Certain diets cause a marked reduction in milk fat production in ruminants. Commonly referred to as milk fat depression (MFD), the mechanism involves an interrelationship between rumen microbial processes and tissue metabolism. Numerous theories to explain this interrelationship have been proposed and investigations offer little support for theories that are based on a limitation in the supply of lipogenic precursors. Rather, the basis involves alterations in rumen biohydrogenation of dietary polyunsaturated fatty acids and a specific inhibition of mammary synthesis of milk fat. The biohydrogenation theory proposes that under certain dietary conditions, typical pathways of rumen biohydrogenation are altered to produce unique fatty acid intermediates that inhibit milk fat synthesis. Trans-10, cis-12 conjugated linoleic acid (CLA) has been identified as one example that is correlated with the reduction in milk fat. Investigations with pure isomers have shown that trans-10, cis-12 CLA is a potent inhibitor of milk fat synthesis, and similar to diet-induced MFD, the mechanism involves a coordinated reduction in mRNA abundance for key enzymes involved in the biochemical pathways of fat synthesis. A more complete identification of these naturally produced inhibitors of fat synthesis and delineation of cellular mechanisms may offer broader opportunities for application and understanding of the regulation of lipid metabolism.

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Nutritional regulation of milk fat synthesis.

Nutritional Factors Influencing the Nitrogen Composition of Bovine Milk: A Review

Altering Milk Composition by Feeding

https://kenanaonline.com/files/0028/28692/Citrus%20by-products%20as%20ruminant%20feeds.pdf

Citrus by-products as ruminant feeds: A review

Protein-Energy Interrelationships in Dairy Cows

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.

A comparison of starchy and fibrous concentrates for milk production, energy utilization and hay intake by Friesian cows

1. Sixteen first-calf Friesian heifers were used in a continous treatment design experiment. For 2 weeks after calving they were given a 750 g concentrate, 250 g hay/kg ration with 169 g crude protein (nitrogen X 6.25; CP)/kg dry matter (DM). They were then divided into two groups of eight and given a high-protein (223 g CP/kg DM) or low-protein (107 g CP/kg DM) ration at the rate of 10.8 kg concentrate + 3.6 kg hay for 8 weeks. 2. Milk yield and composition, live weight and blood composition were monitored throughout. A digestibility trial was carried out with six animals on each treatment. 3. The low protein ration reduced DM, organic matter, energy and fibre digestibility significantly (P less than 0.001) so that intakes of digestible energy were not equal and the low-protein group lost more weight than the high-protein group. 4. Milk yield and the fat content of milk were lower in heifers given the low-protein ration (P less than 0.01). The lactose content of the milk was not affected and protein content only slightly reduced (P less than 0.01) by low-protein feeding. When the heifers were all changed onto an adequate protein (190 g CP/kg DM) ration in mid-lactation, those which had previously been under-fed protein appeared to recover in milk yield to the point they might have been expected to reach is given an adequate-protein ration throughout. 5. Concentrations of urea (P less than 0.001) and albumin (P less than 0.05) were reduced by underfeeding protein, but albumin concentration was affected less by diet than by stage of lactation. Blood concentrations of total protein, glucose, sodium, potassium, calcium, inorganic phosphate, iron, copper, haemoglobin and packed cell volume were unaffected by treatment. Blood magnesium concentration was slightly lower (P less than 0.01) with low-protein feeding.

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The effect of a low-protein ration on milk yield and plasma metabolites in Friesian heifers during early lactation.

1. The results are reported of four feeding experiments in which lactating cows were given fixed rations of hay and high-cereal concentrates at different meal frequencies. 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 was given twice daily. The diets contained 600–900 g concentrate /kg.2. In all the experiments, more frequent feeding of these low-roughage diets reduced milk fat depression and increased milk fat yield. In each experiment the increase was greater with the diet containing the lower proportion of hay. There was no significant effect on milk yield, the protein or lactose contents of the milk or live-weight gain. Digestibility of dry matter, organic matter and energy was increased in one of the experiments but not in two others in which it was measured.3. More frequent feeding was calculated to increase the net energy secreted in milk and there was a tendency for it to increase the net energy in live-weight gain with diets containing 600 or 700 g concentrate/kg but to decrease it with diets containing 800 or 900 g concentrate/kg. These results are discussed in relation to theories of energy partition in lactating cows.4. It is concluded that at fixed feed intakes, the main response to increased meal frequency is likely to be a reduction in milk fat depression and that this will be confined to diets containing not more than about 200 g modified acid-detergent fibre/kg dry matter.

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Feeding frequency for lactating cows: effects on digestion, milk production and energy utilization.

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.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600081284

Influence of plane of nutrition and diet composition on rumen fermentation and energy utilization by dairy cows

Sixteen Friesian cows were used in Expt 1 to measure the effect of substituting urea-N with fishmeal-N either in early lactation (Part 1) or in mid-lactation (Part 2). In Part 1 (days 15-84 of lactation) the major N constituent of the concentrate was urea (U), urea-N: fishmeal-N in the ratio 2:1 (UF) or 1:2 (FU), or fishmeal (F). In Part 2 (days 84-175 of lactation) only urea (UM) and fishmeal (FM) were used. Replacement of urea-N with fishmeal-N significantly (P less than 0.05) increased yield of milk protein both in early and in mid-lactation. At both stages of lactation the cows were, by calculation, in positive energy balance. In mid-lactation replacement of urea-N by fishmeal-N significantly depressed (P less than 0.001) the concentration of fat in milk. Blood urea concentration decreased with increasing fishmeal inclusion (P less than 0.05) from U to FU. In Expt 2 the diets used in Expt 1, Part 1, were offered at a maintenance level of feeding to non-pregnant, non-lactating heifers in a 4 X 4 Latin square design experiment. Digestibility of dry matter, organic matter and cell-wall constituents increased progressively (P less than 0.05) with the first two increments of fishmeal inclusion. A major effect of replacing urea-N with fishmeal-N was to increase digestible organic matter intake (DOMI) and differences in DOMI between treatments in Expt 1, Part 1, accounted for observed differences in performance.

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D. J. Napper

Papers

A comparison of starchy and fibrous concentrates for milk production, energy utilization and hay intake by Friesian cows

The effect of a low-protein ration on milk yield and plasma metabolites in Friesian heifers during early lactation.

Feeding frequency for lactating cows: effects on digestion, milk production and energy utilization.

Influence of plane of nutrition and diet composition on rumen fermentation and energy utilization by dairy cows

Performance of dairy cows offered isonitrogenous diets containing urea or fishmeal in early and in mid-lactation