Supplementation of early and mid-season Italian ryegrass (Lolium mu/tiflorum cv. Midmar) with maize meal, maize meal with NaHC03 buffer, or maize meal plus combinations of slowly degradable protein was studied. Supplements were administered via a rumen fistula. The effect on intake and digestion of ryegrass was measured. In another experiment, the effect of maize meal supplementation on in sacco degradation of some typical South African forages was investigated. Maize meal depressed intake and digestibility of early season ryegrass, but not of mid-season ryegrass. Additions of the buffer did not alleviate the depression, but additions of a slowly degradable protein source did. In other forages, maize meal depressed in sacco degradation of lucerne and kikuyu (Pennisetum clandestinum), but not of Panicum maximum, Smuts finger (Digitaria eriantha spp. eriantha), mature Midmar ryegrass and Eragrostis curvula. In general, energy supplementation markedly depressed cell wall degradation below forage NDF levels of 55-60%, but not above these levels, whether examined within or between forage species. Also, degradation in the absence of supplements was markedly slower in forages containing more than 55-60% NDF than in forages with lower NDF values. The present results support the literature in suggesting that energy intake on forages with more than 55-60% NDF is unlikely to be sufficient to sustain satisfactory animal production, because of slow fermentation and long rumen retention times. Such forages may, however, be supplemented with energy without interfering much with fibre digestion.