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Factors affecting gestation length in dairy cattle
THERIOGENOLOGY
FACTORS AFFECTING GESTATION LENGTH IN DAIRY CATTLE R.H. Foote Department of Animal Science, Cornell University 14853 Ithaca, New York Received for publication:
11/20/80
ABSTRACT Data were collected through a special record keeping system for 2,254 herds that used artificial insemination, 37% of which were enrolled in Dairy Herd Improvement Associations. Information was available on a total of 35,162 gestations, consisting of 24,367 Holstein, 5,849 Guernsey, 2,872 Jersey, 1,667 Ayrshire and 407 Brown Swiss pregnancies resulting in single births plus 930 multiple births. Breeds, twinning, sex of calf, parity of the cow and time of day on which estrus occurred were associated with differences in gestation length. Month of insemination had no effect. Mates and sires of the cows both affected gestation length slightly, but significantly. Selection of sires for high milk production of their daughters over a 23-year period appears to have had little, if any, effect on gestation length, as the mean gestation length has increased only one day, from 278 to 279 days for Holsteins during that time. INTRODUCTION The gestation length in dairy cattle, and factors affecting it, have been reviewed extensively (1, 2, 3, 4, 5, 6). Breeds, parity of dam, sex of the calf and twinning are among factors found consistently to affect gestation length. ,Much of the previously published information was derived from studies of herd books, experiment station farms and private herds that keep records for many years. The original objective of the present study was to obtain a data set that was more representative of the dairy cattle population enrolled in artificial insemination (AI), but not necessarily recorded in other programs. An extensive data set which is believed to be unique,was obtained (7). More recently, a selected profile was obtained (8) on cows in AI in the same geographical area after several generations of selection for high milk production, but where there is believed to have been no direct selection for gestation length. Gestation lengths in these two studies were compared. Dairymen are concerned with premature births or prolonged gestation, because these affect calf losses and difficult births. Additional interest was generated by recent reports (9, 10)
The author is grateful to Eastern A.I. Coop., Inc.for partial financial support, to Dr. L.H. Wade11 for statistical assistance and to H. Polan for manuscript preparation.
JUNE 1981 VOL. 15 NO. 6
553
THERIOGENOLOGY
that the mate of the cow (sire of the fetus), as well as sire of the cow, may affect both the current and subsequent lactation. These effects could be related, in part, to gestation length. MATERIALS AND METHODS Details of the population which was sampled has been described previously (7). Basically, we aided 2,254 herd owners in New York State and surrounding areas to keep breeding and calving records on all cows for 1 year, 1951-52. This was at a time during which it was possible to maintain much more control over the time semen was used for breeding, because the unfrozen semen had to be used within a few days of collection. Also, all inseminations were performed by professional inseminators, with the services reported to the semen producing organization. Currently, similar studies would be virtually impossible. The sample was comprised of 69% Holsteins, 17% Guernseys, 8% Jerseys, 5% Ayshires and 1% Brown Swiss. Overall, 37% of the herds were enrolled in Dairy Herd Improvement, and 48% of the animals were registered. A substantial proportion of animals were available in this project from herds which normally would not be studied because of lack of records. Pertinent data potentially available on each cow included identification, breeding information, time of insemination, abortion or calving data, sex(es) and viability of progeny, whether first calf heifer or not, and sire and dam. It was remarkable that useable information was obtained on over 95% of the cows in the herds that continued for the full year. A few herds were eliminated, because they discontinued operating during the year. Analyses of variance was done within breeds to test each of the main effects, which are given in Tables 1 and 2, along with month of conception. The statistical model which was used to estimate variance components included the mean, assumed random effects of sire of cow, mate of cow (sire of fetus) and herd, fixed effects of sex and viability of calf, and certain first order interactions and errors. Paternal half-sib information was used to calculate genetic relationships. RESULTS AND DISCUSSION . General. Mean gestation lengths (Table 1) generally agree with previous reports (1, 2, 3, 4, 5, 6, 11). Holsteins and Jerseys had the shortest gestation, and Brown Swiss had the longest (Pc.01). Male calves were carried 1 day longer than females, and multiparous cows and cows which were detected to be in estrus in the morning appeared to carry calves 1 day longer than their counterparts (Pc.01). The latter difference probably reflects the fact that most cows which were seen in estrus in the morning are inseminated and conceive the same day. Most "P . M . ' cows are inseminated and conceive the next day. The P.M. cows may tend to be inseminated later in estrus, nearer to the time of ovulation. Twins were carried, on the average, eight days less than the time for single births , and the 7-day difference between twins and single births for Holsteins was significantly different (PC .Ol) from the g-day difference for Channel Island breeds. Also, there
554
JUNE
1981 VOL. 15 NO. 6
THERIOGENOLOGY
TABLE 1. Gestation Length Associated With Breed and Other Variables
Variable
Number of cow&
Gestation length (days)
Single births Holstein
24,367
278
Guernsey
5,849
282
Jersey
2,872
278
Ayrshire
1,667
280
407
286
35,162
279
Holstein
746
271
Guernsey
101
272
Jersey
45
269
Ayrshire
30
276
8
280
930
271
Female
17,428
280
Male
17,995
281
7,046
277
30,635
278
A.M.
