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 propionat. Plasma β-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.

Mean digestible energy (DE) intakes of 147 cows of three parities receiving three levels of DE including one ad libitum (about 2·2, 2·6 and 3·3 multiples of maintenance (MM)) drawn from three mixed diets containing hay and 60, 75 or 90% compound were calculated.

Rumen samples were taken from three-quarters of the cows at monthly intervals throughout the experiment. Molar proportions of volatile fatty acids (VFA) in the rumen were not affected by stage of lactation over the 36 weeks of the experiment. The proportion of acetic acid decreased and that of propionic acid increased with greater intakes and with higher proportions of compound in the diet. The proportion of n-butyric acid was little affected by level of intake but decreased with increasing proportions of compound. In young, non-lactating cattle given the same diets but at lower levels of intake, VFA proportions were unaffected by diet composition at intakes of about 0·7 MM but at intakes of about 1·3 MM the proportion of acetate decreased and that of propionate increased when the proportion of compound was increased from 75 to 90%.

The efficiency of milk energy production in relation to DE or metabolizable energy (ME) above maintenance decreased with increasing level of intake but was little affected by the proportion of compound. Partition towards live weight increased with level of intake in early lactation but not in late lactation. It also increased with higher proportions of compound in mid and late lactation but not in early lactation.

Estimates of the ME requirement for live-weight change (LWC) were in reasonable agreement with recently published standards in early lactation when live weight was decreasing, but later in lactation when live-weight gain was occurring, a much higher value was calculated which is difficult to reconcile with these standards.

Rumen VFA proportions were related to dietary fibre concentration and level of intake additively. The relationship to VFA proportions was close for milk fat concentration, but less so for energy partition towards live weight and none was apparent for the efficiency of ME utilization for milk energy production. It is suggested that both the reduction in milk fat concentration and the increase in milk yield in response to reductions in the fibre content of diets may be independently related to the increase in the proportion of propionate in the rumen VFA.

It is concluded that further progress in studies of the dietary factors affecting the efficiency of milk production will require measurements of nutrient uptake from the digestive tract and description of milk production and LWC in terms of their chemical composition rather than energy alone.

Ninety-nine lactating British Friesian cows, in their second and subsequent lactations, were divided into two groups receiving ad libitum grass silage. The cows in group A were offered silage individually through Calan Broadbent gates while those in group B were given 24 h/day access to a self-feed silage clamp. The crude protein (CP), acid-detergent fibre (ADF), neutral-detergent fibre (NDF), in vitro digestible organic matter and estimated metabolizable energy (ME) values in the D.M. of the silage were 140, 373, 584, 610g/kg D.M. and 9·7 MJ/kg D.M. respectively. The cows in each group received either 11kg/day fresh weight of concentrate S in which the principal energy source was cereal starch or 9 kg/day fresh weight of concentrate F containing mixed high quality fibre sources plus 2 kg/day fresh weight of concentrate S. The CP, ADF, NDF, starch and sugar concentrations in concentrates S and F were 203, 71, 204, 444, and 44 and 202, 147, 329, 116 and 149g/kg D.M., respectively.

The only significant difference produced by method of silage allocation was that groupfed cows gained more live weight (P < 0·01) than those fed individually.

Over the whole experimental period the mean silage D.M. intake of cows fed concentrate F in group A was 0–5 kg/day higher than those fed concentrate S. However, in early lactation the benefit to silage intake in favour of concentrate F was 1 kg D.M./day.

In group A estimated intakes derived from calculated ME inputs and outputs were markedly lower than the observed intakes, but were similar to correspondingly derived estimates for group B.

Although concentrate type did not affect milk yield, concentrate F was associated with a higher fat concentration and yield (P <0–05) but a lower protein concentration (P < 0·001) and a slightly lower yield.

The in vivo digestibility coefficients for D.M. and organic matter determined in lactating dairy cows given concentrates S and F respectively were similar (0·724 v. 0·716 and 0·749 v. 0·742) but that for ADF was significantly (P < 0·001) higher (0·565 v. 0·673) in the ration containing concentrate F than in that containing concentrate S.

Estimates of the mean efficiency of utilization of the production ME made on a weekly basis were unaffected by concentrate type.

