Introduction

The hormones play a very important role in the development and function of the mammary gland (Turner, 1956; Williams, 1960; Erb, 1976; Tucker, 2000). Studies examining the role of estrogens in stimulating mammogenesis in dairy cows showed that estrogen stimulates mammary duct growth, and estrogen and progesterone in combination stimulate lobule-alveolar development of the mammary gland (Tucker, 2000).

Estrogen and Progesterone

Estrogen is involved in initiating lactogenesis in cattle at parturition. Estrogen initiates lactation in two ways-

1. It causes release of prolactin from the anterior pituitary gland into blood (Nagasawa, 1969; Tucker, 2000)
2. Estrogen increases the number of prolactin receptors in mammary cells (Sheth, 1978; Tucker, 2000).

Although combinations of exogenous progesterone and estrogen work synergistically to stimulate lobular-alveolar growth, it is the high levels of progesterone during pregnancy that helps to regulate lobular-alveolar growth and block lactogenesis. Progesterone blocks lactogenesis in several ways. One example of this is how it blocks glucocorticoid receptors in mammary tissues which would suppress the lactogenic activity of glucocorticoids (Tucker, 2000). The exact mechanism of how progesterone accomplishes this is unclear, but removal of the progesterone block, luteolysis; and a decline in progesterone as parturition approaches, allow the onset of lactogenesis (Fulkerson, 1979; Tucker, 2000). The ovarian hormones, estrogen and progesterone, are not the only hormones required for lactogenesis, as prolactin is also needed. Blood prolactin levels in cattle surge several hours prior to parturition, (Ingalls, 1973; Tucker, 2000) and this surge in prolactin is apparently necessary for full lactogenesis. The surge in prolactin can be blocked with the use of bromocriptine, reducing milk yield, but this effect can be reversed with administration of prolactin (Akers et al., 1981).

Glucocorticoids

Glucocorticoids, cortisol being the major glucocorticoid in cattle, also play a role in mammary gland development, leading to alveolar cell differentiation of the gland. Glucocorticoids compete with progesterone for mammary epithelial cell binding sites. Administering glucocorticoids to non lactating cows with a developed mammary gland aids in the induction of lactation because the increase in glucocorticoids displaces progesterone from mammary cell receptors, thus reducing the progesterone block to prolactin receptor synthesis (Tucker, 1965; Fulkerson, 1975). Milk yields from these induced-lactations can be enhanced with the addition of prolactin, offering additional evidence that the hormones (estrogen, progesterone, prolactin, and glucocorticoids) work synergistically in the onset of lactogenesis (Tucker, 2000; Akers, 2002). The use of exogenous hormones to mimic mammary development during pregnancy has led to research to answer to questions regarding how the hormones work synergistically to promote lactogenesis. During these attempts to mimic mammary development with varied dosages of exogenous hormones, it was observed that induced-lactations actually occurred in treated animals, and these results have then led to the use of exogenous hormones to induce lactations in non pregnant cattle.

Techniques to reduce culling rates and replacement costs through the use of hormonal induction of lactation in dairy cattle have been attempted with varied success for more than 60 years (Hancock, 1954; Turner, 1956; Smith 1973). Development of a reliable cost efficient protocol to artificially induce lactation would allow dairy producers to reduce herd culling losses of non breeder cows and lower replacement costs by retaining animals that would otherwise be culled from the dairy herd.

Magliaro et al. (1999) compared the profitability of inducing lactation in non-pregnant cows versus purchasing replacement heifers. Healthy, multiparous, non-pregnant cows were induced into lactation using an estrogen/progesterone protocol. Net present value (NPV) was calculated for a 12-month stream of net incomes for induced animals and peer heifers. Magliaro et al. (1999) and Kensinger et al. (2000) calculated NPV for induced cows versus replacement heifers and found that NPV for induced cows was $520 greater than for replacement heifers. These calculations also considered the administration of bovine somatotropin (BST) to lactation induced cows to enhance milk yields, and estimated that the mean annual NPV advantage for using BST was $261/cow. If a protocol similar to this was approved by FDA, then inducing non-pregnant cows into lactation would be a method for dairy producers to increase profitability (Magliaro et al., 1999). Currently, steroid-based induced-lactation protocols are illegal because of concerns regarding consumer safety and presence of hormones in milk.

