CONSERVATION RESEARCH REPORT 5, U.S. DEPT. OF AGRICULTURE 



spike-tooth harrowed prior to planting to control 

 volunteer sorghum growth and prevent large 

 drying cracks. 



Seeding and Harvesting 



Plots were planted each year with RS-610 

 hybrid grain sorghum in rows 20 inches apart. 

 Planting date, rate, and harvest dates were as 

 follows: 



Planting 

 rate 

 Date of planting (Lb. /acre) Harvest dates 



1956— June 13-14 15 Oct. 4-Nov. 2 



1957— June 22 11 Oct. 29-30 



1958— June 16-17 18 Oct. 6-16 



1959— June 17-18 6 Oct. 10-23 



The rate of planting used in 1959 was considered 

 to be the minimum for maximum yields based on 

 other studies (13). 



Yield Determination and Disposal of Crop 

 Residue 



Yields were determined by hand-cutting heads, 

 which were dried and threshed at a later date, 

 except in 1956. In 1956, 4 rows, 40 feet long, 

 were harvested from each subplot with a small 

 self-propelled combine. In 1957, 1958, and 1959, 

 4 rows, 25 feet long were hand-harvested. After 

 hand-harvesting to determine yields, the rest of 

 the plot area was harvested with a combine. 

 Each year the residue was returned to the indi- 

 vidual plots, except in 1957. In 1957, an offset 

 combine was used that deposited the threshed 

 stalk to the side of the plot. The stubble re- 

 maining after harvest was shredded either in the 

 fall or in the spring and disked into the surface. 



Evapotrcmspiration Determinations 



Evapotranspiration (E t ) was determined from 

 soil samples taken periodically to depths of 4 or 

 6 feet on the F 2 , F 4 , and Fs subplots of each 

 moisture level. Samples were taken by hand in 

 1956, partially by machine in 1957, and by machine 

 in 1958 and 1959 (9). 



Soil sampling sites were marked so that succes- 

 sive cores could be taken about 1 foot or less from 

 the preceding location moving in the same direc- 

 tion each time. After removing the core, the hole 

 was filled with surface soil and tamped. Because 

 of the low intake rates and limited depths of water 



applied, the rate of E t during an irrigation period 

 (from the date of sampling prior to an irrigation 

 to the date of sampling after an irrigation) was 

 calculated as follows: 



Inches 1 + (Irrigation and rainfall) — Inch es 2 

 Days between sampling dates 



= Inches per day 



where inchesi and inches 2 represent the total 

 water in the profile before and after irrigating. 

 The depth of irrigation water applied was gener- 

 ally less than the amount required to bring the 

 soil to field capacity with the exception of the 

 first irrigation in 1956, which was applied to 

 improve the germination and uniformity of 

 stand. 



Values obtained by this procedure for the irri- 

 gation period usually were somewhat larger than 

 those obtained between sampling dates after an 

 irrigation. This method of calculation for the 

 irrigation period assumes that each subplot re- 

 ceives the same depth of water and no deep 

 percolation occurs. Small differences in intake 

 between fertilizer subplots due to small differences 

 in soil moisture content may have occurred from 

 1957 through 1959. The 1959 seasonal total E t 

 was from the first to the last sampling date; for 

 the other years an adjustment was made back to 

 the date of planting. 



Other Measurements 



Nitrogen content of the grain was determined 

 each year from 1956 through 1958 and in 1960. 

 The percentage of protein was obtained by using 

 a constant ratio between nitrogen content and 

 protein content. Detailed evaluation of total 

 nitrogen uptake on three moisture levels with four 

 rates of nitrogen was carried out in 1957 and 1958. 

 The results of the nitrogen study have been pre- 

 viously published (11). Height determinations 

 were made with a surveyor's rod, and the average 

 height of the grain sorghum was observed at full 

 growth. The relative date of heading was deter- 

 mined when the earliest plots were blooming. A 

 numerical rating was used as follows: (1) late boot 

 stage, (2) beginning to head, (3) partially headed, 

 and (4) headed and blooming. The number of 

 heads per unit area was determined as the plots 

 were hand-harvested. Test weight of the grain 

 was determined by standard volumetric and 

 weighing procedures. 



RESULTS AND DISCUSSION 



Evapotranspiration 



The High Plains is not a large homogeneous 

 irrigated area. Irrigated fields are intermixed 

 with rangeland and nonirrigated farmland. The 

 total acreage of irrigated crops in 1954 other than 



wheat, a winter crop, represented only about 10 

 percent of the total land area in the High Plains. 

 In the eight-county area, which had 84 percent 

 of the irrigated grain sorghum in the High Plains 

 in 1954, the total acreage of irrigated crops, 

 other than wheat, represented only 38 percent 