15,238
278
P.M.
5,721
277
Brown Swiss Overall Twins
Brown Swiss Overall Sex of calfw
Age of cow First calf Multiparous Time of heat detection
/ b!
Number of animals with complete information in the several categories differed, so totals differ. Single births only.
JUNE 1981 VOL. 15 NO. 6
555
THERIOGENOLOGY
.FIG. 1.
1600
Frequency distribution of gestation lengths for male and female single births averaged over all breeds. The dual peaks are due primarily to the difference in gestation lengths between Holsteins and Guernseys.
-
lAOO-
*
1200-
I, P !z 5
lOOO-
F 2 z 0
aoo-
8 4 2
600
-
z
GESTATION
LENGTH
[days)
was a significant interaction (Px.01) between the age of cow and the incidence of twinning. There were no breed interactions with sex of calf or age of cow relative to gestation length (P>.O5). The distribution of gestation lengths is shown in Figure 1. Breeds have been combined, because there were too few observations per gestation length by sex of calf in any breed, with the exception of
556
JUNE 1981 VOL. 15 NO. 6
THERIOGENOLOGY
Holsteins, to report each breed separately. Females consistently wer, carried -in utero for a shorter period of time than males. Proportions of the two sexes which were reported outside the range of 265 to 295 days tended to be erratic. This was due to small numbers of progeny and a higher mortality rate. Sex of dead calves was less accurately reported, as sometimes these data were reported with the accompaniment of a question mark. The proportion of calves which were born dead, after different periods of gestation, are in Table 2. Again, for brevity, breeds are combined. Holstein cattle comprise about 70% of the data. Based upon TABLE 2. Viability of Calves After Different Lengths of Pregnancy?./. Length of pregnancy (days)
Number of calves
Born dead (X)
810
6.5
240-249
227
27
250-259
405
22
260-269
1,108
10
270-279
14,915
2.0
280-289
17,542
1.7
290-299
1,397
3.1
393
5.3
36,797
4.0
~240
>300 Overall
a/ Includes single
and multiple births averaged over the five breeds.
the low mortality rate, it would appear that gestation lengths from 270 to 289 should be considered normal. To include most Brown Swiss, the upper limit should be extended to 299. Then the range from 270 to 299 days includes 92% of all births. The percentages of all calves by breeds which were born dead were: for Holsteins, 3.9; Guernseys, 4.3; Jerseys, 4.5; Ayrshire, 4.1 (P>.lO). Too few Brown Swiss calvings were available to provide a meaningful comparison. For single births, over all breeds, 2.5% of the heifer calves and 3.1% of the bull calves were born dead. In multiple births, at least one calf was dead at 12.9% of the parturitions. Season. This had no effect on gestation length, multiple births or abss (P>.lO). The gestation length over all breeds for single births, according to the month of conception, was 279 days in nine months 278 days in August and 280 days in January and May. This agrees
JWE
1981 VOL. 15 NO. 6
557
THERIOGENOLOGY
with several authors (4, 5), but disagrees with Brake1 et al. (2). -T Clearly, any severe environmental stress, which tended to induce premature calving, could be seasonal in nature and cause a seasonal effect, but there was no evidence in the present study that fetal attrition rates differed in different seasons. Mates of the cows. There were 46 Holstein, 26 Guernsey, 22 Jersey and 18 Ayrshire bulls in this category, which contributed to the fetal genotype. The mates of the cows accounted for 1.9, 1.8, 1.6 and 1.3% of the total variation in gestation length for Holsteins, Guernseys, Jerseys and Ayrshires, respectively. While this variance is small, it was larger than any other component. For example, the combined contribution of sex of calf, number of calves per birth, and viability of the calf to the total variation in gestation length in Holsteins was 0.9%. Siresof the COWS. The sire of the cow which was being inseminated also contributed to the variation in gestation length. For example, there were 43 Holstein sires, each with 40 or more daughters in the study. Mean gestation length for these groups of daughters (including all pregnancies) ranged from 273 to 279 days. Within Holsteins and Guernseys, sires of the cows contributed 1.2 and 1.8% of the total variance. This sire component is larger than it was for most other variables. The sample of paternal half-sib groups for the other breeds was too small to accurately estimate these variances. The present study reveals a consistent effect of the sire of the cow and of the sire of the fetus (mate of cow) on gestation length, as has been reported by others (1, 2, 3, 4, 6). Although the effects are smaller than reported by some, selection should result in a change in gestation length. The shortening of the gestation length to produce earlier calving could be of economic benefit in beef cattle. Smaller calves, as a result of shorter gestations, would reduce calving problems, especially in dairy heifers. The effect on optimal days open, days dry, pregnancy requirements, viability of the calf and milk production per day of life are among the factors which would have to be taken into account in an appropriate model in order to determine the potential economic benefit in selection for gestation length. The reported sire and mate effects on lactation through maternal or fetal routes (9, 10) may bear some relationship to the effects on gestation length. The average gestation length for artificially inseminated Holsteins in the two samples,which were collected from the same general geographical area, approximately 23 years apart (7, 8), were 278 and 279 days. During this time, there has been extensive use of AI in the area and intensive selection of sires for artificial breeding on the basis of milk production of their daughters. Thus, if intensive selection for milk production over several generations has had any substantial effect on gestation length, it has been offset by other factors. However, the slight trend toward longer gestation should be monitored. Longer gestations are associated with larger calves and more dystocia. This is a potential problem, particularly when trying to convince dairymen to breed more of their heifers by AI. It would seem desireable to identify high production sires that produce daughters with minimal calving problems as well as those bulls which sire calves 558
JUNE
1981 VOL. 15 NO. 6
associated with few calving problems. Studies of the genetic correlation between milk production and gestation length should be undertaken to determine if the selection for production would alter the gene pool that affects gestation length. REFERENCES
1.
Andersen, H., and Plum, M. Gestation length and birth weight in 48:1224-7235, (1965). cattle and buffaloes: a review, 3. Dairy Sci, -
2.
Brake?, W.J., Rife, D.C., and Shlisbury, S.M. Factors associated with the duration of gestation in dairy cattle. J. Dairy Sci. 35: 179-194. (1952).
3.
Everett, R.M., and Magee, W.T. Maternal ability and genetic ability of birth weight and gestation length. J. Dairy Sci. $&:957-961. (1965).
4.
Gianola, D., and Tyler, W.J. Influences on birth weight and gestation period of Holstein-Friesian cattle. J. Dairy Sci. -57:235-240. (1974).
5.
Jafar, S.M., Chapman, A.B., and Casida, L.E. Causes of variation in length of gestation in dairy cattle. J. Anim. Sci. 2:593-601. (1950).
6.
Touchberry, R.W. and Bereskin, B. Crossbreeding dairy cattle. I. Some effects of crossbreeding on the birth weight and gestation period of dairy cattle. J. Dairy Sci. 49:287-300. (1966).
7.
Foote, R.H. Reproductive performance and problems in New York dairy herds. New York State College of Agriculture and Life Sci. Search 8(2):1-18, (1978).
8.
Cady, R.A. and Van Vleck, L.D. Factors affecting twinning and effects of twinning in Holstein dairy cattle. J. Anim. Sci. 46:950-956. (1978). -
9.
Adkinson, R.N., Uifcox, C.J. and Thatcher, W.W. Effects of sire of fetus upon subsequent production and days open of the dam. J. Dairy Sci. -60:1964-1969. (1977).
10.
Johnson, L.P., and Van Vleck, L.D. Components of variance associated with service sire for milk yield and reproductive traits. J. Dairy Sci. g:754-759. (1979).
11.
Rutledge, J.J. (7975).
Twinning in cattle.
