R. D., A. E. Freeman, and P. J. Berger. 1979. Relationship of reproductive factors with interval and rate of conception. J. Dairy Sci. 62: 74. Shanks, R. D., A. E. Freeman, P. L. Berger, and D. H. Kelley, 1978. Effect of selection for milk production on reproductive and general health of the dairy cow, J. Dairy Sci. 61:1765. Smith, J. W., and J. E. Legates. 1962. Relation of days open and days dry to lactation milk and fat yields. J. Dairy Sci. 45:1192. Spalding, R. W., R. W. Everett, and R. H. Foote. 1975. Fertility in New York artificially inseminated Holstein herds in dairy herd improvement. J. Dairy Sci. 58:718. Stott, G. H. 1961. Female and breed associated with seasonal variation in dairy cattle. J. Dairy Sci. 44:1698. Whitmore, H. L., W. J. Tyler, and L. E. Casida. 1974. Effects of early postpartum breeding in dairy cattle. J. Anim. Sci. 38:339. Wilcox, C. J. 1978. Genetic considerations of economic importance: milk yield, composition and quality. Ch. 2.1 in Large dairy herd management. Univ. Presses Florida, Gainesville. Worstell, D. M., and S. Brody. 1953. Environmental physiology. XX. Comparative physiological reactions of European and Indian cattle to changing temperatures. Missouri Agric. Exp. Stn. Bull. 515:1.
Journal of Dairy Science Vol. 68, No. 7, 1985