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Growth Responses of 1- to 40-Day-Old Calves to Three Levels of Steer-oid® Implant1234
Growth Responses of 1- to 40-Day-Old Calves to Three Levels of Steer-oid® Implant1 ,2,3,4 J. J. Chewning, A. H. Brown, Jr., J. M. Phillips and W. C. Loe Department of Animal Sciences, University of Arkansas, Fayetteville 72701
and 198.7 kg, respectively) than calves implanted October 31 (.64 and 168.9 kg , respectively). Average daily gain and FWT of calves receiving 1/4 dose (.75 and 191.6 kg, respectively) and 112 dose (.73 and 188.3 kg, respectively) were higher (P<.05) than ADG and FWT of the contrOl. ~roup (.67 and 175.9 kg, respectively). Calves receiving a full dose did not differ from the control group. These results indicate that improvements in gains of young calves were achieved with doses lower than those levels of STEER-oid® usedin calves heavier than 180 kg.
Abstract
A field trial was conducted to compare growth responses of 1- to 40-day-old calves to three levels of STEER-oid® implant. A full dose of STEER-oid® consists of 8 pellets and contains 200 mg progesterone, U.S.P ., and 20 mg estradiol benzoate, U.S.P . Calves implanted with 1/4 dose (n:::::35), 1/2 dose (n:::::30) and a full dose (n:::::31) were compared with a control group (n:::::30) receiving no implant. Calves were implanted on October 31, i 984, and December 18, 1984, to meet age restrictions. Mean age at the end of the study was 209 days. Variation in average daily gain (ADG) and final weight (FWT) due to initial weight, initial age, age of dam, sex, sire breed (Charolais vs. Simmental), dam group [Zebu cross (ZX) vs. British and Continental cross ~(BGX}-1T implant date and rlQsagew~ey~dJ Initial weight and initial age influenced (P<.05) both ADG and FWT. Age of dam, sex and sire breed did not influence ADG or FWT. Calves from ZX dams had higher (P<.05) ADG and FWT (.74 and 189.8 kg, respectively) than calves from BeX dams (.68 and 177.8 kg, respectively). Calves implanted December 18 had higher (P< .05) ADG and FWT (.77
Introduction
The use of growth stimulants in the management of suckling-beef calves for more efficient production iswett-established:-&owtll-pr or Ilotin-gimplalits al e one of the most practical management tools used to increase the pounds of calf weaned per cow exposed to breeding. The practice of implanting has routinely been used to improve performance during the preweaning growth phase, postweaning growth phase and in feedlot cattle (Mader et aI., 1985; Fontenot and McClure, 1985; Gill et ai., 1986; Phillips et aI., 1986). Implanting for growth could perhaps be extended if implanting was practiced at an earlier age. This study involved the use of the implant STEER-oid® in suckling calves less than 40 days old. The objectives of the study were 1) to compare the growth responses of calves implanted at less than 40 days of age to three levels of STEER-oid® and 2) to evaluate the influence of sex, age of dam, sire breed and dam group on calf performance.
1 Published with the approval of the Director of the Arkansas Agricul.. tural Experiment Statio n s . . 2Supported in part by Boehnnger Ingelhelm An imal Health, Inc., SI. Joseph, MO. R • • 3STEER-oid®, Boehringer Ingelhelm Animal Health, Inc., SI. Joseph, MO, Ivy Laboratories, Inc., Overland Park, KS. . 4The mention of firm names or trade products does not Imply that they are endorsed by the Arkansas Agricultural Experiment Station over other firms or similar products not mentioned. Reviewed by W. M. Larson and F. H. Baker.
