J. Anim. Sci. 2003. 81:1693-1699
© 2003 American Society of Animal Science
Evaluation of age of dam effects on maternal performance of multilactation daughters from high- and low-milk EPD sires at three locations in the southern United States1
J. F. Baker*,2 and
M. E. Boyd
* Department of Animal and Dairy Science, the University of Georgia, Tifton 31793 and
and
Department of Animal and Dairy Sciences, Mississippi State University, Mississippi State 39762
2 Correspondence:
P.O. Box 748 (phone: 229-386-3364; fax: 229-386-3219; E-mail:
jfbaker{at}tifton.uga.edu).
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Abstract
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Angus bulls (n = 16) selected for either high- or low-milk EPD but similar growth EPD were mated within location at random to Angus cows. Daughters were bred to calve at 2 yr of age and annually until 6 yr of age. Milk yield was measured four times during lactation with a portable milking machine to estimate 12-h milk yield. Milk was collected for analysis of the percentage of fat and protein. A mixed model procedure was used to analyze the weaning weight, milk yield, and milk component data. The model for weaning weight included location, genetic line of sire, gender of calf, and age of dam. Calf age at weaning was used as a covariate. The model for the milk yield and components included location, genetic line of sire, gender of calf, period, and age of dam. Random effects for all models included sire of dam nested within line, sire of calf, and year. Genetic line was a significant source of variation for milk yield (P < 0.01) and weaning weight (P < 0.01) but not for percentage of fat or protein. Location was significant for milk yield (P < 0.01), fat (P < 0.01), protein (P < 0.01), and weaning weight (P < 0.01). The interaction of line with location was not significant except for percentage of protein (P < 0.01). Age of dam was significant for milk yield (P < 0.01), weaning weight (P < 0.01), and percentage of protein (P < 0.01), but not for percentage of fat (P = 0.29). Line difference for mean weaning weight was 18.1 kg, which is similar to the difference between lines for milk EPD (19 kg). Weaning weights from high-milk EPD line daughters were heavier (P < 0.01) than low-milk EPD line daughters at each age of dam evaluated. Cows nursed by males had higher milk yields (4.33 kg/12 h) than cows nursed by heifers (4.0 kg/12 h). The difference in yields for gender was significant for 2-, 3-, and 5-yr-old cows, but not for 4- (P < 0.052) and 6-yr old (P < 0.15) cows. Correlation coefficients between weaning weight and weaning EPD, milk EPD, and total maternal EPD were greater than zero (P < 0.01) (0.76, 0.65, and 0.89, respectively). Daughters of sires with high-milk EPD produced more milk at each age and weaned heavier calves than daughters of sires with low-milk EPD. These results confirm the value of milk EPD for improvement of weaning weights in beef cattle and also validate age of dam effects on milk yield and the associated effects on weaning weights.
Key Words: Beef Cattle • Milk Yield • Predicted Difference • Progeny Testing
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Introduction
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Beef cattle breed associations and the Beef Improvement Federation Guidelines (1996) recommend adjusting the weaning weight of calves depending upon the age of dam. It is generally assumed that as dams mature, their milk yield also increases (Gregory et al., 1992; van Oijen et al., 1993). Minick et al. (2001) reported that dam age within year was associated with calf weight, but in their study, age of dam did not affect milk production. However, they also indicated that the range of dam ages was limited to 3-, 4-, and 5-yr-old cows. Other studies have reported the importance of milk yield on calf growth but often with only one age of dam (Kress et al., 1990; Freetly and Cundiff, 1998) or without reporting age of dam effects on milk production (Diaz et al., 1992).
