The aim of the study was to determine the optimum protein inclusion level in diets containing an ideal amino acid balance for growing pigs. The performance of commercial crossbred grower-pigs was studied in two trials in which the experimental animals were housed either in commercial-type group housing (experiment1) or in individual pens (experiment 2). Treatments consisted of diets containing 14%, 16%, 18% or 20% crude protein. Lysine, tryptophan, threonine and total sulphur-containing amino acids were included in all diets at levels equivalent to that supplied by the 18% crude protein diet. In experiment one, 144 pigs (72 boars and 72 gilts) of initial mass 30 kg were fed until slaughter at 70 kg. In experiment two, 80 pigs (40 boars and 40 gilts) of live masses ranging from 17.5 to 31.1 kg were fed until slaughter at masses between 64 and 105.5 kg. There were no differences between treatments (p > 0,05) for daily gain, feed conversion ratio, daily intake or carcass classification, but feed conversion ratio differed between sexes in experiment two (p < 0.05). It was concluded that protein inclusion levels in pig growth diets could be decreased from 18% to 14% without any detrimental effect on performance, provided that the digestible essential amino acid composition is adjusted to meet requirements. This practice is however not economically viable for South African pig producers at the prevailing price of synthetic amino acids.

A study was conducted with 192 pigs of three sex types with treatments according to slaughter weight (65 kg to 144 kg). The main statistical differences observed were for slaughter weight with significant (P < 0.05) and highly significant (P < 0.01) differences describing more than 10% of variance observed for all characteristics (liveweight, warm carcass weight, dressing percentage, eye muscle area, subcutaneous fat thickness, intramuscular fat area, subcutaneous fat : eye muscle area, intramuscular fat : eye muscle area, fat thickness, muscle depth, carcass length, ham circumference, ham length and chest depth) measured. Sex differences (P < 0.05) were also observed for dressing percentage, fat measurements and muscle depth measured between the 5th and 6th lumbar vertebrae. Sex type differences, in all instances, accounted for 10% or less variance except for subcutaneous fat : eye muscle area ratio (18.47%). It was shown that although significant sex type differences (P < 0.05) existed; slaughter weight had the largest effect on carcass characteristics in the given circumstances accounting for most of the observed variance.

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The effects of four diets containing different protein sources on the performance and gut histology of piglets were compared. The diets contained as the main protein sources either: milk powder plus fishmeal, milk powder plus high-protein (48% crude protein) soyabean oil-cake meal, fishmeal plus high-protein soyabean oil-cake meal or high-protein soyabean oil-cake meal alone. No differences (p > 0.05) were observed between treatments for average daily gain, feed conversion ratio, intake or intestinal villus height. It was concluded that high-protein soyabean oil-cake meal can be utilised successfully in diets for weaner pigs either in combination with other protein sources or as main protein source without detrimental effects on production parameters or gut histology. These results apply to diets are formulated on the basis of digestible amino acids using soyabean oil-cake meal has been correctly processed.