Ninety-two British Friesians (56 cows and 36 heifers) were divided into two groups during weeks 3–10 of lactation and received ad libitum either complete diet H (65% concentrates, 35% grass silage, 11·7 MJ ME/kg D.M.) or diet M (50% concentrates, 50% grass silage, 11·2 MJ ME/kg D.M.). In weeks 11· 20 half the animals of each group continued to receive the same diet (HH or MM) while the other half were changed from diet H to M (HM) or from diet M to L (ML). Diet L contained 35% concentrates, 65% grass silage and 10·7 MJ ME/kg D.M. The crude protein concentration of all diets was maintained at about 157 g/kg D.M. by the addition of soya-bean meal to diets M and L.

During weeks 3–10 the mean D.M. intake (kg/day), milk yield (kg/day), milk protein (g/kg) and live-weight change (kg/day) for animals on diets H and M were 15·0, 21·7, 33·3, 0·50 and 12·9, 20·2, 31·5, 0·35 respectively. Differences were significant for D.M. intake, milk yield and milk protein concentration. Although milk-fat concentration of cows given diet H (40·2 g/kg) was similar to that of cows given diet M (39·9 g/kg), heifers receiving diet H had a significantly lower value (37·0 g/kg) than those given diet M (39·8 g/kg).

When animals were changed from diet H to M or from M to L at week 11, D.M. intake, milk yield and milk-protein concentration decreased. However, milk-fat concentration and live-weight change were generally unaffected.

During the complete experimental period, weeks 3–20, animals on treatments HH, HM, MM and ML had mean D.M. intakes and milk yields of 15·3, 14·2, 13·2, 12·4 kg/day and 19·7, 18·5, 17·8, 17·0 kg/day. Milk-protein concentration, was closely related to D.M. intake, but only with heifers did diet H significantly depress milk-fat concentration. The highest live-weight gain was recorded on treatment HH (0·40 kg/day) and the lowost on treatment ML (0·29 kg/day) with intermediate values for treatments HM and MM.

In vivodigestibility coefficients of D.M. and organic matter for diet H (0·731, 0·757) were significantly higher than those of diet L (0·707, 0·734) with intermediate values for diet M. However, for acid-detergent fibre the digestibility coefficient for diet H was significantly lower (0·562) than for diet L (0·662). The digestibility coefficients for nitrogen in all three diets were similar.

Estimates of mean efficiency of utilization of ME for lactation were made on a weekly basis and were 0·56 and 0·64 for animals on diets H and M in weeks 3·10. During weeks 11·20 the mean values for animals on treatments HH, HM, MM and ML were 0·46, 0·53, 0·51 and 0·59, respectively.

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.

It is concluded that at concentrate intakes of about 10 kg/day, the source of carbohydrate in the concentrates has little effect on milk production when the concentrates are of similar ME concentration. However, at higher levels of concentrate inclusion, although the diets may have similar ME concentrations, important differences in the yields of fat, protein and lactose occur due to carbohydrate source and these can be related to differences in rumen fermentation.

Seventy-six first-calf cows (heifers), 32 second·calf cows and 39 adult cows participated in a trial in which three diets composed of 60:40, 75:25, 90:10 proportions of compound to hay (hay dry-matter digestibility 0·6) were given for the first 36 weeks of lactation in amounts to provide daily an estimated 125, 156 MJ and ad libitum digestible energy (DE) for heifers, and 156, 187 MJ and ad libitum DE for the other parities. The ad libitum intakes were 150 and 200 MJ DE daily for heifers and for older cows respectively. Greater compound content of the diet and larger daily allowance of DE were associated with increased occurrence of mastitis and lameness but not digestive upsets. Dry·matter (D.M.) digestibility was increased with a greater proportion of compound in the diet but was not affected by the amount consumed or stage of lactation. Milk yield and live·weight change were increased linearly and additively by a greater intake of digestible dry matter and by a greater proportion of compound in the diet, throughout the lactation save for no effect of diet composition on live·weight change in early lactation. On equal feed intakes older cows gave more milk and lost more live weight than heifers in early lactation but not thereafter; responses to additional feed were equal for all parities studied. Milk fat concentration was reduced both by increased allowance and by the high compound diet. Milk solids·not·fat (SNF) concentration was not affected by the treatments. Yield of milk fat was not affected by amount eaten and was reduced by the 90:10 diet. Yields of SNF and milk energy were increased by amount eaten and by increased compound in the diet. Thirty-two primiparous cows continued on their experimental treatments without interruption to the end of the second lactation. In the second lactation the effects on yields of milk, milk energy, and SNF were magnified indicating a cumulative effect from lactation to lactation. Little cumulative effect was observed on either milk fat yield, or concentrations of fat and SNF in milk. More generous feeding was associated with reduced milk output/unit D.M. consumed in the first but not the second lactation. More generous feeding improved live·weight change less in the second than in the first lactation. No cumulative effects over two lactations were observed consequent upon variation in diet composition.