Management effects significantly influence milk yield (Buskirk et al., 1995; Freetly and Cundiff, 1998), but often these studies have not been able to consider sire of dam EPD or have been limited to single years or locations. Baker et al. (2003) reported significant genetic line and location effects on milk yield but only reported first lactation records. This study measured milk yield, milk components, and weaning weights from multiple lactations of two genetic lines of female represented by daughters of Angus sires selected for either high- or low-milk EPD (mEPD) at three locations in the southeastern United States. The objectives were to: 1) evaluate age of dam effects for 12-h estimated milk production, 2) evaluate age of dam effects for weaning weights for two genetic lines, 3) estimate correlation coefficients between milk variables, weaning weights, and sire EPD, and 4) determine the importance of interactions between genetic line, location and age of dam for milk yield, milk components, and weaning weight.
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Materials and Methods
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Records for active sires from the American Angus Association were used to rank bulls for both the highest and lowest milk EPD. Nine high- and nine low-milk EPD sires were selected over a 5-yr period with accuracy values greater than 0.6. Secondary selection pressure was also included to minimize differences in growth EPD for birth, weaning, and yearling traits. Mean EPD for the bulls when selected are reported in Table 1
along with the differences between means. Mean EPD weighted by the number of generation one (G1) daughter records are also presented in Table 1. Accuracy values for the EPD were all greater than 0.6 at the time of selection and current accuracy values are greater than 0.86 with one exception; the accuracy for yearling weight for one low-mEPD bull is still only 0.80. The difference between the high and low weighted averages for milk EPD would be the best estimate for the differences in weaning weight between offspring from daughters of the two lines.
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Table 1. Unweighted and weighted EPD (kg) for high- and low-milk EPD lines based on the number of daughters milked within line
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There were three distinct locations where G1 daughters were produced: two locations in Georgia (GA1 and GA2) and one in Mississippi (MS). Location effects were not quantified but do represent different nutritional and management levels. It is known that the three locations have different forages and that management of the forages would vary. The breeding/calving seasons were similar for the three locations. Calving began in late December in Georgia, late January in Mississippi and was finished by early March each year at all locations.
Location effects also included genetic differences in the generation 0 (G0) cows that were the result of previous breeding decisions at each location. There was no known genetic tie between G0 cows at Mississippi and Georgia. However, the G0 at the two Georgia locations would be expected to have similarity in their pedigrees due to historical use of sires at each location. Cows at one Georgia location were registered, but at the other location were not registered and were considered purebred. The G0 cows at each location were randomly mated by artificial insemination to either a high or low mEPD sire. The EPD for G0 dams were not used in the mating decisions. Therefore, it was assumed that G0 dams represented a random sample of Angus females that were available and sires from the high and low lines each had a random sample of potential mates. The list of sires within each line, the number of locations milking daughters from each sire, the year in which daughters were milked, and the number of milk records per year are presented in Table 2.
Initial matings were in 1991 and continued for four additional years. Each location determined how many G0 cows to breed for the project. The research protocol and facilities were approved by the Institutional Animal Care and Use Committees for each state university. Generation 1 heifers from the project sires were reared and managed together at each location. Heifers were developed according to traditional management practices at each location with the stipulation that all G1 heifers would be bred to have their first calf at approximately 2 yr of age. Initial G1 heifer mating was by AI to a common sire selected to represent an average Angus sire unrelated to the sires within the high and low lines. A second unrelated sire was selected to use for breeding to the G1 daughters beginning with those born in yr 3 of the project. Natural-service bulls were also used at each location following the AI period, the length of which varied by location. The AI sires with accuracy values greater than 0.6 for growth and milk EPD were selected from commercial sources.
Procedures for estimation of milk yield were standardized across locations. Each G1 cow was milked four times during each lactation. The first milking date occurred when calf age ranged between 45 and 75 d of age. The fourth milking occurred just prior to weaning. Second and third milking dates were determined to create approximate equal intervals between the first and fourth. The day prior to milking, the cow and her calf (generation 2, G2) were weighed and separated at approximately 1300. Pairs were placed together again at about 2000 for approximately 30 min. The cows and calves had access to water and grass or hay while separated. The next day, milk yield was determined by use of a portable milking machine after the overnight (12 h) separation from their calves. Approximately 10 min prior to attachment of the milking machine, each cow was injected (i.m.) with 2 mL (10mg/mL) of acepromazine. The udder was cleaned if necessary to remove excess dirt. Immediately prior to attachment of the milking unit, the cow received 100 USP units of oxytocin (i.v.) to facilitate milk flow and extraction. When milk flow ceased or appeared to be finished, the machine was removed and the teats were dipped with a standard dairy teat dip. Milk was weighed and approximately 100-mL samples were obtained and preserved with 2-bromo-2-nitropropane-1,3-diol tablets. Milk samples were submitted to Dairy Herd Improvement Association laboratories appropriate for each location for the percentage of protein and fat analyses.
