GRAZING AND FORAGE PRODUCTION ON NATIONAL FORESTS 11 



and small yield of forage on the plots harvested earliest in the 

 season (Series I). The average yield of air-dry forage on June 20 

 of the plants in Series I was only a little more than a third of that on 

 July 10 of plants in Series III, in which the average height at 

 the time of the first cutting was about 6 inches. Furthermore, the 

 average total volume of the June 1 and June 20 cuttings of Series 

 I was but 73 per cent of the volume of the first (June 20) cutting 

 in Series III. In every species the weight of the forage yield for 

 the two cuttings increased sharply with delay in the first cropping. 



A comparison of the average height growth of the leafage of the 

 untreated plants of Series I with the total growth in height of the 

 harvested specimens on June 20 shows a difference of 43.6 per cent 

 in favor of the uncropped plants. This same relationship is less 

 conspicuously brought out in Series II, where the height growth 

 was 28.6 per cent greater for the untreated than for the harvested 

 plants on June 30. In the third series the treated plants produced 

 slightly more height growth than did the untreated ones by July 10. 



When bunch grasses and other non-turf-forming vegetation 

 which have produced from 2 to 4 inches of growth in the spring- 

 are cropped down so that only 1 or 2 inches of the leafage, or food- 

 manufacturing surface, remains, it frequently results that the plant 

 can not elaborate sufficient food both to replace the lost growth and 

 to replenish the stored foods in roots and crown. On the basis 

 of numerous microscopic examinations of the food-storage tissues 

 and of other observations, the difference in the rate of growth and 

 quantity of herbage produced by plants cropped with equal close- 

 ness but of different height and luxuriance of herbage when har- 

 vested may be explained as follows: 



In early spring the entire plant, being without green leafage, de- 

 pends for nourishment on the food which was elaborated the pre- 

 vious summer and stored in the cells of the plant's roots and crown. 

 By the time the plant has attained a height of from 2 to 4 inches it 

 has usually exhausted much of the food stored up during the pre- 

 vious year. The leafage is then so young that the small green bodies 

 known as chloroplasts, where the starch grains are formed in the 

 presence of sunlight, may be only partly developed. If the plant 

 is clipped at that time to within 1 inch of the ground, the leaf area 

 that is left is so limited and the digestive power so weak that 

 the elaboration of food is almost nil. But when a plant such as vio- 

 let wheat grass or mountain brome has attained a height of from 6 

 to 8 inches, sufficient leafage and fully developed cells are available 

 so that a comparatively large quantity of food is elaborated by day 

 and transferred by night to all parts of the plant. When this more 

 developed leafage is cropped the plant has available in its under- 

 ground storage tissues sufficient food to push forth the additional 

 leafage necessary for manufacture of food for the plant as a whole. 



HEIGHT OF PRINCIPAL SPECIES AS AN INDICATOR OF RANGE READINESS 



Since the degree to which the vigor of forage plants is impaired 

 by cropping depends on their degree of development Avhen cropped, 

 height growth of forage cover may serve as an index of the proper 

 time for the opening of the grazing season. 



