To obtain information on the physiological mechanisms controlling the partition of dietary energy between body weight and milk production in lactating cattle, the concentrations of hormones (prolactin, growth hormone, insulin and thyroxine) and metabolites (glucose, non-esterified fatty acids, \g=b\-hydroxybutyric acid and l-lactic acid) in plasma obtained from eight high-yielding and seven low-yielding cows, matched for diet and stage of lactation, were compared. Blood samples were taken via a jugular catheter, throughout four 48 h periods at various times during lactation (days 40, 80, 120 and 180) and also when the animals were dry.The milk yield rose for 7 weeks after parturition, reaching peak values of 24\m=.\8and 10\m=.\1 kg/day in the high-and low-yielding groups respectively. The body weights of the l ow\ x=req-\ yielding animals rose steadily, whereas cows in the high-yielding group lost weight up to week 14 of lactation and thereafter gradually gained weight. No differences were found between the groups in the digestibility of the diets, although analysis of the rumen fluid indicated small differences in the proportions of volatile fatty acids. Milk analysis suggested that the protein content of milk obtained from low-yielding cows was greater than that of milk from high-yielding animals.Throughout lactation, the concentrations of growth hormone (P < 0\m=.\001), non-esterified fatty acids (P < 0\ m=. \ 01) and \g=b\-hydroxybutyric acid (P < 0\ m=. \ 05)were higher in the high-yielding than in the low-yielding group, whereas the concentration of insulin was higher (P < 0\ m=. \ 01) in the low-yielding animals. The level of thyroxine was higher (P < 0\ m=. \ 05) in the low-yielding group when the animals were dry. A comparison of peak lactation (days 40 and 80) with the dry period demonstrated that the concentration of prolactin was higher in both groups hi gh\x=req-\ yielding, P < 0\ m=. \ 01 ; low-yielding, P < 0\m=.\001) when the animals were dry and the concentration of thyroxine in the low-yielding cows was higher (P < 0\ m=. \ 01) when the animals were dry. Significant reductions in the concentrations of growth hormone (P < 0\m=.\001) and non-esterified fatty acids (P < 0\m=.\001) were accompanied by an increase in the concentration of glucose (P < 0\ m=. \ 01) when the high-yielding animals were dry.

Twenty-four Friesian cows were allocated to one of four diets for weeks 3-14 of lactation following 2 weeks on a common diet. The diets (kg air-dry feed/day) were 7-2 kg hay and 10-8 kg either starchy or fibrous concentrates (60S and 60F) or 3-5 kg hay and 14-0 kg either starchy or fibrous concentrates (80S and 80F). Rumen samples were taken by stomach tube in weeks 10 and 12 of lactation and the digestibility of the diets was measured with four cows per treatment during weeks 13 and 14. The digestibility of the same feeds was also measured in sheep at maintenance.The principal carbohydrate constituents of the concentrates were barley, wheat and cassava in the starchy concentrates and citrus pulp, sugar-beet pulp and wheat feed in the fibrous concentrates. The concentrates were designed to have similar concentrations of metabolizable energy (ME) and the diets were planned to provide similar intakes of digestible energy and crude protein.Milk yield and composition were very similar for treatments 60S and 60F. With the higher proportion of starchy concentrates (80S), milk yield was about 20% greater than on 60S, fat concentration fell severely but protein and lactose concentrations were unaffected. With the higher proportion of fibrous concentrates (80F), milk yield and the protein and lactose concentration were similar to values on 60F but fat concentration was lower, though not nearly so low as on 80S. Milk energy yield was reduced by the higher proportion of concentrates but was unaffected by type of carbohydrates. Live-weight changes were small.In both the sheep, consuming at maintenance, and the lactating cows consuming at about 3 times maintenance, digestibility of dry matter, organic matter and energy was higher with the higher concentrate diets but was unaffected by type of concentrate. The digestibility of fibre was greater with the fibrous concentrates but the effect of level of concentrate inclusion was inconsistent. Digestibility coefficients were consistently lower for the lactating cows than for the sheep.The proportion of acetic acid in the rumen volatile fatty acids in the cows was higher and the proportion of propionic acid was lower with the fibrous concentrates. The differences were much greater with the higher proportion of concentrates.During weeks 15-22 of lactation the cows were reallocated to concentrate treatments and given hay ad libitum. Hay intake was about 1 kg/day higher with the fibrous concentrates but the difference was not significant. Hay intake fell by about 0'6 kg/kg concentrate intake for both concentrate types. No significant differences in milk yield or composition were established, probably because of incomplete adaptation even after 8 weeks.