Data collected for analyses included calving date, periodic body weights, milk yield, milk fat percentage, and milk protein percentage. Mixed model procedures (SAS Version 8.0, SAS Inst., Inc., Cary, NC) were used to analyze the data. The model for weaning weight was a repeated records model with subject being individual G1 dams and included fixed effects of mEPD line (high or low), location (GA1, GA2, or MS), age of dam (2, 3, 4, 5, or 6 yr), and G2 gender of calf (male or female). The G2 calf age at weaning was used as a covariate (mean weaning age = 208 d). Random effects included in the analysis of weaning weight included year, sire of G1 dam nested within mEPD line, and sire of G2 calf nested within year. Two-factor interactions included line x location, line x age of dam, and gender (male or female) x age of dam. None of the locations used creep feed nor were any calves implanted. One location in Georgia castrated bull calves at or near birth; the other two locations reared bull calves to weaning as intact males. The model for milk yield, milk fat percentage, and milk protein percentage was a repeated records model, with subject being individual G1 dams nested within line and year. The model included the following as fixed effects: mEPD line, location, gender of calf, age of dam, and milk yield period (1st, 2nd, 3rd, or 4th). The two-factor interactions included line x location, line x period, period x location, line x age of dam, and calf gender x age of dam. The postpartum interval at each measurement period was included as a covariate. The overall simple mean postpartum intervals were 56, 101, 156, and 198 d for the four measurements. Random effects included in the analysis of the milk variables included year, sire of G1 dam nested within mEPD line, and sire of G2 calf nested within year. Estimation method was REML, with a heterogeneous compound symmetry structure for the variance/covariance components. Additional analyses included estimation of correlation coefficients among selected variables using PROC CORR SAS procedures. To generate the coefficients, the model presented earlier was modified to include the sire nested within line as a fixed effect and the option of least squares means for sire. Therefore, the correlation coefficient is the measure of relationship between a sire’s EPD and the least squares means for milk yield, total milk yield, and weaning weights. Pearson correlation coefficients were estimated for several pairs of variables. In addition to the variables measured directly or obtained from American Angus Association database of EPD an estimate of total milk yield was generated. The total milk yield was calculated as the sum of the 12-h yield at each period multiplied by the number of days postpartum between periods. Therefore, a part-whole relationship exists between 12-h yield and total milk yield.
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Results and Discussion
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Milk Yield Analysis
Line and location were significant sources of variation for 12-h milk yield. The interactions of line x location and line x period were not significant. The interaction of line x age of dam was important (P = 0.031). Calf gender and age of dam were both significant sources of variation. The calf gender x age of dam interaction was not significant. Yield least squares means for line are presented in Table 3. Location and age of dam least squares means for 12-h milk yield are presented in Table 4. High genetic line daughters produced significantly more milk at each location than low genetic line daughters. The three locations were also significantly different for yield. Daughters in Mississippi had higher yields (P < 0.05) than daughters at the two locations in Georgia within both the high and low genetic lines.
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Table 3. Least squares means and standard errors for 12-h milk yield, percentage of milk fat, percentage of milk protein, and weaning weight by milk EPD line
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Milk yields increased significantly as the daughters became older (Table 4
) for both the high and low genetic lines. The interaction of line x age of dam is the result of a change in magnitude and not a change in rank. All five ages were significantly different for the high genetic line, whereas the 5- and 6-yr-old daughters were similar (P < 0.147) for the low genetic line. Gregory et al. (1992) reported a significant interaction of age of cow x stage of lactation. In the current study, the two-factor interaction of measurement period x age of dam was not significant with cow ages ranging from 2 to 6 yr. In the study by Gregory et al. (1992), the 3- and 4-yr-old cows had a greater decline in 12-h yield than cows more than 5 yr old. They did not indicate how much older than 5 yr the cows were, but approximately 50% of their records were in this category.