24
25 TABLE 1. Distribution of calves by dosage of STEER-oid® and implant date, dam group and sire breed.
Materials and Methods
A field trial was conducted to compare growth responses of 1- to 40-day-old calves to three levels of STEER-oid ® implant. Research was conducted at the Southwest Research and Extension Center, Hope, AR, in the fall of 1984 and the winter and spring of 1985. A full dose of STEER-oid® consisted of 8 pellets and contained 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P. A full dose is equivalent to the recommended c!')sage for calves weighing 397 to 794 kg (Taylor, 1984). The 112 dose is equivalent to a full dose of Synovex-C®s which contains 100 mg progesterone U.S.P. and 10 mg estradiol valerate U.S.P., with the exception that estradiol valerate has slower absorption than estradiol benzoate (Benson, 1976). Calves implanted with 1/4 dose (n=35), 1/2 dose (n=30) and a full dose (n=31) were compared with a control group (n=30). The youngest calf implanted on day 1 was approximately 12 h old at implantation. Calves were implanted subcutaneously in the top middle one-third of the ear according to the procedure outlined by Taylor (1984). Two implant dates (October 31, 1984, and December 18, 1984) were used to meet age restrictions and to more efficiently utilize personnel and equipment. Calves were born September through December 1984, and their mean age at the end of the trial was 209 day. Calves were progeny of either Zebu cross (ZX) cows or British and Continental (BCX) cows and were sired by either Charolais or Simmental bulls. Zebu cross cows were crosses among Brahman and Santa Gertrudis breeds with Angus, Charolais, Hereford, Red Poll and Simmental. British and Continental cross cows were crosses among Angus, Charolais, Hereford, Jersey, Red Poll and Simmental. Information on distribution of calves by dam breed group, implant date and sire breed is in Table 1. Management of the cows was typical of procedures for commercial cow herds in the Southeast. The herd grazed mixed sward pastures during the summer and were fed hay and a protein supple-
~
SSynovex-C@, Syntex corp., 4800 Westtown Pky, West Des Moines
,
Variable
0
1/4
Dosagea 1/2 1 (Full) Total
Implant date 10/31/84 12118/84
19 11
15 20
15 15
13 18
62 64
16
19
12
19
66
14
16
18
12
60
5 25
5 30
16 14
14 17
40 86
Dam group Zebu crosses British and Continental crosses Sire breed Charolais Simmental
aFull dose consisted of 8 pellets; 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P.
ment during the winter. Pastures contained approximately 50% tall fescue (Festuca arundinacea) and 50% bermudagrass (Cynodon dactylon) overseeded with white clover (Trifolium repens). Within 24 h of birth, calves were eartagged with their dam's identification number, weighed and sire breed determined. Breed of sire was determined from phenotypic genetic markers of color, head and ear shape and body profile. During the breeding period (December 1 to March 1), cows were exposed to Charolais and Simmental bulls. A pair of bulls were exposed to 45 cows. Calves were weighed at the beginning of the trial and at 28 day inteNals. Calves in the two implant date groups were weighed and the trial terminated on May 7 and July 2, 1985, respectively. Personnel at the station made routine subjective evaluations of the calves for side effects or anatomical changes due to the exogenous hormones. Data were analyzed by least-squares analysis of variance using the General Linear Models (GLM) procedure in the Statistical Analysis Systems (SAS, 1982). The model used in the analysis was Xijklm = J..l + B1 Wijklm + B2AiJklmn + B3Dijklmn + Si + Pj + Gk + MI + T m + Eijklmn where
Xijklmn = final weight or cumulative average daily gain (ADG) of the ijklmnth calf, )l = the mean intercept,
26
CHEWNING ET AL.
