38 BULLETIN 1170, U. S. DEPARTMENT OF AGRICULTURE. 



plants generally reduces the value of the total vegetation of this 

 region. It is estimated that the palatability for cattle of the vege- 

 tation as a whole is approximately 75 per cent, but that 95 per cent 

 of it will be eaten when cattle are forced to graze a short range. 



CAUSES OF THE DETERIORATION OF NATIVE PASTURES BY 

 GRAZING WITH CATTLE. 



The main causes of the deterioration of native pastures when 

 grazed by cattle are (1) too early grazing in the spring, (2) con- 

 tinuous grazing, and (3) overgrazing. 



While it is the universal practice to turn the cattle into the pasture 

 as soon as the snow is off the ground in the spring, this fact can 

 not obviate the injury to the vegetation. This injury is caused by the 

 trampling of the cattle in the wet soil and by their picking off the 

 young vegetation as soon as it starts growth. Grazing in the north- 

 ern Great Plains should not begin before May 1 to May 15. By 

 that time the vegetation has usually made enough growth so that it 

 can keep ahead of the grazing. The grazing experiment at Mandan 

 has never been started before May 15. There are conditions under 

 which cattle may be turned on the native pastures early in the spring 

 without serious results except the damage done by trampling. These 

 conditions are found in pastures that are annually undergrazed 

 and therefore have sufficient old growth to maintain the cattle for 

 a short period. 



While continuous grazing at the proper rate may not directly 

 'm jure the vegetation as a whole, some species are likely to be 

 weakened because of overgrazing, while others may become so 

 coarse that they are avoided because of undergrazing. Cattle will 

 eat certain species in preference to others each season, which will 

 result in severe injury to the preferred species. This point is well 

 illustrated in the case of big bluestem in the 100-acre pasture. A 

 small area of this grass occurs in a depression. Each spring the 

 cattle have grazed this grass and kept it short, until at the present 

 time the area has become infested with weeds and its grazing value 

 reduced. 



Overgrazing is brought about by trying to feed too many cattle 

 upon too few acres. This is the most common cause of native-pas- 

 ture deterioration. If continuous overgrazing is practiced long 

 enough it will eventually mean the abandonment of the land for 

 pasture purposes. The cumulative effect of overgrazing is brought 

 about by the continual removal of the vegetation year after year 

 without affording it an opportunity to produce a crop normally. 

 This condition will sooner or later mean the death of most of the 

 valuable grazing plants. 



The system of clef erred and rotation grazing is designed to reduce 

 to a minimum the causes of deterioration of native pastures. This 

 is effected by allowing each division of the pasture to bear its burden 

 of early-spring grazing equally with other divisions and also allows 

 each division to share equally the benefits of normal growth and 

 grazing after the maturity of the vegetation. 



The following citations have a direct bearing upon the above 

 points : 



