ABSTRACT. This study investigated the effect of reel index and type of combine harvester on header losses when harvesting the long grain paddy variety

Lower performances of traditional threshing methods, labour shortage, reduced turn-around time and use of high yielding varieties have inevitably forced farmers to shift into mechanical grain threshing. Recently introduced four-wheel tractor driven high capacity combined paddy thresher gained popularity among Sri Lankan farmers. However, this thresher has not been properly evaluated. This study evaluated the field performances and economics of this thresher and monitored the emission of chaff and dust to compare the commercial makes, grain types and operational conditions. This study was carried out in the North Central Province of Sri Lanka, during the Maha season of 2008/2009. Air monitoring on inhaled air was done by filtration technique, using special masks and surrounding environment by gravity techniques, using Polyurethane foam. As per the RNAM test code procedures, three commercial makes of high capacity combined paddy threshers were evaluated under different operational conditions for long and short grain paddy varieties with two replicates. The combined paddy threshers showed 1.8% damaged grain, 0.2% blown grains, 1.6% grain losses, 96.7% threshing recovery, 98.8% threshing efficiency, 90.7% cleaning efficiency, 1178 kg/h corrected output capacity and Rs. 2744 actual cost of threshing per metric ton of output at 14% moisture content. Though this thresher is suited for Sri Lankan conditions, it is associated with considerable amount of chaff and dust content such as; 2.04 mg/cm 2 /h in inhaled air and 35.59 mg/cm 2 /h in surrounding environment. The performance, economics and chaff and dust emission vary with make of the machine, grain type and operational conditions. Grain moisture is the most dominantly affected factor for the thresher performance.

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Grain quality directly affects the profitability of a crop. This study investigated the field operational conditions of combine harvesters with respect to the quality of the grain. A total of six brands of combine harvesters were tested in 10 different fields in Batticaloa and Ampara districts. The damages imposed on grains were determined based on the milling quality of the samples. The holistic view of data from farmers' field trials indicated that the combine forward speed, tip velocity of threshing drums and grain moisture content highly influenced the physical damages caused to the paddy grains during milling. Threshing unit tip velocity of the combine harvesters were found to be in the range of 10.35-39.51 m/s. The tip velocity of 10.35 m/s gave the lowest broken percentage of 15.38% and a higher head rice yield of 49.45%. Relatively low percentages of broken rice of 19.32% and 27.45% were obtained at high forward speeds of 1.36 and 2.5 m/s, respectively. This could be due to the creation of cushioning effect on the grains by increasing feed rate. High impacting forces on the grains at increased tip velocities caused the grain fissures which decreased the quality of seed paddy. The lowest grain moisture content of 15.9% yielded a relatively high broken rice percentage of 33.98 kg/ha at the tip velocity of 24.64 m/s. The highest grain damage of 39.94 kg/ha was obtained during milling at the tip velocity of 39.51 m/s at the moisture content of 20.9%.