B1
= the partial regression of Xijklmn on initial
weight (Wijkimn), B2 = the partial regression of Xijklmn on initial age (Aijklmn), B3 = the partial regression of Xijklmn on age of dam (Dijklmn)' Si = the effect of the jlh sex, Pj = the effect of the sire breed, Gk = the effect of the kth dam group, M1 = the effect of the ph implant date, T m = the effect of the mth dosage level and Eijklmn = the random error associated with the ijklmnth calf plus residual interactions.
r
All of the two- and three-way interactions were included in a preliminary analysis, found to be nonsignificant and, therefore, deleted from the model. Results and Discussion
Mean-squares from the analysis of variance of ADG and final weight (FWT) are in Table 2. Initial weight of calves and initial age of calves influenced (P<.05) both ADG and FWT. Age of dam, sex and sire breed did not influence ADG and FWT. Dam breed group, implant date and dosage affected (P<.05) ADG and FWT. Least-squares means and standard errors for ADG and FWT of calves by dam breed group, implant date and dosage of STEER-oid® are in Table 3. Calves from ZX dams had higher (P<.05) ·-A86- -and FWT (. 74-and--1.ge.-8--kg ,respectively) than calves from BCX dams (.68 and 177.8 kg, respectively). Higher gains and heavier weaning weights of the calves from ZX cross cows are
TABLE 2. Mean squares from the analysis of variance of average daily gain (ADG) and final weight (FWT) of calves given STEER-oil®. Mean squares
Source
Degrees of freedom
ADG. kg
FWT, kg
Initial weight Initial age Age of dam Sex of calves Sire breed Dam group Implant date Dosage Residual
1 1 1 1 1 1 1 1 115
.1498' .1537' .0023 .0236 .0000 .1107' .3128' .0451' .0081
23044.25' 5663.99' 85.34 873.01 .82 4060.67' 17588.46' 1671.75' 300.98
'Significant at the .05 level of probability.
consistent with results reported by Damon et al. (1959), Turner (1969) and Brown et al. (1985) . Calves implanted December 18 had higher (p<.05) ADG and FWT (.77 and 198.7 kg, respectively) than calves implanted October 31 (.64 and 168.9 kg, respectively) . Calves implanted December 18 were younger and had the advantage of their mother's milk until forage growth in the spring whereas calves implanted October 31 likely experienced a lag in their mother's milk production before forage growth in the spring, consequently reducing weight gain. Average daily gain and FWT of calves receiving 1/4 dose (.75 and 191.6 kg, respectively) and 1/2 dose (.73 and 188.3 kg, respectively) were higher (P<.05) than ADG and FWT of the control group (.67 and 175.9 kg, respectively) . Calves receiving a full dose did not differ from the control group. Perhaps this was due to the interaction of the hypothalamic releasing factor and the higher level of exogenous hormone implanted (Genuth, 1983). No side-effects or anatomical changes were noted during the early part of the study; however, heifers receiving 114 dose had more udder development than heifers in the other groups when the study was terminated. Udder development was likely due to increased body condition (fatness) resulting from heavier body weights and not a direct result of the hormones. These results indicate that improvements in gains of young calves were
TABLE 3. Least-squares means and standard errors for average daily gain (ADG) and final weight (FWT) of calves by dam breed group, im· plant date and dosage level of STEER-oid®. Treatment Dam breed group Zebu cross British and Continental cross Implant dated October 31. 1984 December 18, 1984 Dosage levele 0 1/4 1/2 1 (Full)
Number of calves
ADG. kg
FWT. kg
66
.74 ±.Q1a
189.8 ± 2.3a
60
,68 ± .01 b
177.8 ± 2.4 b
62 64
.64 ± .01 b .77 ± .01 a
168.9 ± 2.6 b 198.7 ± 2.6a•
30 35 30 31
.67 ± .02 c .75 ± .02a .73 ± .02 ab .68 ± .02 bc
175.9 ± 3.6c 191.6 ± 3.3a 188.3 ± 3.2ab 179.4 ± 3.2 bc
a,b,CColumn means in the same treatment that do not share a common superscript differ at the .05 level of probability. dTrial terminated for the two implant dates on May 7. 1985. and July 2, 1985. respectively. 9Full dose consisted of 8 pellets; 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P.
