Classically, the minimum requirement of a nutrient is defined as that amount which will prevent the development of any of the signs of dietary deficiency.
Conversely, an intake that does not allow a maximum growth rate, or results in metabolic changes indicative of the deficiency conditions, is defined as an in-take below the minimal requirement, i.e. a deficient inin-take or a deficient diet.
The determination of the minimum requirement requires groups of animals which are fed experimental diets containing increasing levels of the nutrient in question from zero or deficient amounts to above maximum levels. The smal-lest significant change in response that can be measured is dependent upon the precision and sensitivity of the experimental method and the variation between animals.
As for the determination of minimum requirements of EFAs in pigs, the vari-ous deficiency symptoms described in section 2.2 have been used as criteria for an adequate or inadequate supply. Most studies only imply two or three distinct
levels of dietary EFAs, which in all cases is linoleate, provided as the methyl ester or usually as an oil (olive oil, safflower oil, soya bean oil, corn oil). Only few studies involve more levels from which the minimum requirement may be derived.
In the studies of Hill et al. (1961) the minimum requirement for dietary linoleate was determined with sixty-six miniature swine fed purified diets based on glucose and casein varying in lonoleate level from 0.02 to 12.9% of energy intake. Thirteen different levels of linoleate were implied. Basal diets were sup-plemented with ethyl linoleate or corn oil. The linoleate requirement was de-duced from the plot of triene/tetraene ratio of heart and liver lipids versus diet-ary linoleate concentration. From these curves and the weight gains, the dietdiet-ary requirement was stated to be near 2 energy%. The pigs were taken by hysterec-tomy and deprived of sow's colostrum and milk. They were reared on the ex-perimental diets from one to three weeks of age until an age of 56 to 77 days for most of the pigs.
Leat (1961a;b) raised 6 cross-bred pigs from weaning at 3 days of age for 21 weeks (90 kg live weight) on diets based on cassava, dried skim milk, extracted palm kernel cake and dried brewer's yeast supplemented with olive oil provid-ing from 0.03 to 3.5% of the total dietary energy as linoleate (one pig per group). The relationship between dietary linoleate intake and the trienoic/te-traenoic acid ratio in the fatty acids of plasma, liver and heart lipids indicated a minimum linoleate requirement of 0.95% of the total dietary energy.
From the studies of Leat (1962), however, it appears that the requirement of the pig for EFAs is not constant throughout the growth period, but is maximal in the first 16 weeks of life. During this period the minimum requirement for EFAs was found to be between 1 and 2% of dietary energy estimated from the relationship between linoleate intake and the trienoic/tetraenoic acid ratio of serum lipids. When the first 24 weeks of life were examined in entity, however, 1% of the dietary energy was sufficient to prevent the metabolic lesion from developing. Animals and diets were similar to those used previously (Leat, 1961a). All dietary levels were sufficient to secure normal growth rate. The minimum requirement for linoleate to prevent a normal skin development appeared to lie between 0.07 and 0.28% of dietary energy intake.
Sewell and McDowell (1966) fed three weeks old cross-bred uncastrated male pigs six various levels of linoleate ranging from 0.02 to 4% of the dietary energy for 10 weeks (4 pigs per group). The diets were based on glucose and casein and linoleate was supplied as corn oil. Weight gain was not significantly influenced by the dietary linoleate level. The efficiency of feed utilization was increased as the linoleic acid content of the diet was increased, but the requirement of linoleate using this parameter as an estimate cannot be evaluated since the pigs were group fed and the diets were not iso-energetic. The requirement for
linoleate to prevent dermal lesions was found to be 1 % of dietary energy intake.
A plot of the triene/tetraene ratio of testes lipids versus the dietary linoleate level showed that the linoleate requirement was no more than 2% of the dietary energy intake. As indicated by Sewell and McDowell (1966) no statistical sig-nificance was found between the groups receiving 2% or more of the energy in-take as linoleate, but as pointed out by Riis (1970) an increase in weight gain and a decrease in feed utilization in the studies of Sewell and McDowell (1966) is actually seen until a linoleate intake of 4% of the dietary energy. The discre-pancy in interpretation of the results of the studies by Sewell and McDowell (1966) is apparently due to a different attitude to determination of require-ments. Surely, the interpretation of the results by Sewell and McDowell (1966) is that of determination of minimum requirements, whereas that of Riis (1970) is an evaluation of recommended requirements or allowances. In the latter case the individual differences in sensitivity to deficiency should be encountered.
The various interpretations of the term requirement has been discussed exten-sively (Christensen, 1980; 1983).
The studies of Nørby et al. (1967) show that young boars of the Danish Land-race breed (8 months of age) fed 10,20 or 40% of the dietary energy as butter for 26 weeks corresponding to a dietary linoleate intake of less than 1.8% of the amount of feed had triene/tetraene ratios of plasma lipids between 0.25 and 0.85 indicating a marginal intake of linoleate. At slaughter after about 400 days on the respective diets, however, the triene/tetraene ratios of plasma lipids were about 0.2. These results indicate that the requirement for linoleate is greater for young animals than for adult animals.
2.4 Conclusion
The above mentioned experiments on determination of EFA requirements in pigs show that the magnitude of minimum requirement depends on age and EFA status of the tissues before the experimental period, which again depends on the piglet's supply of EFAs before and after birth, its sex and the criterion chosen for assessing deficiency symptoms. Thus, the minimum linoleate re-quirement of pigs seems to lie between zero and 2% of the dietary energy in-take.
The criteria used for assessment of EFA status in pigs are summarized in Table 2.1.
Table 2.1 Effects (+) or no (-) effects of EFA deficieny in experiments with pigs Tabel 2.1 Effekter (+) eller ingen (-) effekter af EFA mangel i forsøg med grise Daily gain ± Feed conversion efficiency ± High mortality ± Dermal lesions ± Loss of hair ± Organ weights ± Histological changes in organs ± Chemical composition of organs ± Carcass composition ± Meat quality (+) Blood parameters ± 20:3,n-9/20:4,n-6 in plasma and tissue lipids + (+) only measured by one author (Christensen, 1974b)