Cant, J. P., Fredeen, A. H., MacIntyre, T. Gunn, J. and Crowe, N. 1997. Effect of fish oil and monensin on milk composition in dairy cows. Can. J. Anim. Sci. 77: 125-131. Four primparous Holstein cows were used in a 4 × 4 Latin square with 21-d periods to determine the effect of redfish oil and monensin sodium on milk composition. The four dietary treatments were a basal diet (control), the basal diet plus 14.5 mg monensin kg -1 dietary dry matter (M), 2% fish oil (FO), and a combination of fish oil and monensin (FO + M). Total DM intake, measured during the last week of each period was reduced on the two fish oil treatments but an interaction with monensin depressed intakes further. An additive inhibition of rumen fibre degradation is discussed. Differences in yields of milk and lactose were not shown to be significant among treatments. Milk fat content was reduced 29.8% by fish oil supplementation and protein content dropped 5.3%. Protein:fat ratios increased from 0.78 on the control ration to 1.08 on the fish oil treatments. Monensin, on the other hand, only caused a change in milk fat percentage, and that by a 7.5% decline. Fish oil increased the concentrations of 20-and 22-carbon fatty acids in milk, including the n-3 eicosapentaenoic and docosahexaenoic acids which were transferred at 9.3 and 16.2% efficiency from the diet, respectively. Monensin had no effect on milk fatty acid profile. The fish-oil-by-monensin interaction produced the poorest energy intakes and rates of energy deposition into milk. Milk fat consumption has been declining relative to that of protein. This is a major factor that has led to the adoption of milk component pricing systems and quotas based on production of milk fat. With the present quota system and value of milk components, profits for dairy farmers may be increased when the protein-to-fat ratio in milk is elevated above the current average of 0.85. Dietary modification shows the most promise as a management strategy to change milk composition in the short term.Added fat in a dairy ration has previously been shown to significantly reduce milk fat percentage (Nicholson and Sutton 1971;Brumby et al. 1972;Storry et al. 1974) and, although a decrease in milk protein percentage often accompanies the feeding of dietary lipid, protein yields are frequently improved (Sutton 1989;Coppock and Wilks 1991;DePeters and Cant 1992). Fish oil is a locally available fat supplement and is an excellent source of the n-3 fatty acids, eicosapentaenoic acid (EPA; C 20:5 (n-3) and docosahexaenoic acid (DHA; C 22:6 (n-3) . These fatty acids have been shown to impart health benefits to adult humans (Burr 1989) and are essential for normal development and function of the retina and nervous system (Sardesai 1992). There is considerable interest in providing natural sources of these fatty acids in the human diet. Enriching milk with n-3 fatty acids would

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1. A field trial in which a sward, predominantly of S. 24 ryegrass was fertilized with 248, 301, 532 or 589 kg N/ha showed that a maximal yield of 13-1 tonnes/ha of artificially dried grass could be obtained from four harvests in the year.2. The materials harvested ranged in crude protein content from 10-1 to 23-6 % and in apparent digestibility of dry matter from 72 to 76 %. The amounts of the grasses voluntarily consumed by sheep were slightly and significantly lower for those containing the least N x 6-25.3. In feeding trials with twenty-five steers, a barley diet was compared with a diet entirely of dried grass from the 3rd and 4th harvests from an area in the same field which had received 487 kg N/ha. The cattle given the barley diet consumed 8-7 kg/day and gained 1-22 kg/day. Those given dried grass consumed 8-9 kg and gained only 0-85 kg/day. Carcass quality of animals given dried grass was significantly lower than of those given the barley diet.4. Calorimetric trials were made using two herbages from the first harvest (501, 532 kg N/ha) and all four herbages from the third harvest. In each of the six experiments three sheep were each given three amounts of a herbage, and in addition were fasted. Fifty-four calorimetric observations each lasting 5 days and eighteen fasting metabolism measurements were made. 5. With both 1st and 3rd harvests, methane energy losses fell and urinary energy losses increased with the level of N fertilization. Faecal energy losses were little affected by fertilizer application and the metabolizable energy (kcal/g D.M.) was 2-8 (11-7 kJ/g) for 1st harvests and 2-6 (10-9 kJ/g) for 3rd harvests. There was a slight decline in metabolizable energy/g D.M. with N application for the 3rd harvests.6. The net energy values of the grasses for maintenance were 1-99 kcal/g D.M. (8-4 kJ/g) for the 1st harvested material and 1-84 kcal/g D.M. (7-7kJ/g) for the 3rd harvests. Net energy values for fattening, however, were 1-28 kcal/g D.M. (5-4kJ/g) for 1st harvests and 0-87 kcal/g D.M. (3-6kJ/g) for 3rd harvests. There were no significant effects of level of N fertilization.7. The poorer nutritive value for fattening of the 3rd harvested grass, corresponding to an efficiency of utilization of metabolizable energy of only 33%, was contrary to prediction from equations relating nutritive value to either chemical composition or to metabolizable energy. The results of the calorimetric work, however, agreed with those of the feeding trial with cattle.8. Combination of the results of the various trials showed that it was possible to obtain 1400 kg live-weight gain/ha from a system of husbandry based on high fertilizer use and artificial drying of herbage. With a barley system a maximal output/ha would be less than half this amount.

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