Digestibility trials were conducted at two stages of lactation with each of 107 Friesian cows using three diets (60:40, 75:25, 90:10 proportions of compound feed to medium quality hay fed long). Each cow received throughout lactation fixed daily allowances of one of these diets at either 2·26 or 2·63 times average maintenance requirements (MM), or was fed ad libitum, average intake equalling 3·07 MM. The digestibilities of the diets were also measured on immature cattle at 0·76 and 1·33 MM and sheep at 0·65 and 1·27 MM.

For all classes of stock the digestibility of dry matter increased linearly and that of fibre decreased curvilinearly with increasing proportion of compound in the diet. There was no interaction of effects of diet composition and amount consumed on digestibility of feed. The digestibility of both dietary components declined for the young cattle and sheep with increased intake. The digestibilities for lactating cows were lower than for other stock but did not change with amount consumed or milk yield. Digestibility was slightly higher for first and second calf cows than for adult cows; and was higher for organic matter in mid lactation compared with early lactation for adult cows.

These results for dairy cows conflict with the generally accepted interpretation that a linear decline in digestibility occurs from once to five times maintenance intake, embracing all classes of stock.

The influence of physiological state (pregnant, lactating, dry) on body composition and fat partitioning in Friesian cows has been examined. A total of 20 cows, four per physiological state, were slaughtered and their left half carcasses dissected into individual muscles, bones and fat depots. All body parts, including the internal organs and fat depots, were weighed at slaughter.

Muscle tissue and the internal organs showed some weight changes, consistent with a redistribution of tissue towards the udder and gut, and mobilization of muscle, but the major changes in weight associated with changing physiological state occurred in total body fat.

Intermuscular fat made the greatest absolute contribution to changing fat weight, but subcutaneous fat showed the greatest proportional change with changing physiological state. The order of depletion of fat depots during fat loss was approximately the reverse of the order of development found during developmental growth.

The distribution of subcutaneous fat between seven defined regions of the carcass was not affected by differences in total fatness in different physiological states. This, and the high correlation found between fat depth and total body fatness, confirms the view that measures of subcutaneous fat depth, e.g. by ultrasonics, should be useful predictors of the energy status of cows in varying physiological states.

During the first 13 weeks of lactation, four groups of seven cows were given hay and a basal concentrate mix containing 0, 1·7, 3·3 or 5·0 kg/day of a protected lipid supplement (320 g long-chain fatty acid/kg D.M.). The proportion of hay to total concentrate was maintained at 25:75 and cows were fed to appetite.

The total amount of energy digested corrected for endogenous loss (DEc) was partitioned into crude protein energy (CPEc), fatty-acid energy (FAEc), ‘carbohydrate’ (non-protein non-fatty acid) energy (CEC) and non-fatty-acid energy (NFAEc = CPEc + CEc).

Partial efficiency of milk production and live-weight gain was curvilinearly related to FAEc, maximum efficiency occurring at about 30 MJ/day of FAEc.

Net energy utilized was significantly related to DE0c for both of the periods considered, Weeks 2–6 and weeks 7–13. In addition, a curvilinear effect of the ratio of fatty acid energy to non-fatty-acid energy was established. At fixed intakes of digestibl energy, net energy utilized was maximal for ratios of FAEc/NFAEc of 0·14 and 0·18 for weeks 2–6 and 7–13 respectively.

Milk yield was significantly related to DEc for weeks 1–6 and 7–13; milk energy yield was significantly related to DEc for weeks 7–13. The multiple regression relationships between milk yield or milk energy yield and CPEc, FAEc and CEc gave a negative effect of crude protein, possibly indicating that the proportion of total energy supplied as crude protein (18–26% of DEc) was above the optimum requirement.