JUNE 1981 VOL. 1.5 NO. 6
J. Anim. Sci. 40:803-815.
559
FACTORS AFFECTING GESTATION LENGTH IN DAIRY CATTLE R.H. Foote Department of Animal Science, Cornell University 14853 Ithaca, New York Received for publication:
11/20/80
ABSTRACT Data were collected through a special record keeping system for 2,254 herds that used artificial insemination, 37% of which were enrolled in Dairy Herd Improvement Associations. Information was available on a total of 35,162 gestations, consisting of 24,367 Holstein, 5,849 Guernsey, 2,872 Jersey, 1,667 Ayrshire and 407 Brown Swiss pregnancies resulting in single births plus 930 multiple births. Breeds, twinning, sex of calf, parity of the cow and time of day on which estrus occurred were associated with differences in gestation length. Month of insemination had no effect. Mates and sires of the cows both affected gestation length slightly, but significantly. Selection of sires for high milk production of their daughters over a 23-year period appears to have had little, if any, effect on gestation length, as the mean gestation length has increased only one day, from 278 to 279 days for Holsteins during that time. INTRODUCTION The gestation length in dairy cattle, and factors affecting it, have been reviewed extensively (1, 2, 3, 4, 5, 6). Breeds, parity of dam, sex of the calf and twinning are among factors found consistently to affect gestation length. ,Much of the previously published information was derived from studies of herd books, experiment station farms and private herds that keep records for many years. The original objective of the present study was to obtain a data set that was more representative of the dairy cattle population enrolled in artificial insemination (AI), but not necessarily recorded in other programs. An extensive data set which is believed to be unique,was obtained (7). More recently, a selected profile was obtained (8) on cows in AI in the same geographical area after several generations of selection for high milk production, but where there is believed to have been no direct selection for gestation length. Gestation lengths in these two studies were compared. Dairymen are concerned with premature births or prolonged gestation, because these affect calf losses and difficult births. Additional interest was generated by recent reports (9, 10)
The author is grateful to Eastern A.I. Coop., Inc.for partial financial support, to Dr. L.H. Wade11 for statistical assistance and to H. Polan for manuscript preparation.
JUNE 1981 VOL. 15 NO. 6
553
THERIOGENOLOGY
that the mate of the cow (sire of the fetus), as well as sire of the cow, may affect both the current and subsequent lactation. These effects could be related, in part, to gestation length. MATERIALS AND METHODS Details of the population which was sampled has been described previously (7). Basically, we aided 2,254 herd owners in New York State and surrounding areas to keep breeding and calving records on all cows for 1 year, 1951-52. This was at a time during which it was possible to maintain much more control over the time semen was used for breeding, because the unfrozen semen had to be used within a few days of collection. Also, all inseminations were performed by professional inseminators, with the services reported to the semen producing organization. Currently, similar studies would be virtually impossible. The sample was comprised of 69% Holsteins, 17% Guernseys, 8% Jerseys, 5% Ayshires and 1% Brown Swiss. Overall, 37% of the herds were enrolled in Dairy Herd Improvement, and 48% of the animals were registered. A substantial proportion of animals were available in this project from herds which normally would not be studied because of lack of records. Pertinent data potentially available on each cow included identification, breeding information, time of insemination, abortion or calving data, sex(es) and viability of progeny, whether first calf heifer or not, and sire and dam. It was remarkable that useable information was obtained on over 95% of the cows in the herds that continued for the full year. A few herds were eliminated, because they discontinued operating during the year. Analyses of variance was done within breeds to test each of the main effects, which are given in Tables 1 and 2, along with month of conception. The statistical model which was used to estimate variance components included the mean, assumed random effects of sire of cow, mate of cow (sire of fetus) and herd, fixed effects of sex and viability of calf, and certain first order interactions and errors. Paternal half-sib information was used to calculate genetic relationships. RESULTS AND DISCUSSION . General. Mean gestation lengths (Table 1) generally agree with previous reports (1, 2, 3, 4, 5, 6, 11). Holsteins and Jerseys had the shortest gestation, and Brown Swiss had the longest (Pc.01). Male calves were carried 1 day longer than females, and multiparous cows and cows which were detected to be in estrus in the morning appeared to carry calves 1 day longer than their counterparts (Pc.01). The latter difference probably reflects the fact that most cows which were seen in estrus in the morning are inseminated and conceive the same day. Most "P . M . ' cows are inseminated and conceive the next day. The P.M. cows may tend to be inseminated later in estrus, nearer to the time of ovulation. Twins were carried, on the average, eight days less than the time for single births , and the 7-day difference between twins and single births for Holsteins was significantly different (PC .Ol) from the g-day difference for Channel Island breeds. Also, there
554
JUNE
1981 VOL. 15 NO. 6
THERIOGENOLOGY
TABLE 1. Gestation Length Associated With Breed and Other Variables
Variable
Number of cow&
Gestation length (days)
Single births Holstein
24,367
278
Guernsey
5,849
282
Jersey
2,872
278
Ayrshire
1,667
280
407
286
35,162
279
Holstein
746
271
Guernsey
101
272
Jersey
45
269
Ayrshire
30
276
8
280
930
271
Female
17,428
280
Male
17,995
281
7,046
277
30,635
278
A.M.
