16 



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



preplanting irrigation was given, irrigations can 

 be scheduled by (1) observing rainfall that has 

 occurred, (2) estimating probable rainfall based 

 on current forecasts for 4 to 5 days ahead, and 

 (3) utilizing the mean cumulative E t or E t rate 

 curve of figure 2 or 3. For high yields, only 

 about 50 percent of available water can be de- 

 pleted before irrigating the first time when the 

 root system is not fully developed. Thus, the 

 first irrigation should be applied to the entire 

 field before 3 to 3.5 inches are used from the soil. 

 With normal precipitation in June and July and 

 a planting date near June 15, the first irrigation 

 will be needed in late July or early August, 

 depending on actual rainfall. The second irri- 

 gation will be needed about August 15, allowing 

 3 to 3.5 inches to be depleted from the soil after 

 the first irrigation. The third irrigation, and 

 probably the last, would be needed between 

 September 5 and 10. A season with below normal 

 precipitation would require four irrigations with 

 the first one beginning sooner than indicated, 

 whereas a season with above normal precipitation 

 may require only two irrigations, each irrigation 

 requiring 3.5 to 4 inches of water. 



Medium Production Level 



If adequate fertilizer and a preplanting irri- 

 gation are assumed, the first irrigation for the 

 medium production level should be given before 



3.5 to 4 inches of soil moisture are used. With 

 normal climatic conditions, the first irrigation 

 will be needed about the first week in August. 

 The second, and perhaps the last irrigation, 

 should be given when about 5 inches of soil 

 moisture are used after the first irrigation. With 

 about 2.5 inches of average rainfall in August, 

 this irrigation will be needed about September 1. 

 A dry season would require three irrigations and 

 a wet season perhaps only one irrigation, scheduled 

 in a similar manner. 



Low Production Level 



If limited water supply or pumping capacity 

 during the summer is anticipated, irrigation for a 

 low production level may be necessary. The 

 planting rate should be reduced to rates near those 

 used for dryland farming. If a preplanting irriga- 

 tion was given, then the first and only irrigation 

 during the growing season should be given when 

 the plants begin to show signs of severe wilting 

 during the day. Preferably, this irrigation should 

 be delayed until the middle or latter part of August 

 during the milk stage. The yield with this practice 

 will not be high, but it will be greater than dryland 

 yields. Fertilizer requirements will be consider- 

 ably less at this level of production. Lodging 

 caused by charcoal rot may be severe some years 

 with these limited irrigation practices. 



SUMMARY AND CONCLUSIONS 



The 4-year study of irrigated grain sorghum, 

 with six soil moisture levels and each with six 

 fertilizer treatments, showed that seasonal evapo- 

 transpiration (E t ) will average about 22 inches 

 from planting to harvest when irrigating and 

 fertilizing for high yields (fig. 2). Nitrogen fertil- 

 izer increased yields 2 to 2% times more than those 

 from plots receiving no nitrogen, but increased 

 seasonal E t only about 6 percent. 



The rate of E t shortly after planting grain 

 sorghum in June was less than 0.1 inch per day, 

 even though solar radiation and air temperatures 

 were high. As amount of vegetation increased, 

 the E t rate increased rapidly, reaching a maximum 

 of about 0.30 inch per day during the early part 

 of August (fig. 3). During August and until 

 harvest, the E t rate decreased as solar radiation, 

 air temperature, and soil moisture decreased, 

 and as the plants matured. 



Grain sorghum yields were greatly affected by 

 the soil moisture level. Yields were also greatly 

 affected by the rate of nitrogen application in the 

 third and fourth year after beginning to irrigate 

 Pullman soil. To maintain yields of 6,000 to 7,000 



lb. /acre, at least 120 lb. /acre of nitrogen wer 

 required annually after 2 years of irrigation (table 

 4) . With limited irrigations that restricted yields 

 to 2,500 to 3,000 lb. /acre, nitrogen fertilizer was 

 not needed. 



Yields were less than 6,000 to 7,000 lb. per 

 acre when more than 80 percent of the available 

 water in the 0- to 4-foot depth had been depleted 

 before irrigating (fig. 4). Delayed irrigations 

 reduced seasonal E t by 10 to 20 percent, but 

 yields were reduced 20 to 35 percent (fig. 5). 

 Severe lodging caused by charcoal rot and low 

 test weights occurred when soil moisture was 

 adequate early in the season but inadequate late 

 in the season. 



High water use efficiency was greatly dependent 

 on nitrogen fertilizer. Nitrogen fertilizer doubled 

 the production of grain produced per unit of water 

 (appendix table 19). High average water use 

 efficiency occurred when optimum soil moisture 

 was maintained. During years with well-distrib- 

 uted precipitation, lower moisture levels also 

 resulted in high water use efficiencies. 



