In the process of forage balling operation, higher density achievement for produced bales always has been considered. In this study increase in bale density by size reduction of alfalfa stems, was considered. Optimum bale density according to previous research was determined based on percent moisture content and bulk density. Alfalfa size reduction as a procedure to decrease maximum compressing force in baling machines was considered to produce bales with higher densities. To achieve this aim, a prototype chopper device of cylindrical type with two cutter knives was designed, fabricated and evaluated. The machine was designed based on engineering considerations as a unit which can be attached to a windrower. According to flow rate of materials and inlet width space in windrower, three alfalfa chopping units would be used. Since the power requirement of chopping unit was unknown, in this study one prototype unit was fabricated and evaluated. The power and energy consumption and crop losses were determined on a test rig. Experiments were performed using a completely randomized design with three replications. The ratio of shedding weight due to cutting by machine to initial weight of samples was measured and reported as crop loss. Results showed that with increase in cylinder speed, average length of cutting stems were 35, 24 and 15 centimeters for 83, 125 and 167 cylinder RPM, respectively. These results were comparable to calculations made in design procedure of machine. Power consumption was determined by measurement of the electrical current consumed by the test rig electric motor for each experiment. The results showed that with length reduction or with increase in speed of cylinder, power and energy consumption increased but no significant difference was observed. Also the results showed that difference between the calculated losses in three tests were not significant. For determination of machine performance, some mechanical and rheological properties of the alfalfa stems were determined. The confined compression test was conductedon Hamadani alfalfa variety with 70 centimeter stems height with moisture content of 20 percent on a wet basis at four levels of length (25, 33, 50 and 100% of primary length). The experiment was conducted in randomized complete design with three replications. In order to achieve bales with 150 kg-m 3 of density, the stress-strain curve for each test was obtained and the maximum of compression force and time relaxation were determined.Determination of relaxation times is one of important parameters in forage balling and compression process. Results showed that the maximum of compression force increased as stem length decreased, so minimum corresponding value of samples in 25% of primary length was obtained. According to LSD test, the difference in maximum compression forces were highly significant ( p 0.01), but relaxation time difference for various treatments was not significant. Also the results showed that first relaxation times in all the samples were less than 5 seconds. The Maxwell model (relationship between stress and relaxation time of viscoelastic products) was fitted on data values with coefficient of determination of 0.99 and the constant parameters were obtained.