MAMMARY GLAND AND LACTATION 



GK 



that certain dose levels of estrogen in the 

 lactating cow produced long-lasting changes 

 in milk composition characterized by in- 

 creases in the percentages of fat and non- 

 fatty solids. This was regarded as an exam- 

 ple of galactopoiesis and was termed the 

 "enrichment" effect. The effect, however, w^as 

 somewhat erratic and it has recently been 

 re-investigated by Hiitton (1958) who con- 

 firmed and extended the earlier observations. 

 Hutton found that galactopoietic responses 

 (Figs. 10.14 and 10.15) were obtained only 

 within a restricted dose range, the limits 

 of which were affected by the stage of preg- 

 nancy and the breed of the cow. Hutton 

 further concluded that in the normal cow 

 changes in milk composition and yield as- 

 sociated with advancing pregnancy were 

 probably determined by the progressive rise 

 of blood estrogen levels. 



D. THYROID HORMONES 



Studies on the effect of removal of the 

 thyroids on milk secretion have been re- 

 viewed by one of us (Folley, 1952a) ; the 

 evidence strongly suggests that the thyroid 

 glands are not essential for milk secretion, 

 but in their absence the intensity and dura- 

 tion of lactation is reduced. Histologic and 

 cytologic studies of the thyroid of the lac- 

 tating cat suggest that there is a consider- 

 able outpouring of the thyroid secretion in 

 the early stages of lactation (Racadot, 

 1957), and Grosvenor and Turner (1958b) 

 have reported that the thyroid secretion 

 rate is higher in lactating than in nonlactat- 

 ing rats. 



Since the last edition of this l)ook, a great 

 volume of experimental results has been 

 published on the use of thyroid-active ma- 

 terials for increasing the milk yield of cows. 

 These experiments have been extensively 

 reviewed by Blaxter (1952) and Meites 

 (1960) and we need here only touch on the 

 salient points. 



In the early studies i^reparations of dried 

 thyroid gland were fed to cows or injections 

 of DL-thyroxine were given, but the use on 

 a large scale of thyroid-active materials 

 for increasing the milk yield of cows only 

 became feasible when it was shown that 

 certain iodinated proteins exhibited thyroid- 

 like activitv when given in the feed. Al- 



9-9 

 97 



o 9-3 

 ^ 9-1 



8-9 



•'' Guernsey 



Shorthorn 



8-5 



•^U^ri 



I L 



20 40 60 80 100 



Oestradiol monobenzoite (mg) 



Fig. 10.14. Effect of graded doses of estradiol 

 benzoate on percentage of nonfatty solids in milk 

 from cows of three breeds. (From J. B. Hutton, 

 J. Endocrinol., 17, 121-133, 1958.) 



Oestradiol monobenzoate (mg) (arith. scale) 



10 20 30 40 50 



6-25 12-5 250 500 



Oestradiol monobenzoate (mg) (log scale) 



Fig. 10.15. Effect of graded doses of estradiol 

 benzoate on fat content of cows' milk. Upper curve, 

 doses plotted on arithmetic scale. Lower curve, 

 doses plotted on logarithmic scale. (From J. B. 

 Hutton, J. Endocrinol., 17, 121-133, 1958.) 



though these materials were readily made 

 and were economical for large-scale use, they 

 possessed several disadvantages. Their ac- 

 tivity was difficult to assay and standardize, 

 they were frequently unpalatable, and their 

 administration entailed a considerable in- 

 take of iodine which could be undesirable. 

 Nevertheless, a large number of experiments 

 were carried out all over the world with 

 this type of material. In 1949, however, a 

 new and improved method for the synthesis 

 of L-thyroxine was developed (Chalmers, 

 Dickson, Elks and Hems, 1949) and thyrox- 

 ine became available in large quantities. 

 It was then shown jjy Bailey, Bartlett and 

 Folley (1949) that this material was ealac- 



