Abstract The digestibility, and effect on voluntary feed intake, of various forms of cassava tops were determined. For 70 days, 15 entire lambs subdivided into 3 treatment groups, were then fed a basal maintenance ration of Chloris gayana hay with or without 200 g dry matter (DM) of cassavatops hay (CTH) or cassava-tops silage (CTS). The amount of Chloris gayana hay in the treatment groups was adjusted weekly for each animal, in accordance with its weight, in order just to meet maintenance requirements. A mineral supplement was provided and water was available at all times. At the end of the trial, the animals were slaughtered and the gross energy of the carcasses, offal and blood was determined from their contents of protein and fat. The difference between the gross energy of the control animals and those on treatment gave the net energy (NE) of the cassava tops. The results of the first experiments showed that, on average, cassava tops contained 27.1% crude protein (CP), 24.6% crude fibre (CF), 5.9% ether extract (EE) and 782.5 p.p.m. HCN in the DM; hay making seemed the most efficient method of reducing HCN content. Overall, the digestibility coefficients (%) of cassava tops were: organic matter (OM), 67.2; CF, 43.5; CP, 79.5; and EE, 51.7. Feeding cassava tops together with Chloris gayana hay, which was offered to sheep as the basal diet, improved voluntary feed intake by the animals. In the NE study CTS was significantly ( P <0.05) more digestible, and resulted in intake of more metabolisable energy (ME), than CTH. In spite of this superiority, the energy in the CTH was better utilised for gain. The NE values of CTH and CTS were 2.22 and 1.67 MJ kg −1 DM, respectively. This apparent contradiction was explained in terms of thyroid malfunction as shown by the greatly enlarged thyroid glands of the animals feeding on CTS containing high HCN content. It is concluded that, for animal production purposes, CTH should be preferred over CTS.