This study examines the biohydrogenation and utilization of the C20 and C22 polyenoic fatty acids in ruminants. Eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids were not biohydrogenated to any significant extent by rumen microorganisms, whereas C18 polyenoic fatty acids were extensively hydrogenated. The feeding of protected fish oil increased the proportion of 20:5 from 1% to 13-18% and 22:6 from 2% to 7-9% in serum lipids and there were reductions in the proportion of stearic (18:0) and linoleic (18:2) acids. The proportion of 20:5 in muscle phospholipids (PL) increased from 1.5% to 14.7% and 22:6 from 1.0% to 4.2%; these acids were not incorporated into muscle or adipose tissue triacylglycerols (TAG). In the total PL of muscle, the incorporated 20:5 and 22:6 substituted primarily for oleic (18:1) and/or linoleic (18:2) acid, and there was no consistent change in the porportion of arachidonic (20:4) acid.

SUMMARYDissection data from 643 carcasses of castrated male cattle (steers) of 15 breed-type × feeding system groups were used to examine the distribution of total fat (TF) between subcutaneous (SF), intermuscular (IF), kidney knob and channel (KKCF) and cod fat depots. The breed-type groups, which were from cereal or grass/cereal feeding systems, included Ayrshire, Simmental × Ayrshire, British Friesian and Friesian crosses with Aberdeen Angus, Hereford, Limousin, Charolais, South Devon and Simmental. Means for percentage TF in side ranged from 21·4 to 36·2 with a pooled within group SD of 3·87.The growth of each depot relative to TF was examined using the allometric equation. Significant but not large differences existed between groups for the growth coefficients of SF and IF while the coefficients for KKCF differed widely among groups. The coefficient for SF was greater than that for IF in every group (pooled within-group b values±SE were 1·20±·02 and 0·87±0·01 respectively).At constant TF weight, carcasses from Ayrshire and Ayrshire crosses tended to contain less SF and more IF+KKC F than those from Friesian and beef breed × Friesian. Important differences in distribution were recorded between the various beef breed × Friesian groups. The proportion of SF was lower for cattle fed on grass/cereal diets than for cattle of the same breed type fed on cereal diets.The differences in fat distribution led to substantial bias for some groups when the percentages of IF and TF in the side were predicted from percentage SF. The bias was less when both KKCF and SF were used as predictors.

Since 1968, the Meat and Livestock Commission has been carrying out a considerable amount of tissue separation work on samples of cattle and sheep arising from its livestock improvement schemes. The purpose of this work has been to provide data on variation in carcass composition of these animals to relate to variation in production characteristics. From the point of view of the meat market, it is important when fast growing sires are selected for use, particularly by AI, that they should be known to be capable of producing offspring which, under normal feeding conditions, are no fatter than the mass market requires. The latter is known to favour the lean end of the fatness range, although there is still a market for fatter carcasses which are believed to have superior eating characteristics, but may require fat to be trimmed from them before and after they reach the plate. In order to describe market requirements more clearly than at present, the data being collected on variation in carcass composition and its prediction are being used to develop carcass classification schemes for cattle and sheep.

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