The efficiency of conversion of metabolisable protein into milk true protein over a range of metabolisable protein intakes.

The aim of this work was to test the robustness of the 0.68 estimate of the efficiency of conversion of metabolisable protein into true milk protein (Agriculture and Food Research Council (AFRC), 1993) for protein-limiting diets and to determine whether a different value is appropriate for practical rationing. Seventy-two multiparous cows were blocked on the basis of milk energy output per unit of dry matter intake (DMI), and allocated at random to one of four treatments. Treatments supplied metabolisable energy (ME) at a fixed level to individuals within a block, but varied metabolisable protein (MP) supply from 25% below the estimated requirements, through -12.5% and +12.5% up to 25% above requirements for the average performance of animals within blocks at the start of the study. Cows were offered diets to meet their predicted ME requirements for each 3-week period with measurements performed in the last week of each period. Milk protein output was regressed against the estimated MP available for production for each cow and the efficiency of conversion of MP into milk true protein was calculated, assuming a maintenance requirement according to the MP system. The efficiency of conversion of MP into milk true protein decreased with the increasing supply of MP from 0.77 to 0.50. Using an iterative approach to determine the best fit of the data when supply matched requirement resulted in a range of efficiency values between 0.62 and 0.64 g of true milk protein per g of MP.

Effects of breed, diet and muscle on fat deposition and eating quality in pigs.

A study in 192 entire male pigs examined the effects of breed, diet and muscle on growth, fatness, sensory traits and fatty acid composition. There were four breeds: two modern breeds, Duroc and Large White and two traditional breeds, Berkshire and Tamworth. The diets differed in energy:protein ratio, being conventional (C) and low protein (LP) diets, respectively. Muscles investigated were the `white' longissimus dorsi (LD) and the `red' psoas major (PS). Breed influenced growth rate and fatness, the modern breeds being faster-growing with leaner carcasses. However, the concentrations of neutral lipid fatty acids and marbling fat (neutral lipid+phosopholipid fatty acids) were higher in Berkshire and Duroc, in both LD and PS. Relationships between marbling fat and P2 fat thickness showed clear breed effects, with Duroc having high marbling fat at low P2 and Tamworth low marbling fat at high P2. Breed effects on sensory scores given by the trained taste panel to griddled LD and PS steaks were relatively small. Breed affected the fatty acid composition of intramuscular neutral lipid, with high % values for the saturated fatty acids, 14:0 and 16:0 in Berkshire and Tamworth (fat carcasses) and high values for polyunsaturated fatty acids in Duroc and Large White (lean carcasses). Duroc had particularly high concentrations of the long-chain polyunsaturated fatty acids, 20:5n-3 and 22:6n-3 in phospholipid of both muscles. Diet influenced growth rate and fatness, the LP diet slowing growth and producing fatter meat, more so in the two modern breeds, and particularly in intramuscular rather than subcutaneous fat. This diet produced more tender and juicy meat, although pork flavour and flavour liking were reduced. The PS muscle had higher tenderness, juiciness, pork flavour, flavour liking and overall liking scores than LD. The concentration of phospholipid fatty acids was higher in PS than LD but neutral lipid fatty acid content and marbling fat were higher in LD.

Effect of dietary polyunsaturated fatty acids on uterine prostaglandin synthesis in the cow.

Dietary polyunsaturated fatty acid (PUFA) intake in humans can affect the incidence of a variety of diseases including coronary heart disease. Feeding high PUFA diets to cows can alter the PUFA content of milk for human consumption. PUFAs supply the precursors for prostaglandin (PG) synthesis and PGs in turn influence many aspects of reproduction. This study examined the effects of a control (CONT), a high n-6 PUFA diet (derived from protected soya) and a high n-3 diet (derived from protected linseed) on uterine PG synthesis in the lactating dairy cow. Endometrial explants obtained on days 15-17 of the oestrous cycle were cultured for an initial 42 h in vitro in fully defined medium (basal production) and then challenged with control medium, oxytocin (OT; 20 or 200 nM) or calcium ionophore A23187 (CaI; 10 microM). PGF(2 alpha), PGE(2) and 6-keto-PGF(1 alpha) were measured in the spent medium. The experiments were repeated using tissue from two groups of cows, nine in Experiment 1 (three cows per diet) and seven in Experiment 2 (four CONT and three n-6). Results of the two experiments were consistent. The basal concentrations of all three PGs were significantly lower (>50% reduction) in the n-6-fed group in comparison with CONT and n-3 groups. The n-3 diet did not alter basal PGF(2 alpha) and PGE(2) but increased 6-keto-PGF(1 alpha). The n-6 diet also inhibited the ability of the tissue to respond to both OT and CaI, with significant reductions in the stimulated levels of all three PGs. In contrast, the n-3 diet only had minor effects; it did not alter the response to OT but did reduce the long-term response to CaI at 24 h post treatment. In conclusion, dietary PUFA intake can inhibit PG production in bovine endometrial explants, with a more pronounced effect following n-6 rather than n-3 supplementation. These data suggest that a high n-6 diet reduces the endometrial capacity to produce PGs and may therefore have implications for the control of luteolysis and other PG-mediated events such as ovulation.

Nutritional factors affecting the fatty acid composition of bovine milk.

The predominant fatty acids in milk are the long-chain fatty acids myristic, palmitic and stearic. These saturated fatty acids account for 75% of the total fatty acids, with a further 21% occurring as monounsaturated fatty acids of which the most prevalent is oleic acid. Only 4 g/100 g of the milk fatty acids are polyunsaturated, occurring mainly as linoleic and linolenic acids. All milk fatty acids are derived, almost equally, from either de novo synthesis or directly from preformed fatty acids in the diet. There are four main dietary sources of fatty acids: forages, oilseeds, fish oil and fat supplements. The digestive tract exerts a profound influence on the fate of dietary fatty acids. The short-chain saturated free fatty acids are absorbed through the walls of the rumen or abomasum into the bloodstream. The medium- and longer-chain saturated fatty acids pass into the small intestine, diffuse across the membrane wall where they are incorporated into lipoproteins and enter the bloodstream via the lymphatic system. The majority of unsaturated fatty acids are extensively hydrogenated in the rumen. However, recent work has shown that the levels of certain saturated fatty acids can be reduced and the levels of oleic, linoleic and linolenic fatty acids increased by feeding oilseeds rich in mono- or polyunsaturated fatty acids. In addition, work reported here has confirmed that eicosapentaenoic and docosahexaenoic acids can be transferred to milk when a diet containing fish oil is fed, but the transfer efficiencies are low.