At fixed intakes of digestible energy, outputs of milk and milk energy were maximal at ratios of FAEc/NFAEc of 0·16 and 0·15 for weeks 1–6 and 7–13 respectively. The inclusion of a curvilinear effect of total energy (DEc2) gave some indication that utilization of energy for milk production for weeks 7–13 diminished at the highest digestible energy intakes whilst energy utilized for live-weight gain increased.

J. agric. Sci., Camb. (1988), 110, 261–270

The authors regret that there is an error in Fig. 1 of this paper. The values on the x-axis should be reversed to read from left to right: 200, 150, 100 g ADF/kg diet D. M.

Sixty-four multiparous Friesian cows were given one of eight diets from weeks 4–20 of lactation at the former National Institute for Research in Dairying, Shinfield, Reading in 1983–84. The diets consisted of 13·5 kg/day of experimental concentrates plus hay at 4·5 kg/day (fixed) or ad libitum. Concentrates were high-starch (S) based on cereal grains and cassava, high-fibre (F) based on highquality by-products, and two intermediate types containing 2:1 (SF) and 1:2 (FS) starch:fibre, all with a crude protein concentration of c. 180 g/kg DM. The diets were designed to provide similar intakes of digestible energy and crude protein at fixed hay intakes.

With ad libitum feeding, hay intake was lowest on S concentrates. With increasing fibre in the concentrates, the yields of milk, protein and lactose fell but fat concentration and yield were increased. These changes were linear and were unaffected by hay intake. Increasing hay intake to ad libitum had no effect on milk yield but increased the concentrations and yields of fat and protein. It also changed a net liveweight loss on fixed intakes to a gain on ad libitum intakes.

Diet digestibility was measured using sheep at about maintenance and cows at production intakes. Digestibility was higher in the sheep, probably because of the difference in level of intake. In the cows, increasing fibre in the concentrates reduced the digestibility of dry matter, organic matter, energy and nitrogen but increased the digestibility of fibre fractions and ether extract. Level of hay intake had variable effects on mean digestibility and on the relationship with concentrate type.

Increasing the fibre in the concentrates increased the molar proportions of acetate and n-butyrate in the rumen and reduced the proportions of propionate, n-valerate and n-caproate.

It is concluded that, with mixed diets of hay and relatively high proportions of concentrates, changes in the starch:fibre ratio of the concentrates by substitution of high quality by-products for starchy ingredients can induce linear changes in the yields of milk and the principal solids constituents and in milk fat concentration. Since changes in yields of fat and protein went in opposite directions, the optimal concentrate composition depends on the milk composition required by the market. These conclusions may not apply if poorer quality by-products are used.

One hundred and ninety-four lactating British Friesian cows were used over a 2-year period to determine the effects of mixing diet ingredients on dry-matter (D.M.) intake, animal performance, digestibility coefficients, rumen fermentation and efficiency of food utilization. Four treatments were imposed in both years and were complete diet and separate ingredients at ad libitum and restricted levels of feeding.

In Expt 1 a complete diet at ad libitum and restricted levels of feeding was compared with separate ingredients. The diet consisted of 60% concentrates and 40% forage and contained 20, 20, 10 and 50% on a D.M. basis of maize silage, lucerne silage, dried sugar-beet pulp and dairy compound, respectively. Method and level of feeding were the same in Expt 2; however, the diet in the 2nd year consisted of 50% concentrates (40% dairy compound, 10% dried sugar-beet pulp) and 50% grass silage.

In Expt 1 cows fed the complete diet ad libitum consumed 16·5 kg D.M./day which was significantly more than the 14·3 kg/day consumed by those offered the separate ingredients ad libitum. Intakes in Expt 2 were not significantly affected by method of feeding. Much of the difference in Expt 1 was attributed to the fact that many cows rejected lucerne silage when fed in an unmixed form and therefore to keep the concentrate to forage ratio constant other diet ingredients were reduced proportionately.

