GROWING RUBBER IN CALIFORNIA — PERRY 357 



resin is not disadvantageous, and to deresinate it would merely reduce 

 the volume so as to counterbalance the increase in price. 



Getting the best returns from guayule under cultivation is subject to 

 a considerable array of variables. Harvesting and milling costs de- 

 pend largely on the amount of shrub that must be processed to obtain a 

 pound of rubber, and big shrub with high rubber content is an elusive 

 combination. It is when growth slackens or is suspended because of 

 lack of moisture, cold weather, or some similar circumstance that rub- 

 ber percentage begins to rise.. The theory is that the carbon fixed by 

 the leaves is largely used up in growth and seed production when 

 environmental conditions are favorable for those functions, but when 

 conditions are unfavorable and growth and flowering slow down or 

 stop, the plant, continuing to assilimate carbon, seeks to store it in some 

 appropriate form. The compound chosen happens to be the pure 

 hydrocarbon, rubber. 



Thus, if you attempt to force the plant too hard, as by irrigation, you 

 get a magnificent shrub, but one containing a low percentage of rubber. 

 The reason Dr. McCallum chose the Salinas Valley for guayule grow- 

 ing is that there is an invariable alternation of wet winter and spring 

 with a long, dry summer and fall, thus automatically affording the 

 conditions necessary for both plant growth and rubber deposition. On 

 unirrigated land there, the optimum rotation, from an economic stand- 

 point, appears to be about 5 years, and at that age a good field may 

 produce up to 1,800 pounds of rubber per acre. 



Under irrigation, where unseasonable rainfall does not interfere, 

 this rotation may be shortened by properly manipulating the irriga- 

 tion water. Under the stress of the original program it was planned 

 to grow it only 2 years under irrigation, spacing the plants closely 

 (28 inches by 20 inches) to obtain optimum volume on the land. This 

 rotation is not considered economically sound, though actually there 

 has never been sufficient information collected on the behavior of the 

 plant under irrigation to justify definite pronouncements on the 

 subject. 



RESEARCH PROGRAM 



That subject is being studied now, and incidentally one of the bright 

 spots of the program is the fact that a magnificent job of research is 

 being done on it. While the operational phases of the project are 

 under the direction of Paul H. Roberts, who succeeded Major Kelley 

 when Kelley returned to his post at Missoula in 1943, the Bureau 

 of Plant Industry, Soils, and Agricultural Engineering has a corps 

 of scientists under the able leadership of Dr. A. C. Hildreth following 

 almost every imaginable line of investigation in connection with the 

 plant itself ; and the Bureau of Agricultural and Industrial Chemistry 



