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Interspecific competition for phosphorus was studied in continuous lake water cultures of phytoplankton communities from Lake Memphremagog, and its outcome shown to depend primarily on the size of the competitors. Comparison of the activity of the inducible enzyme alkaline phosphatase between the lake and cultures operated at various dilution rates (0.06-0.93 day('-1)) established that in situ phosphorus limitation rarely depressed average community growth rates much below 0.6 day('-1). Growth rates under comparable, or even more severe (0.2-0.3 day('-1)) limitation in culture decreased significantly with increasing cell size. Phosphorus uptake kinetics and cell quotas measured by track autoradiography on the competing species showed that biomass and quota-specific uptake rates varied inversely with cell size in highly significant allometric regressions, correctly predicting that competitive outcome should depend on size. Substituted in a variable internal stores model of phosphorus limited growth, the allometric functions for uptake, cell quota, and maximum growth rate correctly predicted observed growth rates except in competition much stronger than natural (D = 0.06-0.2 day('-1)). Failure of the model to encompass cell senescence and death due to severe nutrient stress appeared to account for the discrepancy. Neither the absolute difference in growth rates due to cell size nor the direction of selection among the naturally co-occurring species varied systematically with the intensity of competition, indicating that spatio-temporal variation of limiting phosphorus supply in nature is unlikely to directly select the size or taxonomic composition of phytoplankton communities.

The effect of a low-Cu diet supplied to gilts during pregnancy and early lactation on dams and their offspring was obtained in seven pairs of first littermate gilts. They were housed in stainless steel metabolism cages in a controlled environment and fed semi-purified diets containing either 2.13 or 12.25 ppm Cu from 30 days of gestation through two weeks of lactation. Feed intake was restricted to 2 kg/day during pregnancy and 4 kg/day in the first two weeks of lactation. Reproductive performance of gilts was measured. Balance studies were conducted at 30, 60 and 100 days of gestation. Mineral levels in the plasma of gilts and piglets, and in colostrum and milk were determined. The low-Cu diet did not affect (p $>$ 0.05) weight changes during pregnancy. The duration of parturition was shorter for low-Cu than for control gilts (3.19h vs 5.71h, p 0.05). Low-Cu gilts had lower plasma Cu in early- and mid-gestation and farrowed piglets with lower plasma Cu and higher plasma Zn concentrations (p 0.05). Plasma Cu and Fe levels of newborn piglets were lower than those of their dams (p 0.05). Low-Cu gilts had lower Cu and higher Zn content in colostrum and also lower Cu in milk than control gilts (p 0.05). Colostrum was richer in Cu and Zn than milk (p 0.05). Higher dry matter digestibility was observed in low-Cu gilts during early gestation (p 0.05). Compared to control gilts, the low-Cu gilts tended to have higher Cu and Zn retention which was achieved by reduction of fecal Cu and Zn excretion. Utilization of minerals appeared to increase as pregnancy progressed.

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Previous work in animal nutrition has focused on single feeds and assumed additivity in ration systems. In the tropics, farmers are likely to feed mixtures of feeds, including tree fodders, which may not be simply additive in nutritional terms. This study has increased our understanding of the mechanisms that determine the associative effects on the in vitro fermentation of mixtures of fodder tree leaves. Associative effects are governed by a synchronisation in the fermentation rates of the components of the mixture. This is in turn dependent on the fermentability of their chemical constituents. Effects were demonstrated by changes in the fermentation kinetics of gas production curves. The chemical components of fodder tree leaves that affect the fermentation, and the time at which the effect occurs, were identified, using two media of different nitrogen contents. The fermentation of mixtures of pure chemical entities in various combinations was then examined. The greatest associative effects were found when the mixture had components of similar fermentability. It is proposed that associative effects are a function of the synchronisation of fermentation of the different components and was shown to occur at the point when the rate was maximal. With two types of protein (casein and bovine serum albumin (BSA)), utilisation of a protein by rumen microbes was shown to be a function of its fermentability and not of its solubility. This is also influenced by the type of associated carbohydrate. Fodder tree leaves were then combined with different pure chemical entities. Associative effects between fodder tree leaves and carbohydrates were shown to occur and the responses were similar to those obtained with mixtures of pure carbohydrates and proteins. The effect of tannins and phenolic compounds was studied using quebracho tannin as a model, and in five of the tree species. They were shown to affect the fermentability of both carbohydrates and proteins. The effect was greater with carbohydrates of medium to low fermentability. They also reacted with both soluble and insoluble protein. Forages with phenolic compounds showed both positive and negative effects. The effects were possible due to a synchrony or asynchrony in the release of protein. In mixtures of leaves from different species, associative effects were related to their fermentability. Again, this appeared to be the result of the synchronisation of the release of nutrients. Associative effects with fodder tree leaves were of a composite nature and can be both positive and negative. The implications of these findings in relation to in vivo digestion and animal production are discussed. Due to the diversity of fodder trees, there is the potential to develop feeding systems based on mixtures which make better use of available resources. This will also contribute to improved efficiency in the management and use of natural resources, and take advantage of natural plant diversity in the tropics.

The purpose of this study was to investigate whether the beneficial effects which the consumption of fish oil imparts on hemostatic function can be modified by the level of dietary selenium. Male Sprague Dawley rats were fed for eight weeks semipurified diets containing 7% corn oil (by weight) or 5.5% fish oil (MaxEPA) plus 1.5% corn oil with or without selenium supplementation. The indicators of selenium status (glutathione peroxidase activity and selenium level) were significantly lower in the rats given inadequate selenium, regardless of the type of fat fed. In the animals fed adequate selenium, these same indicators tended to be lower when fish oil was fed. Although feeding of fish oil increased hepatic and aortic malondialdehyde (MDA), selenium supplementation decreased its level in the liver. Selenium deficiency led to a decrease in the relative weight percent of 22:6 n-3 in aorta and plasma. Increases in the levels of 20:5 n-3, 22:5 n-3, 22:6 n-3, 20:3 n-6 and a decrease in the level of 20:4 n-6 were observed in plasma total lipids and aortic and hepatic phospholipids when fish oil was fed. The increased level of 20:3 n-6 suggests that delta 5-desaturase activity was decreased by fish oil feeding. The level of aortic 6-keto-prostaglandin F₁alpha (6- keto-PGF₁alpha) was highest in the rats fed diets that contained corn oil but no selenium supplementation; selenium supplementation, however, eliminated the difference in the level of 6-keto-PGF₁alpha between fish oil and corn oil fed groups. The levels of thromboxane B₂ (TXB₂) and ADP-induced platelet aggregation were decreased significantly by fish oil feeding and tended to be lower with selenium supplementation. Selenium supplementation did not increase bleeding time while fish oil feeding did. These data indicate that selenium supplementation may decrease fish oil induced lipid peroxidation in liver: this is reflected in the lower hepatic MDA levels in the fish oil fed animals with selenium supplementation and the increased 22:6 n-3 levels in aorta and plasma. Selenium deficiency led to an increase in the level of 6-keto-PGF₁alpha in the rats fed corn oil. Its effects on TXB₂ level and ADP-induced platelet aggregation are marginal. Overall the beneficial effect of selenium supplementation on hemostatic function appears weaker than that of fish oil feeding. / Graduation date: 1990

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