per acre been increased but the feed is being used more ef- 

 ficiently by livestock. Improvement in the breeds of live- 

 stock, improved feeding practices, and a slight increase in the 

 amount of protein feeds have contributed to the rising out- 

 put of livestock products per unit of feed available. During 

 the past quarter of a century the production of livestock 

 products per acre of land and per unit of feed have risen 44 

 and 27 per cent respectively (table 4). 



TABLE 4. INDEX OF PRODUCTION OF LIVESTOCK AND 



LIVESTOCK PRODUCTS PER ACRE AND PER 



UNIT OF FEED* 



1910-14 = 100 



* Feed grains and hay. 



In general, these changes are gradual and occur over long 

 periods of time. The amount of milk produced per cow has in- 

 creased gradually over the past quarter-century. The change 

 from horsepower to tractor power was more rapid and cer- 

 tainly more spectacular. The expansion in the acreage of al- 

 falfa has been slow. The development of hybrid seed corn 

 was gradual, but its adoption by the Corn Belt was sudden 

 and spectacular. This increased the possibilities of expand- 

 ing livestock production. There is a limit to the amount of 

 feed that can be made available to other types of livestock 

 through a reduction in the horse population. There is a limit 

 below which the number of horses is not likely to decline. 

 Furthermore, the amount of feed fed to each horse is ap- 

 proaching an irreducible minimum. 



