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.

An experiment with 25 dairy cows was performed to investigate the effects of different energy intakes on food intake, performance and blood hormone and metabolite levels during the last 70 days of pregnancy and the first 125 days after parturition. Compared with animals fed ad libitum before parturition, cows fed only according to requirements during the same time showed no decrease of food intake at calving. Cows fed at a restricted level also showed a faster increase in food intake, a smaller energy deficiency at the onset of lactation and a smaller weight loss after parturition. Peak milk yield was also lower, but the peak was maintained for a longer period, and during the first 2 months of lactation these cows had higher concentrations of glucose and lactic acid, and lower concentrations of non-esterified fatty acids and ketone bodies in the blood. Weight losses, and decrease of milk production during the 1st weeks of lactation were higher in cows fed only 0-75 of requirements during the first 60 days of lactation than in animals whose energy intake was planned to cover energy requirements, and during the first 2 months of lactation levels of unesterified fatty acids, ketone bodies and urea were higher, whereas levels of insulin, thyroxine and triiodothyronine were lower. Endocrine and metabolic changes seen during early lactation suggested improved glucose homeostasis, diminished fat mobilization and ketogenesis in cows .given reduced amounts of energy during the dry period compared to animals fed ad libitum. To prevent the development of acetonaemia an adequate energy supply during the first part of lactation is, however, of greater importance.

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Berks RG2 p l TThe subjects of nutrient partition during lactation in ruminants and the hormonal control of the relevant aspects of metabolism, particularly in relation to blood hormone concentrations, have been extensively reviewed (Convey, 1974;Bassett, 1978;Trenkle, 1978Trenkle, , 1981Bauman & Currie, 1980; Bell, 1980;Cowie et al. 1980;McDowell, 1983). The purpose of this paper is to describe the metabolic situation in lactating cows; to briefly emphasize the more important aspects of the hormonal control of nutrient partition and to describe two of our recent experiments on the manipulation of hormone secretion. Metabolic status of the high-yielding dairy cowThe capacity of the cow to produce milk is determined by a combination of the growth and development of the mammary gland before and during pregnancy and the animal's ability to supply the substrates for milk synthesis once lactation is established. It is still not clear which of these factors is more important for maximum milk production. There is no doubt that, despite decades of intensive breeding, considerable variation remains in the ability of individual cows to produce what is currently regarded as a good milk yield in the UK (6000-8000 kg milk in a 305 d lactation). Some of the reasons for this variation are obviously genetic in origin and this is easily demonstrated by comparing the changes in milk yield and body-weight that occur during lactation in beef and dairy breeds of cattle offered the same ration (Hart et al. 1975, 1978, 1979). The former produce low yields of milk, in accordance with the smaller mammary gland, and preferentially partition dietary energy to milk production and supplement this by mobilizing considerable amounts of body tissue. This is an important economic consideration because improved milk yield in early lactation is associated with a proportionately greater partitioning of nutrients towards milk at the expense of body reserves at mid-lactation (Broster, 1976).Bauman & Currie (1980) have used the data of others to examine the relationship between energy intake and requirements for lactation in high-yielding cows (average 9534 kg milk containing 332 kg fat in a 305 d lactation). As expected, they found that the peak in milk yield preceded maximum dietary intake by several weeks and that during the first one-third of lactation the cows were in negative energy balance and mobilizing body tissue. In fact the cows did not consume sufficient energy to meet their requirements until approximately 16 weeks of lactation when milk yield had fallen to less than 80% of peak production. They emphasized this situation by calculating that during the first month of lactation the available at https://www.cambridge.org/core/terms. https://doi

