Three energy sources (starch, sucrose and pectin) were used alone (without any N addition), or in combination with either soybean meal or urea as N source to investigate their effect on the in vitro digestion of NDF from alfalfa hay and wheat straw. The ratio of energy source to forage substrate was 50:50 on a hexose equivalent:NDF basis. Forage control treatments, without any energy or N sources added, were also included. The Goering/Van Soest incubation medium was used with slight modification; ammonium bicarbonate and tryptose were omitted (for the treatments with energy sources alone), or replaced by a similar amount of N from either soybean meal or urea. In the forage control treatments, the medium was used without modification. The trial was repeated in 6 runs. Samples were incubated at 39°C for 6 or 30 h. The substrate control data were analyzed per time according to a main effects ANOVA. For the factorial experiment, the data were also analyzed per time, but according to a 3 way factorial ANOVA with substrate, energy source and nitrogen source as factors. The 3 way interactions were interpreted with LSD multiple comparisons and Bonferroni tests. Significance was declared at P < 0.05. After 6 h of fermentation, NDF digestion was higher (P < 0.01) in alfalfa hay (13.6%) than in wheat straw (8.1%), but energy and nitrogen source had no effect in either forage. After 30 h of fermentation, sucrose with soybean meal (35.8%) did not differ from starch and soybean meal (29.0%), but compared with no supplementation (26.4%) it was the only combination that increased NDF digestion of alfalfa hay significantly (P < 0.01). In wheat straw, pectin with soybean meal (16.5%) did not differ from sucrose with soybean meal (12.4%), sucrose with urea (12.6%), pectin alone (15.0%) or starch alone (13.5%), but compared with no supplementation (11.5%) it was the only combination that increased wheat straw NDF digestion significantly (P < 0.02). In the current study, it appeared that sucrose and soybean meal was the best energy-nitrogen combination for alfalfa hay NDF digestion, whereas pectin and soybean meal was the best combination for wheat straw NDF digestion.