Addition of marine oils containing long-chain n-3 polyunsaturated fatty acids to the diet of pregnant sows may reduce piglet mortality. In previous experiments, when marine oils have been fed to pregnant sows, improvements in piglet tissue 22 : 6n-3 status have been accompanied by potentially undesirable decreases in 20 : 4n-6. The objective of the present experiment was to establish an amount of dietary salmon oil which would enhance piglet 22 : 6n-3 status while minimising reductions in 20 : 4n-6. Twenty-four pregnant multiparous sows were used in the experiment which began on day 60 of pregnancy (gestation length 115 d). To give four diets, salmon oil was added in increasing amounts (0, 5, 10 and 20 g/kg diet) to a basal diet; the diets were made isoenergetic by adding palm oil to each diet to give a total of 20 g oil/kg diet. Diets were offered to the sows in fixed amounts (2·5 kg/d) until parturition. Piglet tissue samples (brain, liver and retina) were obtained at birth before consumption of colostrum. The greatest increase in piglet tissue 22 : 6n-3 proportions occurred between 0 and 5 g salmon oil/kg diet, with only small increases between 10 and 20 g salmon oil/kg diet. In contrast, tissue 20 : 4n-6 proportions declined progressively as the amount of salmon oil fed to the sow increased. In brain, the change in the value 22 : 6n-3/22 : 5n-6 was greatest between 0 and 5 g salmon oil/kg diet, whereas in liver the value increased linearly with added salmon oil. In addition, piglet brain weight (g/kg live weight) increased to a maximum at 10 g salmon oil/kg diet. The optimum amount of supplementary salmon oil in the current experiment, defined as that which gave the greatest response in brain 22 : 6n-3 proportions with minimum reduction in 20 : 4n-6,was 10 g salmon oil/kg diet. This corresponds to an intake of approximately 2·4 g 20 : 5n-3 plus 3·6 g 22 : 6n-3/d or 0·6 % digestible energy.

In an attempt to prevent decreases in piglet 20 : 4n-6 status at birth while increasing 22 : 6n-3 status, multiparous sows (eight per treatment) were allocated to one of three different treatments: a basal diet fed from day 63 of pregnancy to term; basal diet supplemented with tuna oil (17·5 g/kg) from day 63 to day 91 and then basal diet alone from day 92 to term; basal diet alone from day 63 to day 91 and then basal diet supplemented with tuna oil from day 92 to term. Tuna oil supplementation increased mainly 22 : 6n-3 intake. Supplementation with tuna oil between day 92 and term increased 22 : 6n-3 to a greater extent in all piglet tissues (brain, liver, retina and the remaining carcass) at birth than supplementation with tuna oil between days 63 and 91. However, while piglet 20 : 4n-6 decreased to a greater extent in liver and carcass when diets were supplemented with tuna oil between days 92 and term than between days 63 and 91, in the brain and retina, the reverse was true; 20 : 4n-6 was decreased to a greater extent between days 63 and 91 than between 92 and term. The effect of pregnancy nutrition on the growth of piglets until 7 d postweaning (35 d of age) was assessed after removing any residual effects of pregnancy treatment by cross-fostering some piglets at birth. Piglets, the diets of whose dams had been supplemented with tuna oil during pregnancy, grew faster during the first 35 d of life than the progeny of sows fed only the basal diet. Feeding tuna oil to sows at different times during pregnancy therefore did not prevent decreases in piglet 20 : 4n-6 status at birth, but did suggest that changes in piglet brain 20 : 4n-6 status between days 63 and 91 of pregnancy were not reversible by later nutrition. Supplementing the diet of the pregnant sow with tuna oil had beneficial effects on postnatal piglet growth.