The precipitation in May and June and the temperature in June are important 

 factors in spring wheat yields in the spring- wheat region generally, although 

 in Montana the critical period for temperature extends into July and, in the case 

 of late grain, hot wind damage is a factor at even later dates. 



In North Dakota during a period of twenty-six years when May and June 

 rainfall was below normal twelve times and above normal thirteen times, the acre 

 yield of spring wheat during the thirteen years with rainfall above normal was 

 nine times above average and four times below. During these same years in South 

 Dakota, yields of spring wheat were ten times above average and three times 

 below. 



A combination of May and June rainfall above normal and June temperature 

 below normal occurred during this same period eleven times in North Dakota and 

 nine times in South Dakota and of these years seventy-three per cent in North 

 Dakota and ninety per cent in South Dakota had yields of spring wheat above 

 average. 



WEATHER AND YIELD RELATIONSHIPS IN MONTANA 



In Montana during the past twenty years, the yield of spring wheat shows a 

 closer relationship with combined May and June rainfall than with any other 

 single factor. In this data where combined May and June rainfall has been eight 

 times above and twelve times below normal, spring wheat yields have been nine 

 times above and eleven times below the adjusted average yield of spring wheat. 

 May rainfall alone was five times above normal and fifteen times below. June 

 rainfall alone was eleven times above normal and nine times below and July rain- 

 fall ten times above and ten times below normal. May, June and July rainfall 

 combined was eight times above normal and twelve times below, or the same as 

 for May and June rainfall combined. 



The combination of May and June rainfall above normal with June tempera- 

 ture below normal occurred six times in this period and of these years sixty-seven 

 per cent had yields above average. For June temperature alone nine years were 

 below normal in this period and of these years fifty-six per cent had yields above 

 average. For July temperature alone eight years were below normal and of these 

 years eighty-seven percent had yields above average. July temperatures above 

 normal occurred twelve times in this period and of these years seventy-five per 

 cent had yields below average. 



From this data it is apparent that combined May and June rainfall together 

 with July temperatures are important factors in determining yields of spring 

 wheat in Montana. A summary of these factors is shown in the following table 

 as plus and minus departures from the monthly normals and averages. 



PRECIPITATION 



TEMPERATURE 



May 

 Rain- 

 Year fall 



1926 — 



1925 - 



1924 _ 



1923 — 



1922 4- 



1921 - 



1920 — 



1919 — 



1918 — 



1917 — 



1916 — 



1915 + 



1914 - 



1913 — 



1912 4- 



1911 -f 



1910 — 



1909 - 



1908 -f 



1907 — 



Sums : 5 + 



15- 

 *No departure from normal. 



June 

 Rain- 

 fall 



July 

 Rain- 

 fall 



May 

 plus 

 June 



May 

 June 

 July 



June 

 Mean 



July 

 Mean 



Avcragre 

 Spring 

 Wheat 

 Yield 



11 + 

 9— 



+ 

 + 



10 + 

 10^ 



8 + 

 12- 



12- 



12+ 



9 + 

 11- 



(See Graphs on following page.) 

 —29— 