15,238
278
P.M.
5,721
277
Brown Swiss Overall Twins
Brown Swiss Overall Sex of calfw
Age of cow First calf Multiparous Time of heat detection
/ b!
Number of animals with complete information in the several categories differed, so totals differ. Single births only.
JUNE 1981 VOL. 15 NO. 6
555
THERIOGENOLOGY
.FIG. 1.
1600
Frequency distribution of gestation lengths for male and female single births averaged over all breeds. The dual peaks are due primarily to the difference in gestation lengths between Holsteins and Guernseys.
-
lAOO-
*
1200-
I, P !z 5
lOOO-
F 2 z 0
aoo-
8 4 2
600
-
z
GESTATION
LENGTH
[days)
was a significant interaction (Px.01) between the age of cow and the incidence of twinning. There were no breed interactions with sex of calf or age of cow relative to gestation length (P>.O5). The distribution of gestation lengths is shown in Figure 1. Breeds have been combined, because there were too few observations per gestation length by sex of calf in any breed, with the exception of
556
JUNE 1981 VOL. 15 NO. 6
THERIOGENOLOGY
Holsteins, to report each breed separately. Females consistently wer, carried -in utero for a shorter period of time than males. Proportions of the two sexes which were reported outside the range of 265 to 295 days tended to be erratic. This was due to small numbers of progeny and a higher mortality rate. Sex of dead calves was less accurately reported, as sometimes these data were reported with the accompaniment of a question mark. The proportion of calves which were born dead, after different periods of gestation, are in Table 2. Again, for brevity, breeds are combined. Holstein cattle comprise about 70% of the data. Based upon TABLE 2. Viability of Calves After Different Lengths of Pregnancy?./. Length of pregnancy (days)
Number of calves
Born dead (X)
810
6.5
240-249
227
27
250-259
405
22
260-269
1,108
10
270-279
14,915
2.0
280-289
17,542
1.7
290-299
1,397
3.1
393
5.3
36,797
4.0
~240
>300 Overall
a/ Includes single
and multiple births averaged over the five breeds.
the low mortality rate, it would appear that gestation lengths from 270 to 289 should be considered normal. To include most Brown Swiss, the upper limit should be extended to 299. Then the range from 270 to 299 days includes 92% of all births. The percentages of all calves by breeds which were born dead were: for Holsteins, 3.9; Guernseys, 4.3; Jerseys, 4.5; Ayrshire, 4.1 (P>.lO). Too few Brown Swiss calvings were available to provide a meaningful comparison. For single births, over all breeds, 2.5% of the heifer calves and 3.1% of the bull calves were born dead. In multiple births, at least one calf was dead at 12.9% of the parturitions. Season. This had no effect on gestation length, multiple births or abss (P>.lO). The gestation length over all breeds for single births, according to the month of conception, was 279 days in nine months 278 days in August and 280 days in January and May. This agrees
JWE
1981 VOL. 15 NO. 6
557
THERIOGENOLOGY
with several authors (4, 5), but disagrees with Brake1 et al. (2). -T Clearly, any severe environmental stress, which tended to induce premature calving, could be seasonal in nature and cause a seasonal effect, but there was no evidence in the present study that fetal attrition rates differed in different seasons. Mates of the cows. There were 46 Holstein, 26 Guernsey, 22 Jersey and 18 Ayrshire bulls in this category, which contributed to the fetal genotype. The mates of the cows accounted for 1.9, 1.8, 1.6 and 1.3% of the total variation in gestation length for Holsteins, Guernseys, Jerseys and Ayrshires, respectively. While this variance is small, it was larger than any other component. For example, the combined contribution of sex of calf, number of calves per birth, and viability of the calf to the total variation in gestation length in Holsteins was 0.9%. Siresof the COWS. The sire of the cow which was being inseminated also contributed to the variation in gestation length. For example, there were 43 Holstein sires, each with 40 or more daughters in the study. Mean gestation length for these groups of daughters (including all pregnancies) ranged