Mixing diet ingredients did not significantly affect milk yields, which were for the complete diet ad libitum and restricted and the separate ingredients ad libitum and restricted 22·2, 22·2; 22·1 and 20·7 kg/day in Expt 1 and 23·6, 20·8; 24·2 and 21·5 kg/ day in Expt 2, respectively. In Expt 1 when the concentrate to forage ratio was 60:40, cows fed the complete diet ad libitum produced milk with a significantly higher milk fat concentration (39·2 g/kg) than those fed the separate ingredients (31·6 g/kg). Similar effects were not recorded in Expt 2 when the concentrate to forage ratio had been reduced to 50:50. Changes in milk protein concentration were more closely related to changes in intake rather than method of feeding. Mixing diet ingredients tended to decrease live-weight loss in early lactation and accelerate live-weight gain in midlactation.

In vivo digestibility coefficients determined in lactating dairy cows showed that in Expt 1 the D.M. and organic-matter digestibility coefficients for the complete diets fed ad libitum (0·689 and 0·713) were lower than those recorded (0·712 and 0·732) for the unmixed diet ad libitum. Although the digestibility coefficient of the acid-detergent fibre of the complete diet was markedly higher at 0·519 compared with 0·478 for the separate ingredients, the difference was not significant. In Expt 2 the only difference in digestibility coefficients was that for nitrogen, which for the complete diet was significantly higher than that of the separate ingredients at both levels of feeding.

At an ad libitum level of feoding in Expt 1, the molar proportions of acetate and butyrate were 620 and 116 mM/l for the complete diet which were slightly higher than 602 and 95 mM/1 for the separate ingredients ad libitum. This diet also had a slightly higher molar proportion of propionate, 249 compared with the 221 mM/l for the complete diet ad libitum. In Expt 2 there wore no significant differences between treatments for either molar proportions of rumen volatile fatty acids or the ratio of lipogenic to non-lipogenic acids.

In a previous experiment it was found that when concentrates were given in large amounts with a fixed ration of hay, cows receiving starchy concentrates gave more milk but with a lower fat concentration than cows receiving the same amount of high quality fibrous concentrates of a similar metabolisable energy (ME) concentration. The purpose of the present experiment was to examine the effects of varying the starch:fibre ratio of the concentrates on milk yield and composition and on hay intake.

Sixty-one Friesian cows in their second or later lactation were given a standard ration for the first two weeks of lactation and were then allocated to one of eight experimental treatments for the following 18 weeks. All the cows were given 13.5 kg/d of one of four concentrates of similar crude protein and estimated ME concentrations. The concentrates varied from high starch (S) based on barley, wheat and cassava to high fibre (F) based on high-quality by-products (citrus pulp, molassed sugar beet pulp and wheatfeed) with two intermediate types (SF and FS).

The effects of administering bovine growth hormone (bGH) on milk production in cattle were examined in two experiments.

In experiment 1, a Latin-square design, three groups of four Friesian cows were injected daily with one of two doses of bGH (6 ng/kg and 30 ug/kg; 0·82 unit/mg) or with saline (control) throughout three successive 21-day periods in mid-lactation. At 30 ug/kg daily bGH stimulated a significant increase in milk yield (P < 0·001) and this response increased with the duration of treatment. Changes in the yields of milk fat, protein and lactose closely followed the changes in milk production. The concentrations of milk fat (P < 0·05) and protein (P < 0·05) rose and fell respectively during the 1st week of injections.' A daily dose of 6 ug/kg bGH failed to stimulate an increase in milk yield but milk lactose content was reduced (P < 0·05) during the 2nd and 3rd weeks of treatment. The proportion of milk serum samples containing detectable quantities of the hormone was increased by the higher dose of bGH.

In experiment 2 the milk yields of four Friesian cows were measured when bGH (21·0 to 37·9 mg/day) or saline was infused intravenously for 6 days. The infusions produced up to eight-fold increases in plasma bGH. These were within physiological limits, and remained at elevated levels throughout the 24-h period. Three of the cows demonstrated a clear increase in milk production.

1. Changes in concentrations of metabolites in blood and rumen fluid were measured in cows when fat and when thin and when given hay and concentrates at a restricted level or ad lib.

2. When fed ad lib., cows consumed 24% more food when thin than when fat.

3. Concentrations in rumen fluid of acetate and propionate after feeding at both levels did not differ significantly between fat and thin cows.

4. Concentrations in blood of the lipogenic substrates acetate and 3-hydroxybutyrate did not differ between fat and thin cows after feeding, but glucose concentration fell more rapidly in thin cows especially when fed ad lib. Propionate rose more in thin cows than in fat cows when fed ad lib. Non-esterified fatty acids were higher in fat cows before feeding, but fell to similar levels to those in thin cows after feeding.

