G28 

 (leaf 2) 



produced although many of these do not mature their seed satis- 

 factorily ; even when well matured the seed has poor viability. Dur- 

 ing drought years practically no flowers or seed are produced. The 

 increase in area by tillering during a given growing season de- 

 pends somewhat upon the intensity of the grazing and the density 

 of the existing vegetation, but chiefly upon the vigor built up in the 

 preceding year. For revegetation by stolons to be highly successful, 

 two successive favorable growing seasons are required, one for the 

 new sets to be produced and the second for the sets to become rooted 

 and established as individual plants. The number of stolons per tuft 

 varies from 1 to 9, but the number produced appears to have no 

 relation to the size of the tuft producing them. The chief value 

 of this method of revegetation is that it makes it possible for new 

 plants to be established at some distance from the parent plant and 

 this results in a greater area increase than would be possible from 

 tillering alone. In general, stolons are not so effective a means of 

 revegetation on grazed areas as is tillering nor are they as effective, 

 either in good or poor years, on grazed as on ungrazed areas. The 

 trampling of the grazing animals breaks off many of the stolons be- 

 fore the new sets can be established. 



Based on quadrat experiments carried on in a 13-year study with 

 four different intensities of grazing use of black grama on the 

 Jornada Experimental Range in southern New Mexico, Nelson (op. 

 tit.} gives the following results: 



Heavy overgrazing year after year has a fivefold effect upon the stand of 

 black grama: (1) The existing stand of black grama rapidly broke down and 

 eventually died; (2) natural revegetation was seriously handicapped and 

 reduced because of the low density and poor vigor of the old plants and 

 excessive trampling of new plants; (3) drought losses were intensified; (4) 

 competition from inferior perennial grasses and weeds increased; and (5) 

 a marked reduction appeared in the annual forage crop. 



Overgrazing during the summer growing season, when the main growth of 

 black grama is made, reduces the height growth even in favorable growing 

 seasons, although not so severely as does heavy, yearlong overgrazing. If 

 this is repeated summer after summer the vigor of the plants is impaired and 

 no spread by tillering or establishment of new plants by stolons can occur even 

 in favorable growing periods. 



Full use in the better years and slight overgrazing in dry years did not per- 

 mit the full recovery of black grama from the losses suffered in drought 

 years, and the lateral spread of the tufts by tillering in the favorable grow- 

 ing years was not so effective as on the ungrazed or conservatively grazed 

 ranges. The average height growth in favorable growing years was only 

 slightly under that of the ungrazed years. In drought years, the species is 

 hindered from attaining its optimum condition. On the whole, this method 

 "prevents the maximum development of the black grama stand and permits 

 the inferior, associated grasses and other species to secure a foothold on the 

 more depleted black grama ranges." 



Conservative grazing, i. e., grazing management on a sustained yield basis, 

 enables forage production of black grama to be maintained "as well as or 

 better than under complete protection from grazing." While revegetation by 

 stolons is not so extensive nor so effective under this method as is lateral 

 spread by tillering, a more even distribution of the smaller tufts over the 

 soil surface results, and the stand as a whole is not reduced. Without doubt, 

 conservative grazing is the most stable and productive system of grazing for 

 black grama. 