Daughters nursed by male calves had higher yields than daughters nursed by heifer calves (P < 0.01). Minick et al. (2001) also reported a significant calf gender effect on milk yield, but only for two of the seven monthly measurements. Least squares means and standard errors for calf gender x age of dam are presented in Table 5. Yields increased significantly as the dams became older for both males and females. Diaz et al. (1992) reported significant differences between high and low maternal sire groups that also included high and low yearling growth lines in a 2 x 2 factorial design. Kress et al. (1990) measured yields only on 4-yr-old dams and reported no differences associated with gender of calf nursing the cow. They did indicate that a difference in yield at 40 and 130 d of lactation approached the significance associated with calf gender. In their study, yield dropped further with dams nursing heifer calves than with steer calves (P > 0.05).
Measurement period was not a significant source of variation for 12-h milk yield. In contrast to this result, other studies have shown a decline in yield when cows were milked several times within a lactation (Buskirk et al., 1995; Freetly and Cundiff, 1998; Minick et al., 2001). Beal et al. (1990) reported high positive correlation coefficients between milk yield measures within a lactation that ranged from 0.84 to 0.92. They suggested this indicates that a single-yield measurement can characterize milk production levels throughout a lactation. In the current study, it was observed that the regression of postpartum interval was significantly greater than zero for milk yield. The statistical model used for 12-h yield was a repeated records model; therefore, the least squares means for the main effects are adjusted for the multiple measurement periods, as well as postpartum interval. The interaction of genetic line x period was not significant.
Weaning Weight Analysis
Genetic line, location, calf gender, and age of dam were significant sources of variation for calf weaning weight. The two-factor interactions of line x location, gender x age of dam, and line x age of dam were not important (P > 0.333). Table 3 presents the overall genetic line least squares means for weaning weight. The difference from Table 3 is 18.1 kg, which is the same as the predicted difference on Table 2 for weighted EPD from when the bulls were selected. The predicted difference between the lines from Table 2 was 16.78 kg for the EPD weighted by the number of daughters milked and based on current (Spring 2002) EPD. The GA1 location had heavier weaning weights than the other two locations, which were similar (Table 6). The differences between lines at each location were similar as was expected since the interaction was not significant. The location for the heavier weaning weights did not correspond to the location with the highest 12-h yield. Fiss and Wilton (1993) reported a linear association between milk yield and gain to weaning. However, they also reported a significantly higher regression coefficient for gain to weaning on milk yield in their low-milk system compared with their high-milk system. In an earlier study (Neville, 1962), it was reported that nutritional regime of the dam influenced milk yield and also net partial efficiency of calf growth. Neville (1962) estimated that under two of the nutritional programs, it took 5.7 kg of milk to increase calf weight by 0.45 kg, and it took 10.7 kg of milk to achieve the same 0.45 kg of calf gain.
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Table 6. Least squares means and standard errors for weaning weight (kg) by milk EPD line within location and age of dam
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Age of dam least squares means for weaning weights and standard errors for both genetic lines are presented in Table 6. Calves from high milk EPD daughters were significantly heavier than calves from low milk EPD daughters at each age of dam measured in this study. Age of dam effects were significant for weaning weight and weights increased as the dams became older. Average weaning weights from 5- and 6-yr-old dams exceeded weights from 2-yr-old dams by 37 kg for the high genetic line and 35 kg for low genetic line. These differences would appear to be in line with the standard adjustment factors outlined for Angus cattle in the Beef Improvement Federation Guidelines (1996). In the study by Minick et al. (2001), cow age affected calf weights in 3 mo, but age of dam was not significant for milk yield. Their cow ages were limited to 3, 4, and 5 yr old. Van Oijen et al. (1993) evaluated 2-, 3-, and 4-yr-old dams across three levels of genetic potential for milk production and reported significant age effects for production.