from 273 to 279 days. Within Holsteins and Guernseys, sires of the cows contributed 1.2 and 1.8% of the total variance. This sire component is larger than it was for most other variables. The sample of paternal half-sib groups for the other breeds was too small to accurately estimate these variances. The present study reveals a consistent effect of the sire of the cow and of the sire of the fetus (mate of cow) on gestation length, as has been reported by others (1, 2, 3, 4, 6). Although the effects are smaller than reported by some, selection should result in a change in gestation length. The shortening of the gestation length to produce earlier calving could be of economic benefit in beef cattle. Smaller calves, as a result of shorter gestations, would reduce calving problems, especially in dairy heifers. The effect on optimal days open, days dry, pregnancy requirements, viability of the calf and milk production per day of life are among the factors which would have to be taken into account in an appropriate model in order to determine the potential economic benefit in selection for gestation length. The reported sire and mate effects on lactation through maternal or fetal routes (9, 10) may bear some relationship to the effects on gestation length. The average gestation length for artificially inseminated Holsteins in the two samples,which were collected from the same general geographical area, approximately 23 years apart (7, 8), were 278 and 279 days. During this time, there has been extensive use of AI in the area and intensive selection of sires for artificial breeding on the basis of milk production of their daughters. Thus, if intensive selection for milk production over several generations has had any substantial effect on gestation length, it has been offset by other factors. However, the slight trend toward longer gestation should be monitored. Longer gestations are associated with larger calves and more dystocia. This is a potential problem, particularly when trying to convince dairymen to breed more of their heifers by AI. It would seem desireable to identify high production sires that produce daughters with minimal calving problems as well as those bulls which sire calves 558
JUNE
1981 VOL. 15 NO. 6
associated with few calving problems. Studies of the genetic correlation between milk production and gestation length should be undertaken to determine if the selection for production would alter the gene pool that affects gestation length. REFERENCES
1.
Andersen, H., and Plum, M. Gestation length and birth weight in 48:1224-7235, (1965). cattle and buffaloes: a review, 3. Dairy Sci, -
2.
Brake?, W.J., Rife, D.C., and Shlisbury, S.M. Factors associated with the duration of gestation in dairy cattle. J. Dairy Sci. 35: 179-194. (1952).
3.
Everett, R.M., and Magee, W.T. Maternal ability and genetic ability of birth weight and gestation length. J. Dairy Sci. $&:957-961. (1965).
4.
Gianola, D., and Tyler, W.J. Influences on birth weight and gestation period of Holstein-Friesian cattle. J. Dairy Sci. -57:235-240. (1974).
5.
Jafar, S.M., Chapman, A.B., and Casida, L.E. Causes of variation in length of gestation in dairy cattle. J. Anim. Sci. 2:593-601. (1950).
6.
Touchberry, R.W. and Bereskin, B. Crossbreeding dairy cattle. I. Some effects of crossbreeding on the birth weight and gestation period of dairy cattle. J. Dairy Sci. 49:287-300. (1966).
7.
Foote, R.H. Reproductive performance and problems in New York dairy herds. New York State College of Agriculture and Life Sci. Search 8(2):1-18, (1978).
8.
Cady, R.A. and Van Vleck, L.D. Factors affecting twinning and effects of twinning in Holstein dairy cattle. J. Anim. Sci. 46:950-956. (1978). -
9.
Adkinson, R.N., Uifcox, C.J. and Thatcher, W.W. Effects of sire of fetus upon subsequent production and days open of the dam. J. Dairy Sci. -60:1964-1969. (1977).
10.
Johnson, L.P., and Van Vleck, L.D. Components of variance associated with service sire for milk yield and reproductive traits. J. Dairy Sci. g:754-759. (1979).
11.
Rutledge, J.J. (7975).
Twinning in cattle.
JUNE 1981 VOL. 1.5 NO. 6
J. Anim. Sci. 40:803-815.
559