1. The present paper reports the effects of dietary modifications on the diurnal pattern of concentrations of certain metabolites and hormones in the peripheral blood of lactating dairy cows. The cows were given fixed rations of hay and high-cereal concentrates in the proportions of 30:70 or 10:90 (w/w). The concentrates were given in either two or six equal meals daily; the hay was given twice daily.2. Previous reports of the same experiment had shown that milk-fat yield and concentration were reduced by increasing the proportion of concentrates in the diet and increased by more frequent feeding of the concentrates. These changes could be explained in part by changes in rumen volatile fatty acid (VFA) proportions and mean daily concentrations of VFA, particularly propionic acid, and insulin in the peripheral blood, but these factors failed to explain all the increase in milk-fat concentration caused by more frequent feeding.3. Analysis of blood samples taken at hourly intervals for 24 h at two stages of lactation showed that, in the cows fed six times daily, the concentrations of metabolites and hormones remained relatively constant over the day. In the cows fed twice daily, the concentrations of VFA, 3-hydroxybutyric acid and insulin all increased after both meals whereas the concentrations of glucose and growth hormone tended to fall. The concentration of nonesterified fatty acids tended to increase overnight and fall rapidly after the morning feed. The concentrations of glucagon, thyroxine and prolactin showed no clear pattern in relation to meals. The postprandial responses of propionate, insulin and growth hormone were greater with the higher concentrate diet. 4.The maximum concentration and the diurnal range of concentrations were reduced by more frequent feeding of both diets in the case of propionic acid and of the higher concentrate diet in the case of insulin, but the effects on insulin concentrations of more frequent feeding of the lower concentrate diet were smaller and not significant. The maximum concentration and the diurnal range of concentrations of growth hormone were unaffected by meal frequency.5. It is concluded that the severity of milk-fat depression in cows fed twice daily is increased by the rapid rise in propionic acid concentration in the peripheral blood after a meal, which in turn increases insulin secretion and may be accompanied by a suppression of growth hormone release. This causes lipogenesis to be diverted towards adipose tissue at the expense of the mammary gland. In cows fed more frequently, VFA absorption is more evenly spread over the day and is not closely associated with changes in insulin or growth hormone concentrations.

1. The present paper reports the effects on rumen fermentation and plasma metabolites and hormones of giving fixed rations of hay and high-cereal concentrates at different meal frequencies to lactating cows. In Expt 1 the total ration was given in two and twenty-four meals daily and in Expts 2 4 the concentrates were given in two and five or six meals and the hay in two meals daily. The diets contained 60&920 g concentrates/kg.2. In Expt I, minimum rumen pH was higher but mean pH was lower when cows were given their ration in twenty-four meals/d rather than two meals/d.3. In all the experiments, the effects of increased meal frequency on the molar proportions of rumen volatile fatty acids (VFA) were small and not significant, although there was a general tendency for the proportion of acetic acid to increase and that of propionic acid to fall. Increasing the proportion of concentrates in the diet reduced the proportion of acetic acid and increased the proportions of propionic and n-valeric acids.4. In Expt 3, more frequent feeding was found to reduce the concentration of non-esterified fatty acids in the blood, but changes in other metabolites were small and not significant. Increasing the proportion of concentrates in the diet reduced the concentrations of acetic acid and 3-hydroxybutyric acid and increased the concentrations of propionic acid and glucose.5. The mean daily concentration of insulin in the blood was reduced by more frequent feeding of the higher-concentrate diet but not of the lower-concentrate diet. The concentration of glucagon also tended to fall with more frequent feeding. Increasing the proportion of concentrates in the diet increased the concentration of insulin.6 . More frequent feeding reduced the depression in milk-fat concentration caused by feeding the low-roughage diets. About three-quarters of the variation in milk-fat concentration could be related to changes in rumen VFA proportions, but the relations for the two meal frequencies had different intercepts although similar curves. The results suggest that milk-fat depression on low-roughage diets with twice-daily feeding was due to a change in rumen VFA proportions accompanied by elevated plasma insulin concentrations. The improvement in milk-fat concentration due to more frequent feeding could be explained partly by the small change in rumen VFA proportions and partly by a reduction in mean plasma insulin concentrations, but these mechanisms did not fully account for the milk-fat responses observed.In a previous paper it was clearly demonstrated that when the concentrates in a low-roughage diet were given in several meals daily rather than in two meals at milking time, the milk-fat depression caused by the diets was reduced . Similar results were obtained by Kaufmann et al. (1975), who suggested that the response could be ascribed to an increase in the ratio of the molar proportions of acetate: propionate in the rumen volatile fatty acids (VFA). Milk-fat depression induced by low-roughage diets is certainly closely related to the ty...

Recording electrodes were implanted in contact with the dura mater overlying the parietal cortex of six female goats, four of which were lactating. After recovery from surgery and complete familiarization with the housing conditions, the personnel and the recording technique, each goat was observed continuously for 24 h with simultaneous recording of the cortical electroencephalogram (EEG). Remote blood sampling was carried out every 30 min without disturbing the animal. Apart from the release of growth hormone (GH) associated with morning milking in two of the goats, there was no consistent relationship between the apparently spontaneous, episodic release of GH and behvaiour, stages of sleep, cortical EEG, air temperature, time of day or night, obvious environmental stimuli which arose from the normal husbandry routine, or the levels of porlactin, insulin, glucose or free fatty acids in the blood. There was also no relationship between the release of prolactin and the stages of sleep.

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.