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 non-esterified 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.