Animal Science 2006, 82: 555–559 Q 2006 British Society of Animal Science
ISSN 1357-7298 DOI: 10.1079/ASC200656
Milk production and reproductive performance of cows induced into lactation and treated with bovine somatotropin M. Mellado1†, E. Nazarre1, L. Olivares1, F. Pastor1 and A. Estrada2 1
Department of Nutrition, University Autonoma Agraria Antonio Narro, Saltillo, Me´xico Faculty of Veterinary Medicine and Animal Science, University Autonoma of Sinaloa, Culiacan, Mexico
2
†
E-mail:
[email protected]
Abstract Milk production and reproductive performance of pluriparous dairy cows was compared for cows whose lactations were induced by hormonal treatment or followed natural calving. The study was conducted on 179 high-yielding dairy cows in a large dairy operation in a hot arid environment of northern Mexico, where bovine somatotropin (bST) was routinely used in all cows. To induce lactation, 98 cows that had previously failed to become pregnant were treated with 500 mg bST on days 1, 8 and 21. From days 2 to 8, cows were treated with oestradiol cypionate (0·30 mg/kg live weight (LW) per day) and progesterone (0·28 mg/kg LW per day). From days 9 to 15, oestradiol cypionate alone was given. PGF2a was given on day 16. Nothing was administered on days 17 and 18, and 15 mg flumetasone was administered from days 19 to 21. Lactation was induced successfully in all 98 cows subjected to the hormone treatment. All cows received 500 mg bST every 14 days throughout lactation from 63 ^ 7 days after lactation had started. Cows with induced lactations produced less ( P , 0·01) milk per 305-day lactation (9599 ^ 1387 kg) than controls (12 302 ^ 1245 kg). Proportion of cows pregnant was similar in induced cows and non-treated cows (71 v. 75%). Induction of lactation was associated with increased numbers of services per pregnancy (5·8 ^ 4·0 v. 4·22 ^ 2·98; P , 0·01) compared with controls; however, impaired reproduction might not necessarily have been related to the hormonal treatment to induce lactation, but due to the nature of the cows utilized for this treatment group. This protocol can be used on dairy farms where bST treatment and prolonged steroid administration of dairy cows is legally permitted. Keywords: dairy cows, lactation curve, lactation duration, milk yield, pregnancy.
Introduction
been variable; generally milk yields have been erratic and normally lower than those expected from cows calving normally (Smith and Schanbacher, 1973; Collier et al., 1975; Peel et al., 1978). Data of Magliaro et al. (2004) indicate that milk production was not different between induced cows treated with bovine somatotropin (bST) and first-lactation cows whose lactation derived from parturition, and that, despite a short period of bST treatment, the procedure was profitable. The current treatments for inducing lactation do not entirely mimic the endocrinological changes associated with pregnancy, parturition and lactation (Collier et al., 1975; Chakriyarat et al., 1978). Our study attempted the modify the traditional short hormonal treatment (7 days) to initiate lactogenesis, with a longer steroid treatment for a more pronounced mammotrophic action of hormones acting during normal mammogenesis (pregnancy) and lactogenesis (parturition). Additionally, corticosteroids, and prostaglandin were used to ‘trigger’ lactogenesis, and bST was included in the protocol for its ability to stimulate milk producing cell proliferation (Capuco et al., 1989; Knight et al., 1990). Thus, this present study investigated a more complex treatment protocol to induce lactation in cattle including
Continuous selection for increased milk yield in dairy cattle has resulted in an antagonistic relationship between increasing production and lowered fertility (Pryce et al., 2004). Lowered fertility has become a major reason for involuntary culling of dairy cows in intensive systems, particularly in northern Mexico where the extremely high ambient temperatures in this area influence production efficiency. A high culling rate of high-yielding cows due to reproductive failure reduces profitability as those cows are sold for meat purposes. Replacement cost would be reduced with fewer cows sold for beef purposes and more replacements available for sale and not to maintain lactating herd size. Although hormonal induction of lactation in commercial dairy operations is questionable in a sound management programme, this practice could be used selectively in cows with fertility problems, but with a high genetic potential for milk production. Protocols for induction of lactation in dairy cattle were established three decades ago, using daily injection of oestadiol-17b and progesterone, but outcomes have
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Mellado, Nazarre, Olivares, Pastor and Estrada After a voluntary waiting period of 50 days, cows observed to be in oestrus were artificially inseminated. Pregnancy was confirmed by palpation per rectum of the uterine content 50 to 70 days after the last AI date recorded. Dates for calving, first service after parturition and subsequent services were recorded. The interval from calving to pregnancy (days open) was recorded.
