This study evaluates the effects of fresh cassava root (CR) and a solid feed-block containing sulfur (S-FB) on fermentation in the rumen, feed utilization, milk yield, and milk composition in lactating dairy cows. Four Holstein-Friesian cows with 470 ± 50.0 kg body weight (BW), 10 ± 2 kg day−1 average milk yield, and 112 ± 15 days-in-milk were studied. A 2 × 2 factorial combination was arranged in a 4 × 4 Latin square design to evaluate the treatment-related effects. The treatments were

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... from a combination of two factors: (1) levels of CR at 10 g kg−1 BW (CR-1) and 15 g kg−1 (CR-1.5) and (2) levels of sulfur supplementation in solid feed-block at 20 g kg−1 (S-FB-2) and 40 g kg−1 (S-FB-4). The results showed that CR and S-FB had no interaction effect on feed intake, digestibility, fermentation, blood metabolites, milk yield, or its composition. Feeding CR up to 15 g kg−1 of the BW significantly increased (p < 0.05) the milk fat concentration while it decreased (p < 0.05) the somatic cell count. The S-FB-4 of the sulfur significantly (p < 0.05) increased the acid detergent fiber when compared with the S-FB-2 of the sulfur. CR could be fed up to 15 g kg−1 of BW with S-FB containing high sulfur (40 g kg−1) in dairy cows without a negative impact.