Feeding phenol-containing plants to ruminants has the potential to mitigate both methane and ammonia formation. In the present study, mostly woody plants, such as the leaves of trees and shrubs, were tested for their influence on in vitro fermentation. The plants selected grow naturally under temperate climatic conditions, are usually available in bulk and do not directly compete with human food production. The detailed screening included whole plants or parts of different plant species reporting their effects on methane and/or ammonia formation. The plant materials were added at 167 mg/g of total dry matter (DM) to a common total mixed ration and incubated for 24 h with the Hohenheim gas test method. The results from in vitro fermentation were also used to determine the net energy of lactation and utilizable crude protein in the complete diets. Thirteen out of 18 test materials did not impair the organic matter (OM) digestibility of the diet. Ammonia concentrations decreased up to 35% when adding any of the test materials. Methane formation per unit of feed DM and per unit of digestible OM was lowered by 13 of the 18 test materials from 12 to 28% and 5 to 20%, respectively. In conclusion, a number of plant materials tested have the potential to mitigate ruminal ammonia and methane formation without adversely affecting digestibility. The leaves of Betula pendula, Corylus avellana, Ribes nigrum, Vitis vinifera and the aerial part of Geum urbanum were particularly promising in this respect.

Ileal apparent digestibility of amino acids (AA) in a cassava root meal and soya-bean meal diet without and with an inclusion (150 g/kg) of sun-dried cassava leaves, ensiled cassava leaves, leucaena leaves and groundnut foliage was studied in a change-over experiment (5 ✕ 5) with post-valve T-caecum cannulated growing pigs.

The ileal apparent digestibility of dietary crude protein (CP) and AA were unaffected by the inclusion of groundnut foliage (P > 0·05). In contrast, when sun-dried and ensiled cassava leaves, and leucaena leaves were included in the diet the ileal apparent digestibility of dietary CP and AA were significantly reduced (P < 0·05).

The estimated ileal apparent digestibilities of essential and non-essential AA in groundnut foliage were higher (P < 0·05) than those of the other leaf products and there were no differences among cassava leaves and leucaena leaves (P > 0·05). On average ileal apparent digestibility of essential AA was 0·71 in groundnut meal and 0·56 in cassava and leucaena leaves.

It was concluded that, under tropical conditions, groundnut foliage has the potential to improve the dietary protein and AA supply and to replace partially soya-bean meal, in low fibre diets for growing pigs. Further, when conventional protein-rich foods are not available, cassava and leucaena leaves may also have the potential to improve the protein and AA supply for pigs.

Growing pigs fitted with post-valve T-caecum cannulas were used in a change-over design experiment (5 ✕ 5) to determine the ileal and total tract apparent digestibility and hindgut disappearance of dietary components and energy in a cassava root and soya-bean meal-based diet with an inclusion rate at 0·15 of the dry matter (DM) of either cassava leaf meal, ensiled cassava leaves, groundnut foliage, or leucaena leaves.

The ileal and total tract apparent digestibilities of organic matter (OM), crude protein, nitrogen-free extractives and energy of the diets were significantly reduced with inclusion of the foliages (P < 0·05). The digestibility of neutral-detergent fibre (NDF) and crude fibre was improved when groundnut foliage was included in the diet (P < 0·05).

The dietary content of NDF was a better predictor of the total tract digestibility of OM (R2 = 0·87) and energy (R2 = 0·90) than the content of crude fibre (R2 = 0·68 for OM digestibility and R2 = 0·72 for energy digestibility).

The disappearance of OM and energy in the hindgut of the foliage diets was 0·36 (s.d. 0·02) and 0·31 (s.d. 0·02), respectively, which was significantly lower (P < 0·05) than on the control diet (0·48 and 0·46, respectively).

Ileal as well as total tract digestibilities of OM in groundnut foliage were higher than for the other leaf products (ileal, 0·55 and 0·42; and total tract, 0·64 and 0·55, respectively). For foliage products of cassava and leucaena an average digestible energy value of 11·4 MJ/kg DM was calculated, whereas for groundnut foliage the corresponding value was 10·9 MJ/kg DM.

It was concluded that, inclusion of foliages in pig diets will reduce the dietary energy content, due to a high fibre content. However, under tropical conditions foliage at low levels of inclusion may be useful in low fibre diets for growing pigs to improve the dietary protein supply.