bST treatment throughout lactation. Milk production and reproductive performance were compared between pluriparous cows induced to lactation and those whose lactation followed normal pregnancy and parturition.
Material and methods The study was conducted in a large commercial dairy farm in a hot arid environment in northern Mexico, where bST was used routinely in all cows. The use of bST is widespread in the dairy industry in Mexico, and the current legislation allows the use of this protocol to induce lactation in cows for milk to be sold for human consumption.
Lactation yields were analysed as repeated measures using the mixed procedure of Statistical Analysis Systems Institute (SAS, 1996). Proportion of cows pregnant were evaluated by chi-square analysis (SAS, 1996). After limiting the number of services per conception to cows with a confirmed pregnancy diagnosis, the effect of induction of lactation on the number of services per conception was evaluated by the bivariate Wilcoxon rank sum test (proc npar1way; SAS, 1996) without adjustment for confounders. Days open for pregnant cows were analysed by the LIFETEST procedure of SAS using both strata and time statements (SAS, 1996). ANOVA
Ninety-eight non-pregnant, dry, healthy pluriparous Holstein cows in good body condition and approximately 575 kg live weight (LW), which had failed to become pregnant with repeated services, and had been subjected to bST treatment in previous lactations, were induced into lactation by a combination of hormonal treatment. Bovine somatotropin (500 mg i.m., Lactotropinaw Eli Lilly Co., Mexico) was given on day 1 of treatment. From day 2 to 8, cows received oestradiol cypionate (ECPew Pfizer Animal Health MX, 0·30 mg/kg LW per day, s.c.) and progesterone (Progestyn A-Ew Lab. Tornell, Mexico; 0·28 mg/kg LW per day, s.c.). Bovine somatotropin was again administered on days 8 and 15. From day 9 to 15, oestradiol cypionate alone was administered. Cows were treated with PGF2a (Lutalyse, Pharmacia and Upjohn, Kalamazoo, MI, 25 mg i.m.) on day 16. Nothing was applied on days 17 and 18. For 3 days consecutively (19 to 21) cows were treated with flumetasone (Flusolw Lab. Tornell, Me´xico, 15 mg i.m.). On day 21 cows received once again bST and were milked.
Results Lactations were induced successfully in all non-pregnant cows with reproductive problems. Milk production data of cows induced into lactation and post-partum lactation are presented in Table 1. The 305-day yields for cows lactating after normal calving were 0·22 greater ( P , 0·01) than those following induced lactations. In both groups lactation length was greater than 400 days. Total milk production was 0·13 greater for post-partum lactations compared with induced lactations and mean daily milk yield during the first 305 days of lactation was greater ( P , 0·01) for induced lactations than post-partum lactations.
Eighty-one cows that calved at about the end of the sequence of hormone treatments served as controls. In order to avoid a bias in this sample, the criteria used to select these animals were based on pluriparous cows with four functional quarters, with no clinical metabolic diseases and with . 9000 kg 305-day lactation yields in their previous lactation. All cows (induced and controls) received 500 mg bST every 14 days, starting at 63 ^ 7 days after calving or initiation of lactation (induced cows) for the duration of lactation.
