in the table for a bench with a contributing 

 area. The data for a bench without a con- 

 tributing area are also averages of three 

 benches. Yield of alfalfa from the benches 

 ranged from 2.59 to 4.46 tons per acre. 

 The 4-year average yield from the benches 

 was more than twice that from the nonbenched 



Grassland 

 study 



slope, or a gain of 2.2 tons per acre. The 

 benches with contributing areas outyielded the 

 benches without by only 0. 1 2 ton. 



Following is a summary of the alfalfa 

 yield results for each of the two studies, and 

 an average for the two: 



Contributing 

 area study 



Average 



Gain, bench 

 over slope 



•Tons per acre- 



Slope 0.90 



Bench without C.A. 1.94 



Bench with C. A. 2.08 



1.58 

 3.74 

 3.86 



1.24 

 2.84 

 2.97 



1.60 

 1.73 



ENGINEERING ASPECTS 



Several engineering aspects affect the 

 problem of laying out a level bench system, and 

 each influences costs and returns. To conserve 

 water efficiently, the benches must be level in 

 all directions, and they should be uniform in 

 width to permit efficient farming operations. 

 These two requirements complicate the layout 

 job, especially on irregular slopes. In general, 

 the cost of the bench is presumed to depend on 

 the amount of earthmoving to be accomplished, 

 which in turn depends on the degree of slope 

 and bench width. The relative complexity of 

 the slope will likewise affect the cost. 



Slope, Bench Width, and Depth of Cut 



On a given slope, the wider the bench and 

 the steeper the slope, the deeper will be the 

 cut necessary to construct the benches, and 

 the deeper the cut, the more earthmoving will 

 be required per acre. For a given width of 

 bench, the steeper the slope, the deeper the 

 cut, and hence the more earth to be moved. 

 Several widths of bench on several ranges in 

 slope were analyzed for comparison. 



To improve efficiency in harvesting oper- 

 ations, the width of bench should be a multiple 

 of the width of the harvesting machinery to be 

 used. Two types of hay-harvesting machines 

 considered were a 7-foot mower and a 14-foot 

 self-propelled swather. With either of these 

 machines, the width of bench should be a mul- 



tiple of 14 feet. For analysis, five multiples of 

 14 were selected: 14, 28, 42, 56, and 70 feet. 

 If different widths of harvesting machines were 

 used, appropriate modification should be made 

 in the bench widths. 



For each of the five widths of bench, costs 

 were calculated for 10 degrees of slope (i.e., 

 1 percent through 10 percent at intervals of 



1 percent). To simplify the analysis, the slope 

 was assumed to be perfectly uniform, though 

 no such slope is likely to be found in actual 

 practice. 



In developing comparative cost data, the 

 first step was to calculate the maximum depth 

 of cut for each slope and each bench width. To 

 build a 14-foot bench on a 1 -percent slope would 

 require a cut of 0.9 inch, while a 70-foot bench 

 on the same slope would require a cut of 

 4.4 inches (table 2).^ On a 10-percent slope, 

 a cut of over 3 1/2 feet would be required for 

 a 70-foot bench, but an 8.9-inch cut would be 

 enough for a 14-foot bench. Where the soil is 

 cut below the A horizon (topsoil), enough top- 

 soil should be reserved to return at least 



2 inches of it to the exposed subsoil areas (3). 



8 The calculations in tables 2 and 3 are based on 

 unpublished data furnished by Dan McLellan, formerly 

 Associate Professor, Agricultural Engineering, North 

 Dakota State University, Fargo. A "balance factor" of 

 1.3 cubic yards of cut for each cubic yard of fill was 

 assumed. 



6 



