This chapter discusses the development of the concepts of homeostasis and homeorhesis in the regulation of metabolism, regulation of nutrient use during lactation, and the mechanisms of nutrient partitioning during lactation in dairy cows.

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PRL involvement in periparturient mammary development and onset of milk secretion was studied in 17 multiparous, monotocous Holstein cows. Mammary glands were obtained 10 days prepartum from untreated cows and 10 days postpartum from untreated cows and cows treated with CB154 (2-Br-a-ergokryptin) or CB154 plus PRL. CB154 was administered for 12 days before expected parturition through 10 days postpartum. PRL was infused continuously for 6 days immediately before parturition in cows receiving CB154 plus PRL. Treatment with CB154 reduced basal serum PRL concentrations approximately 80% and blocked the normal surges in serum PRL concentrations at parturition and during milking. Average milk production in cows given CB154 alone was 11.4 kg/day lower than in controls. Periparturient infusion of PRL in cows treated with CB154 (CB154 plus PRL) prevented reductions in milk production. Treatment with CB154 had no effect on feed intake or periparturient serum concentrations of GH, progesterone, and glucocorticoids. Mammary glands from postpartum controls or cows treated with either CB154 or CB154 plus PRL contained equivalent total amounts of DNA. Inhibition of PRL secretion reduced total mammary RNA content 36% and decreased the RNA/DNA ratio (1.38) relative to ratios in cows that received PRL replacement therapy (2.17) or untreated lactating controls (1.86). Thus, reduced milk yields among cows treated with CB154 were associated with reduced mammary secretory cell activity rather than reduced cell numbers. Biosynthetic rates of fatty acid and lactose synthesis were lower in mammary tissue from cows given CB154. These rates were related to decreased activities of acetyl coenzyme A carboxylase, fatty acid synthetase, and a-lactalbumin. For all mammary biochemical parameters, cows given CB154 plus PRL had values equal to those of controls. We conclude that periparturient secretion of PRL is essential for maximal synthesis of milk in the postpartum period and that PRL plays a critical role in mammary differentiation of key biochemical steps involved in synthesis of milk. {Endocrinology 109: 23, 1981) I N LABORATORY species, PRL is required for initiation and maintenance of lactation (1), but its role in cattle is unclear. For example, reducing concentrations of serum PRL during established lactation does not reduce milk yields in cows (2-4) as it does in nonruminants (5, 6). On the other hand, Karg and Schams (7) reported that cows treated periparturiently with CB154 (2-Br-aergokryptin) exhibited markedly reduced concentrations of plasma PRL and that subsequent milk production was nearly abolished. Johke and Hodate (8) reported that

Branched chain fatty acids (BCFA) have recently been shown to be a major component of the normal human newborn gastrointestinal tract and have long been known to be a component of human milk. Ruminant food products are major sources of fat in the American diet, but there are no studies of milkfat BCFA content in retail milk. We report here the profile and concentrations of BCFA in a representative sampling of retail milk in the 48 contiguous United States (US), and their estimated intake in the American diet. Conventionally produced whole fluid milk samples were obtained from 56 processing plants across the contiguous 48 states. Retail milk samples contain exclusively iso-and anteiso-BCFA with 14-18 carbons. BCFA were 2.05 ± 0.14%, w/w of milkfat fatty acids (mean ± SD), and anteiso-BCFA comprised more than half this total. Based on these data and USDA food availability data, the average per capita BCFA intake of Americans is estimated to be about 220 mg/d from dairy; if current dietary recommendations were followed, BCFA intake would be about 400 mg/d. Adding intake from beef consumption, these estimates rise to approximately 400 and 575 mg/d, respectively. These results indicate that BCFA intake is a substantial fraction of daily fat intake, in amounts exceeding those of many bioactive fatty acids.