27
achieved with doses lower than those levels of STEER-oid® used in calves heavier than 180 kg. Literature Cited 1. Benson, R. C. 1976. Current Obstetric & Gynecologic Diagnosis & Treatment. Lange Medical Publications. Los Altos, CA. p. 465. 2. Brown, C. J., A. H. Brown, W. C. Loe, M. L. Ray, A. E. Spooner, R. W. Parham, H. Huneycutt, and Z. B. Johnson. 1985. A comparison of crossbred cows for feeder calf production. Ark. Agric. Exp. Sta. Bull. 881. 3. Damon, R. A., Jr., S. E. McCraine, R. M. Crown, and C. B. Singletary. 1959. Performance of crossbred beef cattle in the Gulf Coast region. J. Anim. Sci. 18:437-447. 4. Fontenot, J. P., and W. H. McClure. 1984/1985. Animal Science Research Report, Virginia Agricultural Experiment Station. NO.4: 29-32.
5. Genuth, S. M. 1983. The Endocrine System. In Berne, R. M. and M. N. Levy (Ed.). Physiology. The C. V. Mosby Co., SI. Louis, MO. pp 993-1000. 6. Gill, D. R., H. R. Spires, F. E. Bates, B. L. Peverly, and K. S. Lusby. 1986. Responses of fall-born calves to progesterone-estradiol benzoate implants and reimplants. J. Anim. Sci. 62:37-41. 7. Mader, T. L., D. C. Clanton, J. K. Ward, D. E. Panaskie, and G. H. Deutscher. 1985. Effects of pre- and postweaning zeranol implant on steer calf performance. J. Anim. Sci. 61 :546-551. 8. Phillips, W. A., J. B. McLaren, and N. A. Cole. 1986. The effects of a preassembly zeranol implant and post-transfer diet on the health performance and metabolic profile of feeder calves. J. Anim. Sci. 62: 27-36. 9. SAS. 1982. SAS User's Guide. Statistical Analysis Systems Institute, Inc. Cary, NC . 10. Taylor, R. E. 1984. Beef Production and the Beef Industry : A Beef Producer's Perspective. Macmillan Publishing Co., NY. p. 317. 11. Turner, J. W. 1969. Preweaning production differences among reciprocal crossbred beef cows. J. Anim. Sci. 29:857-861.
and 198.7 kg, respectively) than calves implanted October 31 (.64 and 168.9 kg , respectively). Average daily gain and FWT of calves receiving 1/4 dose (.75 and 191.6 kg, respectively) and 112 dose (.73 and 188.3 kg, respectively) were higher (P<.05) than ADG and FWT of the contrOl. ~roup (.67 and 175.9 kg, respectively). Calves receiving a full dose did not differ from the control group. These results indicate that improvements in gains of young calves were achieved with doses lower than those levels of STEER-oid® usedin calves heavier than 180 kg.
Abstract
A field trial was conducted to compare growth responses of 1- to 40-day-old calves to three levels of STEER-oid® implant. A full dose of STEER-oid® consists of 8 pellets and contains 200 mg progesterone, U.S.P ., and 20 mg estradiol benzoate, U.S.P . Calves implanted with 1/4 dose (n:::::35), 1/2 dose (n:::::30) and a full dose (n:::::31) were compared with a control group (n:::::30) receiving no implant. Calves were implanted on October 31, i 984, and December 18, 1984, to meet age restrictions. Mean age at the end of the study was 209 days. Variation in average daily gain (ADG) and final weight (FWT) due to initial weight, initial age, age of dam, sex, sire breed (Charolais vs. Simmental), dam group [Zebu cross (ZX) vs. British and Continental cross ~(BGX}-1T implant date and rlQsagew~ey~dJ Initial weight and initial age influenced (P<.05) both ADG and FWT. Age of dam, sex and sire breed did not influence ADG or FWT. Calves from ZX dams had higher (P<.05) ADG and FWT (.74 and 189.8 kg, respectively) than calves from BeX dams (.68 and 177.8 kg, respectively). Calves implanted December 18 had higher (P< .05) ADG and FWT (.77
Introduction
The use of growth stimulants in the management of suckling-beef calves for more efficient production iswett-established:-&owtll-pr or Ilotin-gimplalits al e one of the most practical management tools used to increase the pounds of calf weaned per cow exposed to breeding. The practice of implanting has routinely been used to improve performance during the preweaning growth phase, postweaning growth phase and in feedlot cattle (Mader et aI., 1985; Fontenot and McClure, 1985; Gill et ai., 1986; Phillips et aI., 1986). Implanting for growth could perhaps be extended if implanting was practiced at an earlier age. This study involved the use of the implant STEER-oid® in suckling calves less than 40 days old. The objectives of the study were 1) to compare the growth responses of calves implanted at less than 40 days of age to three levels of STEER-oid® and 2) to evaluate the influence of sex, age of dam, sire breed and dam group on calf performance.