5. The results are interpreted as indicating a more rapid rate of fatty acid synthesis in adipose tissue of thin cows. This in turn tends to lower blood levels of fatty acid precursors and so enhance the rate of their absorption from the rumen. This in turn permits higher intake of food before short-term chemical regulators of intake start to operate.

1. Seven Friesian heifers in mid-lactation were used in an experiment to measure the effects of including casein, formaldehyde-treated casein or formaldehyde-treated soya-bean meal in the diet on performance and on the concentrations of certain metabolites and hormones in the blood.

2. Milk yield and the concentration of fat, protein and lactose in milk were not affected by the treatments.

3. The concentration of growth hormone (GH) in jugular venous blood was higher with both formaldehyde-treated proteins than it was with untreated casein (P < 0·05). Diet had no effect on insulin, prolactin or thyroxine concentrations in blood.

4. Urea concentration tended to be higher and non-esterified fatty acids lower in blood from heifers offered the diet containing untreated casein, but these differences were not statistically significant. Blood glucose concentration was not affected by dietary treatment.

5. It was concluded that blood GH concentration can be increased by offering protein supplements which will increase total amino acid supply to the intestines in lactating cattle. This appears to be a direct effect on GH status independent of effects on milk yield.

1. Circulating concentrations of some hormones and metabolites and nitrogen balance were measured in lactating beef and dairy cows given daily injections of growth hormone (GH) and were compared to values before injection. Changes in milk yield and composition were recorded in these cows and in additional cows injected with GH fragments.

2. GH, but not GH fragments, raised milk yield while milk composition did not change. GH injection caused a large, rapid increase in the level of the hormone in blood and slower, smaller increases in the concentrations of insulin, prolactin and thyroxine. Blood metabolite levels were unaltered except for a rise in glucose concentration in beef cows during GH injection and a fall in 3-hydroxybutyrate concentration in the same cows after the injections ceased. An increase in N output in milk was partly balanced by a reduction in urinary N so that N retention did not change significantly.

3. GH caused a reduction in food intake in the beef cows.

4. The results are discussed in relation to control of partition of nutrients in the lactating cow, where GH is considered to play an important role. It was concluded that this role may be in increasing the supply of energy metabolites for milk synthesis, rather than a direct effect on the activity of the mammary gland.

1. Pelleted diets containing concentrates and 0, 200, 400 or 600 g chopped straw/kg were fed ad lib. for 5 h daily to four cows. Concentrations of various energy-yielding metabolites were measured in samples of rumen fluid and jugular blood taken before feeding and at intervals after food was offered.

2. After feeding, the pH of rumen fluid decreased rapidly and the total concentration of volatile fatty acids (VFA) increased; the changes were greatest in the diet containing no roughage. The changes were essentially complete by 135 min at approximately the time feeding stopped. Rumen lactate concentrations were always low and increased significantly only in cows given no roughage.

3. There were highly significant relationships between the peak rumen acetate concentration after a meal and the apparent digestibility of the dry matter of that meal, and the amount of material in the rumen at the end of a meal. The latter relationship resulted in there being no difference between treatments in the total amount of acetate present in the rumen after feeding.

4. In the blood, concentrations of acetate, propionate, β-hydroxybutyrate (BHB) and lactate all increased after feeding. The increase was prolonged and maximum values were rarely reached before 4–5 h. The highest concentrations of acetate and BHB were found in cows given 200 g roughage/kg and were twice as great as those in cows given no roughage; lower concentrations were found in cows given 400 or 600 g roughage/kg. This information, together with the rumen concentrations of acetate and butyrate, was interpreted as indicating an inhibition of VFA absorption from the rumen of cows given no roughage.

5. Plasma glucose concentrations decreased rapidly for 4 h after feeding, the decrease being greatest in cows given 200 g roughage/kg. Non-esterified fatty acid concentrations also decreased after feeding 200 and 0 g roughage/kg rations, but concentrations were not high at any time.

6. It is concluded that chemical changes in the rumen correspond to feeding behaviour much more closely than changes in blood and therefore any chemostatic regulation of food intake probably occurs at the former site, with acetate playing a major role. Monitoring energy balance at tissue level is likely to be mediated hormonally, with insulin an important factor.