Page 527 - Anatomy and Physiology of Farm Animals, 8th Edition

512 / Anatomy and Physiology of Farm Animals

also enhanced by glucocorticoids, which short in duration (minutes to an hour). This
periodic and relatively brief surge of prolac-
also increase in the blood prior to parturi-
VetBooks.ir tion in most species. tin is essential to maintain normal lactation
The placenta of ruminants produces a
in most species, but prolactin does not
protein hormone, placental lactogen, or appear to be an essential regulator of lacta-
chorionic somatomammotropin, which is tion in cows. Supplementation with prolac-
similar in structure and function to prolac- tin does not increase milk secretion in cows,
tin. Placental lactogen production increases and lactation is maintained in cows in spite
in late gestation in ruminants and is of severe reductions in blood levels of
believed to be more responsible for mam- prolactin.
mary gland development in these species Growth hormone appears to be espe-
than prolactin from the adenohypophysis. cially important in cows, whose blood
The high metabolic demands of lacta- levels of growth hormone are directly
tion require increased activity of thyroid correlated with the maintenance and level
hormone in the mammary tissue and of milk production. Growth hormone is
removal of the thyroid gland decreases milk also known as “somatotropin” and is syn-
yield in cows. The primary thyroid hor- thesized and secreted by somatotropes of
mone produced in the thyroid gland is the adenohypophysis. Even though growth
3,5,3′,5′‐tetraiodothyronine (T , thyroxine), hormone appears to be a primary regulator
4
but the more biologically active form of the of milk production in cows, neither suck-
hormone is 3,5,3′‐tri‐iodothyronine (T ). ling nor milking are able to produce an
3
Deiodination and therefore conversion of immediate release of growth hormone in
T to T in the mammary gland is consid- the lactating cow. Supplementation with
3
4
ered a principal mechanism by which the growth hormone, or bovine somatotropin
mammary tissue is able to meet the (bST), is able to increase milk production
metabolic demands of lactation and locally in cows by 10 to 40%, or an average of
regulate thyroid hormone homeostasis. 10 pounds of milk per day per cow. This
Normal glucocorticoid hormone levels are increased production is the result of a vari-
also imperative for normal milk production. ety of physiologic changes that promote
Dairy cows, given high exogenous doses of milk production in a healthy cow with
glucocorticoids or exposed to environmen- adequate access to good quality nutrition.
tal or behavioral stress, will experience a These changes include increased mobiliza-
decrease in milk production. Conversely, tion of body energy stores and alterations
removal of the adrenal glands and the in overall protein metabolism in other
endogenous source of the glucocorticoid organs to provide substrates for milk secre-
hormones will also cause lactation to arrest. tion, increased food intake, increased
nutrient absorption from the gastrointesti-
nal tract, and improved efficiency of the
Galactogenesis conversion of nutrients to milk by the
mammary gland. It is presumed that many
The continuation of lactation requires stim- of the effects of growth hormone are medi-
uli to promote milk production and removal ated via insulin‐like growth factors (IGFs),
or inhibition of stimuli that retard milk pro- whose production is directly increased by
duction. Stimulation of the nipples (teats) growth hormone (see Chapter 13). The
by suckling (or milking) elicits an abrupt production of bST using recombinant
increase in blood levels of prolactin. The biotechnology (rbST) was first studied
increased secretion of prolactin is the result in dairy cows in 1981. In rbST‐treated
of a neural reflex mediated through the cows, there was a net improvement in
hypothalamus that regulates prolactin milk output per feed resource input,
release from lactotropes of the adenohypo- commonly termed “productive effi-
physis. The increase in prolactin is relatively ciency.” In 1993, the United States