1 Published with the approval of the Director of the Arkansas Agricul.. tural Experiment Statio n s . . 2Supported in part by Boehnnger Ingelhelm An imal Health, Inc., SI. Joseph, MO. R • • 3STEER-oid®, Boehringer Ingelhelm Animal Health, Inc., SI. Joseph, MO, Ivy Laboratories, Inc., Overland Park, KS. . 4The mention of firm names or trade products does not Imply that they are endorsed by the Arkansas Agricultural Experiment Station over other firms or similar products not mentioned. Reviewed by W. M. Larson and F. H. Baker.
24
25 TABLE 1. Distribution of calves by dosage of STEER-oid® and implant date, dam group and sire breed.
Materials and Methods
A field trial was conducted to compare growth responses of 1- to 40-day-old calves to three levels of STEER-oid ® implant. Research was conducted at the Southwest Research and Extension Center, Hope, AR, in the fall of 1984 and the winter and spring of 1985. A full dose of STEER-oid® consisted of 8 pellets and contained 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P. A full dose is equivalent to the recommended c!')sage for calves weighing 397 to 794 kg (Taylor, 1984). The 112 dose is equivalent to a full dose of Synovex-C®s which contains 100 mg progesterone U.S.P. and 10 mg estradiol valerate U.S.P., with the exception that estradiol valerate has slower absorption than estradiol benzoate (Benson, 1976). Calves implanted with 1/4 dose (n=35), 1/2 dose (n=30) and a full dose (n=31) were compared with a control group (n=30). The youngest calf implanted on day 1 was approximately 12 h old at implantation. Calves were implanted subcutaneously in the top middle one-third of the ear according to the procedure outlined by Taylor (1984). Two implant dates (October 31, 1984, and December 18, 1984) were used to meet age restrictions and to more efficiently utilize personnel and equipment. Calves were born September through December 1984, and their mean age at the end of the trial was 209 day. Calves were progeny of either Zebu cross (ZX) cows or British and Continental (BCX) cows and were sired by either Charolais or Simmental bulls. Zebu cross cows were crosses among Brahman and Santa Gertrudis breeds with Angus, Charolais, Hereford, Red Poll and Simmental. British and Continental cross cows were crosses among Angus, Charolais, Hereford, Jersey, Red Poll and Simmental. Information on distribution of calves by dam breed group, implant date and sire breed is in Table 1. Management of the cows was typical of procedures for commercial cow herds in the Southeast. The herd grazed mixed sward pastures during the summer and were fed hay and a protein supple-
~
SSynovex-C@, Syntex corp., 4800 Westtown Pky, West Des Moines
,
Variable
0
1/4
Dosagea 1/2 1 (Full) Total
Implant date 10/31/84 12118/84
19 11
15 20
15 15
13 18
62 64
16
19
12
19
66
14
16
18
12
60
5 25
5 30
16 14
14 17
40 86
Dam group Zebu crosses British and Continental crosses Sire breed Charolais Simmental
aFull dose consisted of 8 pellets; 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P.
