The effects of dietary algal supplementation, a source of docosahexaenoic acid, on the fatty acid profile of rumen lipids in cattle were evaluated, with special emphasis on CLA and trans fatty acids produced by rumen microbes. A diet based on corn silage was fed with supplements containing the following: 1) no algal meal and fed at 2.1 kg of DM/d (control), 2) algal meal and fed at 1.1 kg of DM/d (low algal meal), 3) algal meal and fed at 2.1 kg of DM/d (medium algal meal), and 4) algal meal and fed at 4.2 kg of DM/d (high algal meal). A modified lipid extraction procedure was developed to analyze the lipid changes in rumen fluid. The percentage of stearic acid (18:0) in rumen fluid was decreased by algal meal supplementation (P < 0.001) compared with control and was linearly dependent on the level of algal meal supplementation (P = 0.005). Total trans-18:1 in rumen fluid of cattle fed the control diet was 19% of total fatty acids. Addition of algal meal increased (P < 0.001) total trans-18:1 up to 43%, mostly due to 18:1 trans-10 that increased (P = 0.002) to 29.5% of total rumen fatty acids. This increase in 18:1 trans-10 seems to suggest a change in the rumen microbial population. Vaccenic acid (18:1 trans-11) increased quadratically (P = 0.005) with increasing level of algal meal supplementation in the diets. The total CLA content was low in the control (<0.9%) and increased with dietary algal meal addition, although not significantly; the greatest level was 1.5% with the medium algal meal diet. The increase of rumenic acid (cis-9, trans-11 CLA) was quadratic (P = 0.05) with algal meal supplementation, whereas trans-10, cis-12 CLA increased linearly with increased level of algal meal from 0.08 to 0.13% (P = 0.03). The ratio of trans-11 (cis-9, trans-11 CLA + 18:1 trans-11) to trans-10 (trans-10, cis-12 CLA + 18:1 trans-10) decreased from 2.45 to 0.77, 0.87, and 0.21 for the control, low algal meal, medium algal meal, and high algal meal diets, respectively. The content of docosahexaenoic acid in rumen fluid increased (P = 0.002) from 0.3 to 1.4% of total fatty acids with increasing level of algal meal supplementation in the diets. Our results suggest that algal meal inhibits the reduction of trans-18:1 to 18:0, giving rise to the high trans-18:1 content. In conclusion, algal meal could be used to increase the concentration in rumen contents of trans-18:1 isomers that serve as precursors for CLA biosynthesis in the tissues of ruminants.

In studies of the effect of fertiliser treatment on the incidence and severity of hypomagnesaemia in grazing cows, determinations of inorganic constituents were made of herbage samples from the swards grazed. From the values obtained, some of the mineral indices of Brouwer3aj b and t'Hart & Kemp2 were calculated for the individual experimental plots and their relationship to the observed incidence of hypomagnesaemia examined. For a given sward in a given season, the severity of hypomagnesaemia in cows grazing plots given different fertiliser treatments, generally increased as the alkaline earth alkalinity (Ca + Mg -P, expressed as mequiv./Ioo g. silica-free herbage dry matter) of the herbage decreased. None of the other mineral indices considered showed a consistent relationship with the severity of hypomagnesaemia.Although the mineral composition of herbage may influence the development of hypomagnesaemia in grazing cattle, the effects of the palatability of herbage on dry matter (and hence magnesium) intake and of food factors other than minerals on the availability of herbage magnesium, preclude the use of mineral composition of herbage to assess the likelihood of the occurrence of hypomagnesaemia in grazing cattle. IntroductionDutch workers have frequently claimed that pastures in Holland which produce a high incidence of hypomagnesaemia and tetany in grazing cows show a characteristic mineral composition (see Sjollema,l t'Hart & Kemp2 and Brouwer3", b). Work at this In~titute~showed that hypomagnesaemia in grazing cattle results from a combination of a low intake and a poor assimilation of magnesium from the grazed herbage, in comparison with typical winter foods. Although there is little published information on the factors that affect the assimilation of feed magnesium, the results of Stewart & Moodie5 suggest that different salts of magnesium are absorbed to different extents and it seems reasonable that the mineral composition of herbage, and of feeds in general, should influence the absorption of magnesium from the gut.Over the past seven years, a number of field investigations of the relationship between pasture management and the incidence and severity of hypomagnesaemia in grazing cattle have been carried out at this Many of the herbage samples taken cluring these investigations have now been analysed for mineral composition in detail and the results are reported in this paper. Using these results, an attempt has been made to relate the mineral composition of herbage to the severity of hypomagnesaemia in cattle grazing the swards.

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