Lactation curves over 350 days of induced and post-partum lactations are compared in Figure 1. Cows induced to lactate took longer to reach peak milk production (125 days) but once maximum milk production was reached (34 kg/day) milk yield tended to parallel that of cows lactating after normal calving and treated with bST. However, there was a significant interaction ( P , 0·01) between milk yield and periods, which suggests that induced cows did not maintain the same tendency in milk yields observed in controls throughout lactation.
All cows were kept as a group and offered a total mixed ration composed of 0·32 lucerne hay, 0·28 maize silage, 0·22 maize grain, 0·17 soya-bean meal, and 0·02 minerals and vitamins (dry-matter basis). The ration was formulated to meet the requirement of their current milk production. They were kept in outside lots with metal shades and with a feeding bunk. Food was delivered to cows 10 times during the day. Cows were milked three times daily at 8-h intervals. bST was routinely used in previous lactations on all cows of this dairy farm, and average 305-days lactation yields in the previous lactation for induced and controls were 9675 (s.d. 1227) and 10 024 (s.d. 1423) kg, respectively. Milk yields were recorded every 14 days throughout lactation. The experiment lasted until all cows were dried off. The longest lactation was 935 days (17 kg milk in last weighing) for an induced cow, and 592 days (20 kg milk in last weighing) for a cow with normal calving.
Data regarding the reproductive performance following induced lactation are presented in Table 2. Services per pregnancy in the previous lactation were abnormally high Table 1 Lactation performance of cows induced into lactation and cows with a post-partum lactation Lactation following calving (no. ¼ 81) Mean
s.d.
Group induced (no. ¼ 98)
Mean
s.d.
Yield of 305-days lactation (kg) 12 302 1245 9599** 1387 Lactation length (days) 420 133 462 112 Total lactation yield (kg) 16 187 4742 14 041 3518 Daily milk yield in 305 days (kg) 40·3 5·2 31·4** 4·5
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Performance of cows induced into lactation the acceptable milk yield observed in the present study contrast with the unsatisfactory milk yields of induced cows with the traditional 7-to 10-day steroid regime (Collier et al., 1975; Tervit et al., 1980; Davis et al., 1982). To our knowledge, this is the first demonstration that cows induced into lactation with the administration of bST in the treatment, and then treated with bST during lactation, reached acceptably high milk production levels. The longer steroid treatment and the use of bST for inducing lactation seem to explain this response. Sejrsen et al. (1986) and Radcliff et al. (2003) have shown that heifers injected with somatotropin presented a large increase in mammary parenchymal mass and total mammary cell numbers. Likewise, Sakamoto et al. (2005) suggest that, in addition to its widely accepted homeorhetic roˆle in vivo, somatotropin also acts on the mammary parenchyma. This mammogenic action of somatotropin remains a puzzle, but the effect of exogenous bST most likely involves insulin-like growth factor-1 (Forsyth, 1996; Sejrsen et al., 1999).
60 Postpartum lactation Milk yield (kg/day)
50 40 30 20
Induced lactation
10 0 0
50
100
150 200 250 Days of lactation
300
350
Figure 1 Milk production curves for induced and control cows. Points are means ^ s.d.
in cows whose lactations derived from parturition. The induced cows also presented a considerable number of unsuccessful attempts to become pregnant, before inducing them into lactation. No significant effect of induced lactation was found for average days to first service. However, a higher ( P , 0·01) number of services per pregnancy was observed in induced cows than in cows with a post-partum lactation. A very high proportion of induced and non-induced cows were not detected in oestrus throughout lactation. Average days open was 58 days shorter in non-treated cows than in induced cows, although this difference was not significant. Finally, proportion of cows pregnant was similar between induced and non-induced cows and 70 live calves resulted from the 98 cows induced successfully to lactate.