ment during the winter. Pastures contained approximately 50% tall fescue (Festuca arundinacea) and 50% bermudagrass (Cynodon dactylon) overseeded with white clover (Trifolium repens). Within 24 h of birth, calves were eartagged with their dam's identification number, weighed and sire breed determined. Breed of sire was determined from phenotypic genetic markers of color, head and ear shape and body profile. During the breeding period (December 1 to March 1), cows were exposed to Charolais and Simmental bulls. A pair of bulls were exposed to 45 cows. Calves were weighed at the beginning of the trial and at 28 day inteNals. Calves in the two implant date groups were weighed and the trial terminated on May 7 and July 2, 1985, respectively. Personnel at the station made routine subjective evaluations of the calves for side effects or anatomical changes due to the exogenous hormones. Data were analyzed by least-squares analysis of variance using the General Linear Models (GLM) procedure in the Statistical Analysis Systems (SAS, 1982). The model used in the analysis was Xijklm = J..l + B1 Wijklm + B2AiJklmn + B3Dijklmn + Si + Pj + Gk + MI + T m + Eijklmn where
Xijklmn = final weight or cumulative average daily gain (ADG) of the ijklmnth calf, )l = the mean intercept,
26
CHEWNING ET AL.
B1
= the partial regression of Xijklmn on initial
weight (Wijkimn), B2 = the partial regression of Xijklmn on initial age (Aijklmn), B3 = the partial regression of Xijklmn on age of dam (Dijklmn)' Si = the effect of the jlh sex, Pj = the effect of the sire breed, Gk = the effect of the kth dam group, M1 = the effect of the ph implant date, T m = the effect of the mth dosage level and Eijklmn = the random error associated with the ijklmnth calf plus residual interactions.
r
All of the two- and three-way interactions were included in a preliminary analysis, found to be nonsignificant and, therefore, deleted from the model. Results and Discussion
Mean-squares from the analysis of variance of ADG and final weight (FWT) are in Table 2. Initial weight of calves and initial age of calves influenced (P<.05) both ADG and FWT. Age of dam, sex and sire breed did not influence ADG and FWT. Dam breed group, implant date and dosage affected (P<.05) ADG and FWT. Least-squares means and standard errors for ADG and FWT of calves by dam breed group, implant date and dosage of STEER-oid® are in Table 3. Calves from ZX dams had higher (P<.05) ·-A86- -and FWT (. 74-and--1.ge.-8--kg ,respectively) than calves from BCX dams (.68 and 177.8 kg, respectively). Higher gains and heavier weaning weights of the calves from ZX cross cows are
TABLE 2. Mean squares from the analysis of variance of average daily gain (ADG) and final weight (FWT) of calves given STEER-oil®. Mean squares
Source
Degrees of freedom
ADG. kg
FWT, kg
Initial weight Initial age Age of dam Sex of calves Sire breed Dam group Implant date Dosage Residual
1 1 1 1 1 1 1 1 115
.1498' .1537' .0023 .0236 .0000 .1107' .3128' .0451' .0081
23044.25' 5663.99' 85.34 873.01 .82 4060.67' 17588.46' 1671.75' 300.98
'Significant at the .05 level of probability.
