The impact on rumen function of supplements traditionally used by smallholder dairy farmers in Tanzania is reported. The supplements included sources of energy and protein (maize bran, molasses, cassava flour and sunflower cake) and an ionic feed additive called “Magadi”. The experimental layout was a 5 x 5 Latin square with 14 days for adaptation to diets, 7 days in vivo digestibility and the last 7 days used for rumen evacuations. Treatments were: poor quality hay with 2.7 kg DM of maize bran (HMB) which was the control reflecting smallholder farmers feeding practice. Other treatments were hay with 2.7 kg DM of maize bran further supplemented with 0.25 kg DM of “Magadi” (HMG), 1.3 kg DM of sugar (molasses) (HMO) or 0.9 kg DM of starch (cassava tuber flour) (HCA). The fifth treatment was hay supplemented with 2.8 kg DM of farm made concentrate mixture containing (%): maize bran (68), sunflower cake (31) and mineral powder (1) on DM basis (HFC). Five mature heifers (1/2 Boran x 1/2 Friesian) were used during the trial. Intake of DM and organic matter was significantly (P=0.01) higher with sugar (HMO), starch (HCA) and nitrogen (HFC) supplements compared to the control (HMB). DM and OM digestibility varied between 49 and 56% and were significantly (P<0.05) higher for rations with nitrogen (HFC), “Magadi” (HMG), sugar (HMO) and starch (HCA) supplementation compared to the control. Rumen fluid pH was not greatly influenced by treatments but rumen fluid ammonia was significantly (P<0.001) higher in HFC compared to other treatments. Mean total VFAs tended to be higher (P<0.1) in HFC (67.0 mM) compared to the other treatments. The molar proportion of acetate was significantly (P=0.05) higher in HFC (68.8%), HMG (68.4%) and HMB (68.0%) compared to HMO and HCA (65% each). Rate and extent of digestible NDF degradation was significantly (P=0.02) higher with nitrogen (HFC) (2.66% h^-1 and 64.2%, respectively) and significantly (P=0.01) lower with starch (HCA) (1.72% h^-1 and 53.0%, respectively) compared to the other treatments. Starch supplementation (HCA) caused the highest passage rate for INDF (2.55% h^-1) and was significantly higher than for the control diet. It is concluded that more efficient utilization of poor quality forages occurred with increased nitrogen availability in the rumen and that the use of sugar, starch and feed additives like “Magadi” can have variable effects on intake and digestibility of poor quality forages. However, if cheap sugar and starch concentrates are available, they can be used without compromising the digestibility of the basal forage markedly.