Mean lactation length of induced cows was over 400 days, with remarkably high levels of milk production (25 kg/day) at the end of lactation. Despite the fact that both groups of cows underwent extended lactation, lactation persistency was remarkably high in both groups. This possibly was due to the fact that frequent milking causes an increase in mammary parenchyma (Henderson et al., 1985), which results in a more persistent lactation curve (Bar-Peled et al., 1995; Knight and Sorensen, 2000). Moreover, administration of bST may enhance persistency by increasing cell proliferation and turn-over, or by reducing the rate of apoptosis (Capuco et al., 2003), and by maintaining total mammary parenchyma weight and lactating alveoli (Baldi et al., 2002). Reproductive performance of cows induced into lactation and cows with a post-partum lactation was poor but, compared with previous studies with cows induced to lactate, our results did not reach the extremely low reproduction rates found by others researchers. In the present study 70 live calves resulted from 98 cows that were induced successfully to lactate, whereas others (Collier et al., 1975) obtained only five live calves from 11 induced cows. Even worse, in another study only two live calves were obtained from 16 induced cows (Harness et al., 1978). Reproductive failures in these studies have been ascribed to the occurrence of static, enlarged ovarian follicles (Collier et al., 1975; Erb et al., 1976) and the occurrence of oestrous behaviour without ovarian structures present (Jordan et al.,
Discussion The success rate in inducing lactation was 100% but the induced cows produced 0·78 of the milk produced by cows with post-partum lactations. The amount of milk produced by the induced cows as a proportion of that produced by cows after normal calving was similar to that reported by Magliaro et al. (2004) but higher than reports of other researchers (Smith and Schanbacher, 1973; Collier et al., 1977; Kensinger et al., 1979), who noted that milk yields of induced cows were proportionately 0·6 to 0·7 of previous production. Moreover, in contrast to previous studies, variation in milk yield among cows induced to lactate was reduced. The high success rate in inducing lactation and
Table 2 Reproductive parameters of cows induced into lactation and cows with a post-partum lactation Lactation following calving (no. ¼ 81)
†
No. of services per cow in previous lactation Average days to first service Services per pregnancy Cows not observed in oestrus (% and no.) Days open Cows pregnant (% and no.) †
Group induced (no. ¼ 98)
Mean
s.d.
Mean
s.d.
5·33 68 4·22 17 (14/81) 206 75 (61/81)
3·65 11 2·98
4·2 71 5·8** 19 (19/98) 264 71 (70/98)
3·0 14 4·0
In the induced cows none of the services resulted in pregnancies.
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133
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Mellado, Nazarre, Olivares, Pastor and Estrada induced cows and heifers that had previously failed to become pregnant.
1981). However, normal cyclic activity has been observed by day 27 post treatment in heifers induced to lactate artificially and 92% of these heifers conceived (Sawyer et al., 1986). To circumvent the formation of abnormal ovarian follicles in induced cows, Jordan et al. (1981) administered GnRH, which resulted in proportion of cows pregnant similar to cows with post-partum lactation. Ninety percent conception rates in induced cows have also been reported (Peel et al., 1978).
Our work shows the effectiveness and consistency of a technique to induce lactation in cows with reproductive problems, which allows cows to achieve 305 days milk yield levels .9000 kg. Despite the fact that cows that were induced had zero conceptions in the previous lactation, a satisfactory proportion of cows eventually conceived and calved. With this technology there is a potential to increase income by reducing the number of replacement heifers and the retention in the herd of cows of high genetic merit.
In the present study reproductive management was less than ideal, which was reflected in a high failure to conceive from regularly spaced services, which led to abnormally high services per pregnancies for both groups of cows. This failure of cows to become pregnant resulted in extremely long intervals between commencement of lactation and conception. Previous studies have also found that interval from treatment to induce lactation to conception in cows has been unacceptably long (Smith and Schanbacher, 1973).
Acknowledgements The authors wish to thank Alfonso R. Nava Cruz for providing facilities, cows and drugs to carry out the present study.
