Our objective was to know the right slit width for iron (Fe) concentration of forage sorghum, sorghum hybrid (Sorghum bicolor (L.) Moench), and also to discern which water treatment sludge (WTS) were good for ruminant's health with the feeding sorghum on the present study. The present experiment was carried out on a randomized block design with four treatments; Control, alum sludge compost, alum sludge + NPK (nitrogen, phosphorus, potassium fertilizers), alum sludge compost + NPK (nitrogen, phosphorus, potassium fertilizers). Sorghum hybrid was harvested, and iron content of it was analyzed with an atomic absorption spectrophotometer on background correction (BGC) mode. In order to analyze the iron (Fe) content of the sorghum with the spectrophotometer, three different slit widths conditions were used; 0.15, 0.20 and 0.25 nm. Absorbance and background values were obtained during the Fe analyses with the apparatus. When the background value is small, it is preferred for some trace metals' analyses. Both (AM/BS) ratio (mean of the absorbance values to the standard deviation of back ground values) and (AS/BS) ratio, were larger on 0.25 nm slit than those on 0.15 and 0.20 nm slit, and, from our experiment, the condition seemed better on the 0.25 nm slit for the iron analysis with the spectrophotometer. Therefore, the sorghum hybrid grown on (Alum+NPK) and on (Compost only) might be dangerous for ruminants because of their higher values than 200 mg Fe/kg DM (dry matter).Key words: Absorbance, alum sludge, atomic absorption spectrophotometer, background, forage sorghum hybrid, iron, slit. INTRODUCTIONIn Korea, the sorghum hybrid was used as forage plant for ruminants. Fribourg (1985) described that summer annual grasses, for example sorghum, provide high quality feed in summer when well managed, but poor management results in misuse of land resources, little regrowth *Corresponding author. E-mail: kimsd@joongbu.ac.kr. Tel: later in the season, and hungry livestock. All the body systems depend on minerals in order to function properly (Nelson, 1979). For ruminants, iron (Fe) was considered to be necessary more than Cu, cobalt (Co), or iodine (I) among trace elements as follows; Fe (30 to 200 mg/kg dry matter (DM)> Zn (50 mg)> Mn (40 mg)> Cu (5 to 10 mg)> I (0.12 to 0.80 mg)> Co (0.1 mg) (McDonald et al., 1985).Until recently, (flame) atomic absorption spectroscopy (AA) was the most widely used of all atomic spectral methods because of its simplicity, effectiveness, and relatively low cost. The position of preeminence is now being challenged, however, by inductively coupled plasma spectroscopy (ICP), an emission method (Skoog, 1985). When a calibration curve is linear upon a range of metal's standard solutions with an atomic absorption spectrometer, they generally start the analysis of a metal. And the condition of slit width is not an important thing. While atomic absorption spectrophotometry (AA) is historically an older method than Inductively Coupled Plasma (ICP...