Gender of calf was significant for weaning weight and the least squares means and standard errors for male and female calves are presented in Table 7. The overall advantage for male calves was 16 kg. Weaning weights for males exceeded females at each age except for 6-yr-old dams (P < 0.15).
Milk Component Analyses
Genetic line was not important for percentage of milk fat (P > 0.683) or protein (P > 0.333). Least squares means and standard errors for line are presented in Table 1. However, location was significant for both percentage milk fat and protein. The two-factor interaction of line x location for percentage of milk protein was significant and approached being significant for percentage of milk fat (P = 0.060). Milk fat least squares means for Mississippi were larger than for either Georgia location. Differences between lines within each location were small; means for the GA1 location were the most different. This is the reason why the interaction approached being significant for milk fat. Mississippi had the largest mean milk fat, followed by GA1 and GA2 (3.36 ± 0.04, 3.23 ± 0.04, and 3.11 ± 0.04, respectively). The significant interaction for line and location is the result of the Mississippi low genetic line mean being greater than the Mississippi high genetic line (3.43 ± 0.05 and 3.28 ± 0.05, respectively), whereas the lines within the two Georgia locations were not different. The percentage of milk protein means was different for all three locations. The smaller standard errors for protein means compared with the fat mean standard errors in this study were also seen in a report of just first lactation records at six locations (Baker et.al. 2003). In a study by Buskirk et al. (1995), the standard errors of the least squares means for fat and protein percentage were similar in magnitude, suggesting more similar variation than in this study. Diaz et. al., (1992) reported standard deviations, and in their data set, the standard deviation for percentage of protein was much smaller than the standard deviation for percentage of fat.
Measures of Association Between Variables
Table 8 presents correlation coefficients between a sire’s EPD and yield estimates. The negative correlation coefficients in Table 8 between wean EPD and the other variables are unusual, but not unexpected, due to the experimental design. In the selection of sires, attempts were made to minimize the differences in the growth EPD between the genetic lines. The low-milk EPD genetic lines sires had a slight advantage for average growth EPD compared with the high-milk EPD line. Therefore, the negative correlation coefficients were the result of the experimental procedures for sire selection.
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Table 8. Pearson correlation coefficients between weaning EPD, milk EPD, total maternal EPD, and least squares means for total milk yield, and weaning weight
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The magnitudes of the other coefficients in Table 8 are of more interest. A strong positive relationship was observed between the milk EPD and 12-h milk yield, total milk yield, and weaning weight. The correlation coefficient between weaning weight and the two estimates of yield are also significantly greater than zero (Table 8). Buskirk et al. (1995) reported positive correlation coefficients between milk yield and calf weights at four ages. The correlation among breed group means for 12-h milk yield and 200-d weight was 0.91 in a study by Gregory et al. (1992). Table 9 presents correlation coefficients between 12-h milk yields for each period with weaning weight and with yield at the other periods. The overall association between yield and weaning weight was higher in magnitude (Table 8) than in the study by Buskirk et al. (1995). However, the relationships between weaning weight and yields at each milking period (Table 9) were similar in magnitude to the values reported by Buskirk et al. (1995), which ranged from 0.21 to 0.49 for three measurements before weaning. These correlation coefficient values are similar to those of Marston et al. (1992), which ranged from 0.30 to 0.47 when considering several herds together for both Angus and Simmental breeds. The composite milk yield estimates and weaning weight are more highly correlated (Table 8) than are weaning weight and any individual period yield estimate (Table 9). However, the correlation coefficients of weaning weight and yield at each period are all positive and significantly greater than zero. In addition yield at any one period is also positively associated with yield at other periods and these coefficients range from 0.71 to 0.83 (Table 9). Marshall and Long (1993) reported rather low, positive residual correlation coefficients between milk yield, weaning weight, milk EPD, and total maternal EPD.
