Impact of forage legumes on nutritive value and growth performance in pigs reared in traditional farming systems in tropical Africa

Forage legumes (FL) are used in tropical countries to feed pigs, either by reducing allowance of well-balanced diets and supplementing with FL or by incorporating FL in unbalanced fibre-rich diets. The impact of both strategies on animal and economical performances was studied in 2 growth experiments. In Exp. 1, 4 groups of 8 growing pigs (25.5 +- 4.2 kg) were fed under one of 4 dietary treatment: a well-balanced diet (CONTROL) ad libitum or 80% of the ad libitum level of CONTROL + ad libitum freshly cut foliage of one of 3 FL species (Vigna unguiculata, Stylosanthes guinanensis, or Psophocarpens scandens). Similarly, in Exp. 2, 4 groups of 6 pigs (22.6 +- 3.7 kg) were fed one of 4 experimental diets: the well-balanced diet (CONTROL), an unbalanced traditional diet rich in fibre and made of bran, brewers grain, and corn (TRAD) or 80% TRAD diet supplemented with 20% of one of 2 FL hays (V. unguiculata or S. guinanensis). Animals were regularly weighed and feed intake was monitored. After 90d, animals were slaughtered and carcass composition and economic traits were recorded to calculate production costs and economical value. In addition, nutritive value of the FL and the diets was assessed by means of an in vitro model of the pig digestive tract combining an enzymatic hydrolysis to a fermentation with a fecal inoculum. Results indicate that pigs fed diets with FL (Exp.1) and TRAD with and without FL (Exp.2) had reduced ADG, final live and carcass weights (P<0.05) compared to CONTROL pigs. Surprisingly , dry matter intake (DMI) was also reduced by 10% with those diets compared to CONTROL (P<0.001) in both experiments. In Exp. 1, FCR and carcass dressing were not affected by the 20% reduction in CONTROL diet and supplementation with fresh FL, while in Exp. 2 those parameters were negatively affected in TRAD with and without FL compared to CONTROL pigs. In vitro data showed that differences in nutritive values explained most differences in growth performances and carcass traits, but not the decrease in intake, since passage rate and voluntary intake are not taken into account in the in vitro model. Economical assessment showed that fresh foliage of FL could increase incomes, as long as their production costs remain marginal, but the incorporation of FL hays could not improve economical performances in pigs fed traditional unbalanced fibre-rich diets.