Early protocols for induction of lactation consisted of long term hormone treatment of 120 to 180 days and resulted in low milk yields and low rates of success (Hancock, 1954; Turner, 1956; Williams, 1960). Smith and Schanbacher (1973) utilized a seven day injection protocol using a combination of the hormones 17β-estradiol (0.1 mg/kg BW/d), and progesterone (0.25 mg/kg BW/d) and were able to successfully initiate lactation in 60% of treated animals (6 of 9 cows and 1 heifer) after the first series of injections with this shorter protocol. Although Smith and Schanbacher (1973) were successful in inducing lactation in a majority of animals with the shorter protocol, they also experienced a 40% failure rate. This high failure rate left opportunities for other scientists to examine protocols to improve success rate of artificial induction of lactation in nonpregnant and nonlactating dairy cows. Researchers attempted to increase the success of induction of lactation by extending the duration of hormonal treatment (Erb, 1976a; Erb, 1976b; Peel, 1978) or doubling the daily dose of steroids used (DeLouis et al., 1978), and several included use of either exogenous dexamethasone (Collier, 1975; Collier, 1976; Chakriyarat, 1978) and/or reserpine (Collier, 1977; Peel, 1978; Lembowicz, 1982) in the induction schemes. Exogenous reserpine can elicit an increase in blood prolactin concentration that lasts for several hours in cattle (Bauman et al., 1977), thus imitating the dramatic increase in prolactin seen several days prepartum in pregnant cows (Convey et al., 1974). It is also well documented that plasma glucocorticoids increase dramatically at the time of parturition in ruminant species (Convey, 1974; Heald, 1974; Collier, 1975), and administration of the synthetic glucocorticoid, dexamethasone, can mimic increased glucocorticoid levels (Collier et al., 1975).

Long term treatments using injections of estrogen alone or in combination with progesterone were thought to be important in mimicking pregnancy in the ruminant, and this long-term steroid stimulation was also thought necessary for complete mammary cell differentiation and development (Folley, 1944; Turner, 1956; Meites, 1961; Chakriyarat, 1978). Smith and Schanbacher (1973) were able to prove this theory wrong utilizing higher dosages of estrogen and progesterone injections for 7 days, but other researchers (Erb, 1976b; Peel, 1978) speculated that a longer duration of treatment was needed to improve success rates (the number of animals producing a defined milk yield) and to eliminate the variation in milk yields experienced using the 7 days treatment. Increasing the injection period from 7 days to 11 days, or 12 days, as performed by Peel et al.(1978) and Erb et al. (1976b) did not improve success rates (success > 1 kg/d within 24 after start of milking) or eliminate the variation in milk yields reported between animals induced into lactation. Lembowicz et al.(1978) were able to successfully reduce the 7-day estrogen and progesterone protocol to 5.5 day, or even 3.5 day, in multiparous polish black and white cows, so that its application would be more practical in commercial herds. In that study, doses of 17-β estradiol and progesterone, as outlined by Smith and Schanbacher (1973), were used in addition to eight single injections of reserpine on days 9 to 16 at a dose of 22.5 mg/cow/d. The lower milk yields resulting from this modified procedure were not significantly different from those of cows induced into lactation with the 7 days treatment, but the advantages were its simplicity and reduction of estrus-like excitement (mounting) that would be beneficial to producers (Lembowicz et al.,1982).

It is believed that the increase in circulating glucocorticoids at parturition is a necessary factor in initiating lactogenesis (Collier et al., 1975), thus leading researchers to examine the addition of exogenous synthetic glucocorticoid, dexamethasone, to induction protocols. Collier and co-workers (1975) modified the 7 days treatment outlined by Smith and Schanbacher (1973), by administering 3 single injections of dexamethasone (20 mg/cow/d) to 6 heifers and 10 cows on days 18, 19, and 20 of the induction protocol. This trial resulted in a 69% success rate (success > 9 kg milk/d at peak yield), but it was concluded that these results were similar to those reported by Smith and Schanbacher (1973) and that dexamethasone did not substantially improve the success rate. Chakriyarat et al. (1978), using 19 dairy cows of varied breed and age, examined the addition of 3 single injections of dexamethasone (.028 mg/kg BW/d) on days 18, 19, and 20 of the 7 days estrogen-progesterone induction protocol. These researchers reported that addition of dexamethasone injections increased the number of cows (9 of 11; 82%) successfully induced into lactation compared with cows induced into lactation without dexamethasone (3 of 11; 27%) (Chakriyarat et al., 1978). Eleven holstein cows and 9 guernsey cows were induced into lactation with 17-β estradiol (0.10 mg/kg BW/d) and progesterone (0.25 mg/kg BW/d) for 21 days, in addition to 3 single injections of dexamethasone (.028 mg/kg BW/d) on days 31 to 34, again improving success rate (success > 5 kg milk/d) but not milk yields compared with cows not receiving dexamethasone injections (Fleming et al., 1986). These results suggest that glucocorticoids may play an important role in initiating lactogenesis in the induced cow but apparently do not enhance milk yields. Collier and co-workers (1977) continued to pursue modifications to the 7 days estrogen/progesterone protocol with the goal of improving success rates and decreasing the variation in milk yields among animals, by incorporating the addition of reserpine injections. It was hypothesized that prolactin may be a limiting component of the lactogenic complex. In cows that fails to lactate following the 7 days estrogen and progesterone treatment (Collier et al., 1977). Reserpine injections were administered to nonpregnant cows either on days 13, 14, 15, and 16, or days 8, 10, 12, and 14 of the experiment. Days 13, 14, 15, and 16 were chosen so that prolactin levels in induced cows would mimic the increase in prolactin that occurs in pregnant cows during the period immediately prior to parturition (Convey, 1974; Collier, 1977).