The correlation coefficients between the milk components and weaning weight, as presented in Table 10, are not as informative as between weaning weight and milk yield. The relationships between weaning weight and the components vary in magnitude and sign with many not different from zero. The relationships between the components at each period are mostly positive in sign, but often not different than zero.
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Implications
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Results validated that expected progeny differences were observed for weaning weights for dams through 6 yr of age. The genetic line x location interaction was not significant for the four traits evaluated: 12-h milk yield, weaning weight, percentage of milk fat, or percentage of milk protein. This lack of significance supports the use of expected progeny differences in different environments and herds of different genetic merit. These results were generated from a designed study with good linkage of sires across locations. The difference in weaning weights predicted by milk expected progeny differences (19 kg) was very close to those measured in the calves (18.1 kg) after properly accounting for fixed and random effects.
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Footnotes
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1 Contribution to multistate project S-243. 
Received for publication May 31, 2002.
Accepted for publication February 26, 2003.
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Literature Cited
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Baker, J. F., M. E. Boyd, A. H. Brown, D. E. Franke, and C. E. Thompson. 2003. Evaluation of maternal performance of daughters from high and low milk EPD sires. J. Anim. Sci. 81:(In press).
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BIF. 1996. Guidelines for Uniform Beef Improvement Programs. 7th ed. Beef Improv. Fed., Northwest Research Extension Center, Colby, KS.
Buskirk, D. D., D. B. Faulker, and F. A. Ireland. 1995. Increased postweaning gain of beef heifers enhances fertility and milk production. J. Anim. Sci. 73:937–946.[Abstract]
Diaz, C., D. R. Notter, and W. E. Beal. 1992. Relationship between milk expected progeny differences of Polled Hereford sires and actual milk production of their crossbred daughters. J. Anim. Sci. 70:396–402.[Abstract]
Fiss, C. F., and J. W. Wilton. 1993. Contribution of breed, cow weight, and milk yield to the preweaning, feedlot, and carcass traits of calves in three beef breeding systems. J. Anim. Sci. 71:2874–2884.[Abstract]
Freetly, H. C., and L. V. Cundiff. 1998. Reproductive performance, calf growth, and milk production of first-calf heifers sired by seven breeds and raised on different levels of nutrition. J. Anim. Sci. 76:1513–1522.[Abstract/Free Full Text]
Gregory, K. E., L. V. Cundiff, and R. M. Koch. 1992. Effects of breed and retained heterosis on milk yield and 200-day weight in advanced generations of composite populations of beef cattle. J. Anim. Sci. 70:2366–2372.[Abstract]
Kress, D. D., D. E. Doornbos, and D. C. Anderson. 1990. Performance of crosses among Hereford, Angus, and Simmental cattle with different levels of Simmental breeding: V. Calf production, milk production, and reproduction of three- to eight-year-old dams. J. Anim. Sci. 68:1910–1921.[Abstract]
Marshall, D. M., and M. B. Long. 1993. Relationship of beef sire expected progeny difference to maternal performance of crossbred daughters. J. Anim. Sci. 71:2371–2374.[Abstract]
Marston, T. T., D. D. Simms, R. R. Schalles, K. O. Zoellner, L. C. Martin, and G. M. Fink. 1992. Relationship of milk production, milk expected progeny difference, and calf weaning weight in Angus and Simmental cow-calf pairs. J. Anim. Sci. 70:3304–3310.[Abstract]
Minick, J. A., D. S. Buchanan, and S. D. Rupert. 2001. Milk production of crossbred daughters of high- and low-milk EPD Angus and Hereford bulls. J. Anim. Sci. 79:1386–1393.[Abstract/Free Full Text]
Neville, W. E., Jr. 1962. Influence of dam’s milk production and other factors in 120- and 240-day weights of Hereford calves. J. Anim. Sci. 21:315–320.[Abstract/Free Full Text]
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Abstract
Introduction