consistent with results reported by Damon et al. (1959), Turner (1969) and Brown et al. (1985) . Calves implanted December 18 had higher (p<.05) ADG and FWT (.77 and 198.7 kg, respectively) than calves implanted October 31 (.64 and 168.9 kg, respectively) . Calves implanted December 18 were younger and had the advantage of their mother's milk until forage growth in the spring whereas calves implanted October 31 likely experienced a lag in their mother's milk production before forage growth in the spring, consequently reducing weight gain. Average daily gain and FWT of calves receiving 1/4 dose (.75 and 191.6 kg, respectively) and 1/2 dose (.73 and 188.3 kg, respectively) were higher (P<.05) than ADG and FWT of the control group (.67 and 175.9 kg, respectively) . Calves receiving a full dose did not differ from the control group. Perhaps this was due to the interaction of the hypothalamic releasing factor and the higher level of exogenous hormone implanted (Genuth, 1983). No side-effects or anatomical changes were noted during the early part of the study; however, heifers receiving 114 dose had more udder development than heifers in the other groups when the study was terminated. Udder development was likely due to increased body condition (fatness) resulting from heavier body weights and not a direct result of the hormones. These results indicate that improvements in gains of young calves were
TABLE 3. Least-squares means and standard errors for average daily gain (ADG) and final weight (FWT) of calves by dam breed group, im· plant date and dosage level of STEER-oid®. Treatment Dam breed group Zebu cross British and Continental cross Implant dated October 31. 1984 December 18, 1984 Dosage levele 0 1/4 1/2 1 (Full)
Number of calves
ADG. kg
FWT. kg
66
.74 ±.Q1a
189.8 ± 2.3a
60
,68 ± .01 b
177.8 ± 2.4 b
62 64
.64 ± .01 b .77 ± .01 a
168.9 ± 2.6 b 198.7 ± 2.6a•
30 35 30 31
.67 ± .02 c .75 ± .02a .73 ± .02 ab .68 ± .02 bc
175.9 ± 3.6c 191.6 ± 3.3a 188.3 ± 3.2ab 179.4 ± 3.2 bc
a,b,CColumn means in the same treatment that do not share a common superscript differ at the .05 level of probability. dTrial terminated for the two implant dates on May 7. 1985. and July 2, 1985. respectively. 9Full dose consisted of 8 pellets; 200 mg progesterone, U.S.P., and 20 mg estradiol benzoate, U.S.P.
27
achieved with doses lower than those levels of STEER-oid® used in calves heavier than 180 kg. Literature Cited 1. Benson, R. C. 1976. Current Obstetric & Gynecologic Diagnosis & Treatment. Lange Medical Publications. Los Altos, CA. p. 465. 2. Brown, C. J., A. H. Brown, W. C. Loe, M. L. Ray, A. E. Spooner, R. W. Parham, H. Huneycutt, and Z. B. Johnson. 1985. A comparison of crossbred cows for feeder calf production. Ark. Agric. Exp. Sta. Bull. 881. 3. Damon, R. A., Jr., S. E. McCraine, R. M. Crown, and C. B. Singletary. 1959. Performance of crossbred beef cattle in the Gulf Coast region. J. Anim. Sci. 18:437-447. 4. Fontenot, J. P., and W. H. McClure. 1984/1985. Animal Science Research Report, Virginia Agricultural Experiment Station. NO.4: 29-32.
5. Genuth, S. M. 1983. The Endocrine System. In Berne, R. M. and M. N. Levy (Ed.). Physiology. The C. V. Mosby Co., SI. Louis, MO. pp 993-1000. 6. Gill, D. R., H. R. Spires, F. E. Bates, B. L. Peverly, and K. S. Lusby. 1986. Responses of fall-born calves to progesterone-estradiol benzoate implants and reimplants. J. Anim. Sci. 62:37-41. 7. Mader, T. L., D. C. Clanton, J. K. Ward, D. E. Panaskie, and G. H. Deutscher. 1985. Effects of pre- and postweaning zeranol implant on steer calf performance. J. Anim. Sci. 61 :546-551. 8. Phillips, W. A., J. B. McLaren, and N. A. Cole. 1986. The effects of a preassembly zeranol implant and post-transfer diet on the health performance and metabolic profile of feeder calves. J. Anim. Sci. 62: 27-36. 9. SAS. 1982. SAS User's Guide. Statistical Analysis Systems Institute, Inc. Cary, NC . 10. Taylor, R. E. 1984. Beef Production and the Beef Industry : A Beef Producer's Perspective. Macmillan Publishing Co., NY. p. 317. 11. Turner, J. W. 1969. Preweaning production differences among reciprocal crossbred beef cows. J. Anim. Sci. 29:857-861.