References
The bST treatment used to improve lactation performance may have influenced the reproductive performance of cows. An extensive review on the effect of bST on the reproductive performance of dairy cattle indicates that the use of this hormone increases the risk of a cow failing to conceive by approximately 0·4 (Dohoo et al., 2003). However, recent studies with large numbers of cows and herds indicate that bST treatment does not affect (Judge et al., 1999; Collier et al., 2001) or even enhances fertility in dairy cows (Luna-Dominguez et al., 2000; Moreira et al., 2001 and 2002; Santos et al., 2004).
Baldi, A., Modina, S., Cheli, F., Gandolfi, F., Pinotti, L., Baraldi Scesi, L., Fantuz, F. and Dell’Orto, V. 2002. Bovine somatotropin administration to dairy goats in late lactation: Effects on mammary gland function, composition and morphology. Journal of Dairy Science 85: 1093-1102. Bar-Peled, U., Maltz, E., Bruckental, I., Folman, Y., Kali, Y., Gacitua, H., Lehrer, A. R., Knight, C. H., Robinzon, B., Voet, H. and Tagari, H. 1995. Relationship between frequent milking or suckling in early lactation and milk production of high producing dairy cows. Journal of Dairy Science 78: 2726-2736. Capuco, A. V., Ellis, S. E., Hale, S. A., Long, E., Erdman, R. A., Zhao, X. and Paape, M. J. 2003. Lactation persistency: insights from mammary cell proliferation studies. Journal of Animal Science 81: 18-31. Capuco, A. V., Keys, J. E. and Smith, J. J. 1989. Somatotrophin increases thyroxine-50 -monodeiodinasa activity in lactating mammary tissue of the cows. Journal of Endocrinology 121: 205-211.
The exceedingly high proportion of cows showing no oestrous behaviour during lactation (over 17%) points toward an inappropriate oestrous detection programme. This failure to detect cows in oestrus might be mediated by attenuation of expression of oestrus in bST-treated cows. Previous studies have shown that oestrous detection rate has been lower in cows receiving bST (Kirby et al., 1997; Santos et al., 2004) and heifers treated with bST had a delayed onset of oestrus, fewer mounts, and oestrus of shorter duration (Lefebvre and Block, 1992). Additional factors that might have caused the poor reproductive performance of both groups of cows were a combination of environmental variables. Maximum ambient temperature in spring and summer in this area fluctuates between 35 and 418C (Mellado and Meza-Herrera, 2002) which severely disrupts the reproductive performance of dairy cows (DeRensis and Scaramuzzi, 2003).
Chakriyarat, S., Head, H. H., Thatcher, W. W., Neal, F. C. and Wilcox, C. J. 1978. Induction of lactation: lactational, physiological, and hormone response in the bovine. Journal of Dairy Science 61: 1715-1724. Collier, R. J., Bauman, D. E. and Hays, R. L. 1977. Effect of reserpine on milk production and serum prolactin of cows hormonally induced into lactation. Journal of Dairy Science 60: 896-901. Collier, R. J., Bauman, D. E. and Hays, R. L. 1975. Milk production and reproductive performance of cows hormonally induced into lactation. Journal of Dairy Science 58: 1524-1527. Collier, R. J., Byatt, J. C., Denham, S. C., Eppard, P. J., Fabellar, A. C., Hintz, R. L., McGrath, M. F., McLaughlin, C. L., Shearer, J. K., Veenhuizen, J. and Vicini, J. L. 2001. Effects of sustained release bovine somatotropine (sometribove) on animal health in commercial dairy herds. Journal of Dairy Science 84: 1098-1108. Davis, S. R., Welch, R. A. S., Pearce, M. G. and Peterson, A. J. 1982. Induction of lactation in nonpregnant cows by estradiol-17-b and progesterone from an intravaginal sponge. Journal of Dairy Science 66: 450-457.