Reserpine was administered on days 8, 10, 11, and 12 based on results of an earlier study that indicated that mammary tissue from induced cows was undergoing cellular changes associated with lactogenesis by day 8 and continuing through day 16 of treatment (Collier, 1976; Croom, 1976; Collier, 1977). Use of reserpine to cause prolactin release reduced variation in milk yields between animals and increased the success rate (success > 9 kg milk/d at peak yield) from 40 to 100% in cows administered reserpine on days 13 to 16, and from 75 to 100% in cows receiving reserpine injections on d 8, 10, 12, and 14 (Collier et al., 1977). Peel et al., (1978), utilizing dairy cows of mixed breed, demonstrated that administering reserpine (5 mg/d) on days 1, 6, 11, 16, and 21, in addition to the 7 day protocol described by Smith and Schanbacher (1973), did not increase milk yields, but increased the proportion of cows responding to lactation induction treatment when compared with controls receiving 17-β estradiol and progesterone injections alone.

Several modifications to the 7 day estrogen and progesterone treatment protocol have yielded higher success rates (success defined by the scientist, but usually > 9 kg milk/d peak yield), but little improvement in reducing variation in milk yields among animals have been reported (Collier, 1977; Chakriyarat, 1978; Peel, 1978; Fleming, 1986). Further research into development of new protocols for induction of lactation is required in order to continue improving the success rate, eliminate variability in milk yields between cows, and increase milk yields. During normal lactogenesis, progesterone concentrations in plasma begin to decrease approximately one week before parturition, and estrogen concentrations, already elevated during pregnancy, start to increase dramatically during the final weeks before calving (Fulkerson, 1979). Initiating a lactation induction scheme with all cows in a uniform phase of the estrous cycle may eliminate variation among animals that may be related to differences in concentrations of estrogen, progesterone, glucocorticoids, and prolactin at the time of initiation of lactation. Also, since progesterone competes with glucocorticoids for binding sites in mammary tissue (Fulkerson, 1979), removing the progesterone source (corpus luteum) by administering prostaglandin PGF2α may allow induced cows to be more responsive to reserpine and dexamethasone treatment following the initial estrogen and progesterone protocol. Induced luteolysis allows glucocorticoids to displace progesterone from binding sites in mammary tissue, thereby removing the progesterone block to lactogenesis (Fulkerson, 1979). Thus, use of PGF2α to induce luteolysis offers the opportunity to initiate a lactation induction protocol during very different stages of the estrous cycle; estrus and metestrus when progesterone is low and estrogen is elevated or diestrus when progesterone is the dominant steroid hormone. Exogenous PGF2α may then be used following the initial estrogen-progesterone therapy to insure luteolysis and depress circulating progesterone at the time of glucocorticoid and reserpine administration.

Somatotropin

The effect of pituitary extracts from cattle on milk yield in dairy cows was first reported in 1937 (Asimov and Krouze, 1937). The mean increase in milk yield from four treatment groups (all receiving the same dosage of pituitary extract) was 11 L. Young (1947) augmented the findings of Asmimov and Krouse (1937) and demonstrated in lactating dairy cows that somatotropin was the galactopoietic factor in pituitary extracts that stimulated milk yield in dairy cows. The amount of bovine pituitary extract needed to conduct extensive studies was limited because only small amounts of bovine somatotropin could be purified from each pituitary gland (5 to 15 mg) (Peel and Bauman 1987). Due to limitations in availability of pituitary derived somatotropin, studies could only involve small numbers of cows treated for a few days (Bauman, 1992). Breakthroughs in biotechnology in the early 1980’s enabled somatotropin to be produced by recombinant DNA technology. This resulted in the first study in 1982 in which recombinantly derived somatotropin was administered to domestic animals (Bauman et al., 1982). Daily injections of 25 mg and 50 mg increases milk yield in a dose dependent manner in buffaloes (Ludri et al., 1989). Bauman et al. (1985) reported that increased milk yield associated with somatotropin treatment caused a decrease in energy balance and voluntary feed intake increased with increasing dose of somatotropin to compensate for the decrease in energy balance.

Thyroproteins

Thyroprotein is a hormone containing product which is produced by iodination of casein under laboratory conditions. The active agent is thyroxine or an iodinated amino acid which has thyroxine- like properties. Thyroprotein feeding 50 days postpartum increase milk production by 7% (Schmidt et al., 1971), but result in marked reduction in body weight and condition, increases in services/conception and longer calving intervals than control cows (Schmidt et al., 1971). Literature indicates that thyroprotein feeding might best be utilized, if at all, in overly fat cows after peak lactation. Further, commercial suitability of thyroxine for increasing milk yield is limited because of reported decrease in milk quality following the withdrawal of treatment, and increased concentrations of thyroid hormones in milk (Astrup, 1985).

Conclusion

The estrogen, progesterone and prolactin play a very important role in lactogenesis. Glucocorticoids also play a role in mammary gland development leading to alveolar cell differentiation of the gland. The use of PGF2α to induce luteolysis offers the opportunity to initiate a lactation induction protocol during different stages of the estrous cycle. The modest increase in milk yield with somatotropin and thyroprotein supplementation is encouraging for further experimentation regarding the physiology of these hormones in milk induction.

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