Supplements of protected soya-bean-tallow were incorporated at 0, 1-7, 3-0 or 5-0 kg/day into a hay-concentrate diet and fed ad libitum to Friesian cows during the first 13 weeks of lactation.Intakes of supplement during weeks 1-6 were 0,1 • 34, 2-25 and 3-18 kg/day and during weeks 7-13, 0, 1-64, 3-10 and 4-24 kg/day on each of the respective treatments.As the lipid supplement increased there was a reduced intake of hay and concentrates, giving rise to reduced intakes of total, protein, and particularly 'carbohydrate' (non-protein, non-fatty-acid) energy. Apparent digestibilities, particularly of fibre, decreased as fatty-acid intake increased. After correcting for endogenous losses, true digestibilities of organic matter, carbohydrate, protein, non-fatty-acid lipid, total energy, calcium, magnesium and phosphorus were unaffected by the amount of supplement given, whereas fibre digestibility was reduced by increasing the supplement. Fatty acid digestibility was little affected up to an intake of 1-4 kg/day but above this there was a sharp reduction.Yields of milk, milk energy, protein, fat and lactose were increased at low lipid supplementation but reduced at the highest supplementation when amounts of protein and carbohydrate digested were lowest.Lipid supplementation reduced concentrations of rumen total volatile fatty acids in association with reduced, proportions of butyrate and. increased proportions of propionate. Plasma ^-hydroxybutyrate and magnesium were reduced and plasma free fatty acids increased in cows given lipid supplemented diets. There were no differences between treatments in plasma glucose, lactate or calcium nor in live-weight changes.

Potential of the bovine mammary gland to synthesize milk probably is determined fully at parturition. Realization of that potential depends on the gland receiving adequate amounts of nutrients in the correct proportions. Provided the cow is fed adequately in relation to her potential yield, supply to the body of those nutrients that are absorbed directly from the gut is not likely to limit milk synthesis. Glucose and long-chain fatty acids, however, are not absorbed from the gut in adequate amounts, and metabolic factors (primarily hormonal regulation) determining the supply of these may be important limits to milk synthesis. Even when amounts and proportions of metabolites entering circulation are adequate for maintenance and maximum milk synthesis, the cow's potential cannot be realized fully unless nutrients are channeled selectively towards the mammary gland. This occurs naturally in the high-yielding cow in early lactation and probably can be maintained artificially into midlactation by use of growth hormone. Consequences of this on milk output, health, and reproductive performance are considered.

I. Six adult, non-pregnant, non-lactating, Friesian cows were used when fat and when thin to measure differences in the voluntary intakes of straw, hay and hay plus concentrates caused by the fatness of the animals. Measurements of digestibility, time of retention of food in the digestive tract, rate of breakdown of cotton threads in the ventral sac of the rumen and amounts of digesta in the reticule-rumen were included.2. The mean voluntary intakes of straw were similar for fat and thin cows. In absolute terms, thin cows consumed 31 % more hay and 23 yo more hay and concentrates than fat cows; in relation to metabolic body size ( W 0 9 , these differences were 76% and 52% respectively.3. Small decreases in digestibility of these diets by the thin cows, reflected in slight reductions in the rate of loss of weight of cotton threads placed in the rumen, did not alter the significance of the differences in intake between fat and thin cows.4. Small changes in time of retention of food in the digestive tract suggested that the capacity of the tract may have been greatest in the thin cows.5 . The presence of a greater amount of digesta in the reticulo-rumen of thin cows than in that of fat cows after eating hay supports the suggestion of a greater gut capacity in these animals. In both fat and thin cows, the capacity of thereticulo-rumen didnot appearto havelimited the intake of the hay and concentrate diet. In both groups the lowest levels of rumen fill were observed after straw was given.6. The results are discussed in terms of possible mechanisms which may operate to reduce the voluntary intake of medium-and good-quality diets as cows become fatter. When poorquality roughages are given, other factors appear to conceal any differences in intake which may exist between fat and thin cows.It is well known that in adult man and adult animals body-weight is usually maintained at more or less constant levels, sometimes for very long periods of time. Thus a long-term regulation of food intake must have an important and precise role. As a possible mechanism bringing about such control, [Kennedy, 1953) postulated a ' lipostatic' theory of regulation of voluntary food intake. This theory proposes that some compound, released from the fat depots in proportion to their total size, inhibits food intake. Thus, over a period of time, food intake is gradually reduced as the animal becomes fatter, until eventually only the maintenance requirement of the animal will be met, and the animal will thus gain no further weight.The quantitative effects of such a mechanism in ruminants are not clear. Schinckel (1960) observed a progressive decline in the ad lib. intake of grazing sheep, which was associated with increasing bodily fatness. However, this was confounded with an increase in environmental temperature during the last part of the experiment. He refers also to a syndrome in extremely fat sheep in which some unknown factor precipitates a complete refusal to eat. The animals lose weight and usually die in Japan.