The number of services per conception was greater for the induced cows than cows with a post-partum lactation. This was not surprising because cows selected to be induced were not a randomly selected group of cows. These animals had failed to conceive during the previous lactation, which might imply that they had reproductive problems that precluded them from becoming pregnant. However, pregnancy percentages for the induced cows were acceptable, given their previous inability to get pregnant. These findings are in agreement with results of other researchers (Fulkerson, 1978; Jordan et al., 1981; Lembowicz et al., 1982; Magliaro et al., 2004) who have also reported successful breeding in
DeRensis, F. and Scaramuzzi, R. J. 2003. Heat stress and seasonal effects on reproduction in the dairy cow – a review. Theriogenology 60: 1139-1151. Dohoo, I. R., DesCoteaux, L., Leslie, K., Fredeen, A., Shewfelt, W., Preston, A. and Dowling, P. 2003. A meta-analysis review of the effects of recombinant bovine somatotropin. 2. Effects on animal health, reproductive performance, and culling. Canadian Journal of Veterinary Research 67: 252-264. Erb, R. E., Monk, E. L., Mollett, T. A., Malven, P. V. and Callahan, C. J. 1976. Estrogen, progesterone, prolactin, and
558
Performance of cows induced into lactation other changes associated, with bovine lactation induced with estradiol-17b and progesterone. Journal of Animal Science 42: 644-654.
Moreira, F., Badinga, L., Burnley, C. and Thatcher, W. W. 2002. Bovine somatotropin increases embryonic development in superovulated cows and improves post-transfer pregnancy rates when given to lactating recipient cows. Theriogenology 57: 1371-1387. Moreira, F., Orlandi, C. A., Risco, C. A., Mattos, R., Lopez, F. and Thatcher, W. W. 2001. Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cows. Journal of Dairy Science 84: 1646-1659.
Forsyth, I. A. 1996. The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants: action and interaction with hormones. Journal of Dairy Science 79: 1085-1096. Fulkerson, W. J. 1978. Artificial induction of lactation: A comparative study in heifers. Australian Journal of Biological Science 3: 65-71.
Peel, C. J., Taylor, J. W., Robinson, I. B., McGowan, A. A., Holley, R. D. and Findlay, J. K. 1978. The importance of prolactin and the milking stimulus in the artificial induction of lactation in cows. Australian Journal of Biological Science 31: 187-195.
Harness, J. R., Anderson, R. R., Thompson, L. J., Early, D. M. and Younis, A. K. 1978. Induction of lactation by two techniques: Success rate, milk composition, estrogen and progesterone in serum and milk, and ovarian effects. Journal of Dairy Science 61: 1725-1735.
Pryce, J. E., Royal, M. D., Garnsworthy, P. C. and Mao, I. L. 2004. Fertility in the high-producing dairy cow. Livestock Production Science 86: 125-135. Radcliff, R. P., VandeHaar, M. J., Skidmore, A. L., Chapin, L. T., Radke, B. R., Lloyd, J. W., Stanisiewski, E. P. and Tucker, H. A. 2003. Effect of diet and bovine somatotropin on heifer growth and mammary development. Journal of Dairy Science 80: 1996-2003.
Henderson, A. J., Blatchford, D. R. and Peaker, M. 1985. The effects of long-term thrice-daily milking on milk secretion in the goat: Evidence for mammary growth. Quarterly Journal of Experimental Physiology 70: 557-565. Jordan, D. L., Erb, R. E., Malven, P. V., Callahan, C. J. and Veenhuizen, E. L. 1981. Artificial induction of lactation in cattle: Effect of modified treatments on milk yield, fertility and hormones in blood plasma and milk. Theriogenology 16: 315-329.
