MSc. Dissertation of Crop Science 2018Nitrogen deficiency and planting low yielding varieties are common problems at Kilombero Estate mill area, and are known to be the major causes of yield decline. An experiment was laid out in split plot in a randomized complete block design (RCBD) with three replications to evaluate the effect of N- fertilizer on yield and quality of introduced varieties in the commercial farms at Kilombero sugar estate during the 2015/16 season. The main plots were varieties and the sub plots were N-fertilizer rates. Tested varieties were N41 (at first ratoon crop), R 579 (at third ratoon crop) and N25 (at fourth ratoon crop). Each variety was applied with different N-fertilizer rates as urea fertilizer. The compared rates were 0, 150, 200, 250, 300, 350, 400 and 450 kg N/ha. Results indicated that N had an effect on yields of cane and sugar (t/ha) quality except purity, pol % and sucrose %. There was an increase in yield of cane and sugar, plant leaf nutrients concentration and plant nutrients uptake with increased N-rates. Variety N 41R1 had highest N utilization efficiency, nutrient removed, yield and quality followed by R 579 R3 and finally N 25 R4. Highest average yields of cane and sugar (t/ha) were found with 400 kg N/ha followed by 350 and 300 kg N/ha, then declined at 450 kg N/ha. Effect of N application was significant (P< 0.001) on cane and sugar yields. Differences were not significant between N-rates on quality parameters of pol, sucrose and purity percentage cane. The interaction of N41R1 x N fertilizer rates of T7, T6, T5, T4 and R 579 R3T7 had higher yields of cane and sugar followed by interaction of R 579 R3 x N -fertilizer rates of T6 and T5, while the least yield was with N25 R4 x N fertilizer rates of T7, T6 and T5. Development of soil specific nutrient management guidelines for the Kilombero farms is vital so as to recommend optimum fertilizer application levels on introduced sugarcane varieties to ensure profitability, net benefits, cane (t/ha), total benefit and total variable costs were significantly and positively correlated among themselves. The benefit cost ratioiii was negatively correlated with the other economic variables but significantly so with all except net benefit. Most of the cane yield components and nutrients removal were significantly and positively associated. It is concluded that N-fertilizer rates of 300- 400kgn/ha should be used for sugarcane production at Kilombero. A combination of N41 RI was superior in most of sugarcane quality and yield variables. The interactions between variety-ratoon with N-fertilizer rates suggest that N-rates have differential effects of variety-ratoon combination. The highest benefit cost ratio was not necessarily associated with higher net benefit.