Santos, J. E. P., Juchem, S. O., Cerri, R. L. A., Galvao, K. N., Chebel, R. C., Thatcher, W. W., Dei, C. S. and Bilby, C. R. 2004. Effect of bST and reproductive management on reproductive performance of Holstein Dairy cows. Journal of Dairy Science 87: 868-881. Sakamoto, K., Komatsu, T., Kobayashi, T., Rose, M. T., Aso, H., Hagino, A. and Obara, Y. 2005. Growth hormone acts on the synthesis and secretion of alpha-casein in bovine mammary epithelial cells. Journal of Dairy Research 72: 264-270.
Judge, L. J., Bartlett, P. C., Lloyd, J. W. and Erskine, R. J. 1999. Recombinant bovine somatotropin: association with reproductive performance in dairy cattle. Theriogenology 52: 481-496. Kensinger, R. S., Bauman, D. E. and Collier, R. J. 1979. Season and treatment effects on serum prolactin and milk yield during induced lactation. Journal of Dairy Science 62: 1880-1888. Kirby, C. J., Wilson, S. J. and Lucy, M. C. 1997. Response of dairy cows treated with bovine somatotropin to a luteoliyic dose of prostaglandin F2a. Journal of Dairy Science 80: 286-294.
Sawyer, G. J., Fulkerson, W. J., Martin, G. B. and Christine, G. 1986. Artificial induction of lactation in cattle: initiation of lactation and estrogen and progesterone concentrations in milk. Journal of Dairy Science 69: 1536-1544.
Knight, C. H., Fowler, P. A. and Wilde, C. J. 1990. Galactopoietic and mammogenic effects of long-term treatment with bovine growth hormone and thrice daily milking in goats. Journal of Endocrinology 127: 129-138.
Sejrsen, K., Foldager, J., Sorensen, M. T., Akers, R. M. and Bauman, D. E. 1986. Effect of exogenous bovine somatotropin on pubertal mammary development in heifers. Journal of Dairy Science 69: 1528-1535. Sejrsen, K., Purup, S., Vestergaard, M., Weber, M. S. and Knight, C. H. 1999. Growth hormone and mammary development. Domestic Animal Endocrinology 17: 117-129.
Knight, C. H. and Sorensen, A. 2000. Manipulation of lactation persistency with maintenance of milk quality. Journal of DairyScience 83: (suppl. 1) 24. Lefebvre, D. M. and Block, E. 1992. Effect of recombinant bovine somatotropin on estradiol-induced estrous behaviour in ovariectomized heifers. Journal of Dairy Science 75: 1461-1464.
Smith, K. L. and Schanbacher, F. L. 1973. Hormone induce lactation in the bovine. I. Lactational performance following injections of 17b-estradiol and progesterone. Journal of Dairy Science 56: 738743.
Lembowicz, K., Rabek, A. and Skrzeczkowski, L. 1982. Hormonal induction of lactation in the cow. British Veterinary Journal 138: 203-208. Luna-Dominguez, J. E., Enns, R. M., Armstrong, D. V. and Ax, R. L. 2000. Reproductive performance of Holstein cows receiving somatotropin. Journal of Dairy Science 83: 1451-1455. Magliaro, A. L., Kensinger, R. S., Ford, S. A., O’Connor, M. L., Muller, L. D. and Graboski, R. 2004. Induced lactation in nonpregnant cows and response to bovine somatotropin. Journal of Dairy Science 87: 3290-3297.
Statistical Analysis Systems Institute. 1996. SAS system (release 6·12). SAS Institute Inc., Cary, NC. Tervit, H. R., Fairclough, R. J., McGowan, L. T., MacKensie, D. D. S., MacMillan, K. L. and Peterson, A. J. 1980. Induction of lactation in dry dairy cattle. New Zealand Veterinary Journal 28: 15-19.
Mellado, M. and Meza-Herrera, C. A. 2002. Influence of season and environment on fertility of goats in a hot-arid environment. Journal of Agricultural Science 138: 97-102.
(Received 10 January 2006—Accepted 22 March 2006)
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