58 CHAPTER IV 



Injurious Effects of Weeds.— (1) Over the greater part 

 of South Africa the most serious of these is the fact that they 

 utilise soil moisture and thus decrease the supply of water 

 available for the crop plants. A ton of Lamb's Quarters (G. 

 album), according to American investigation, has a water re- 

 quirement of over 800 tons, or roughly 8 inches of water. 

 When a loss of this magnitude is added to those due to evapora- 

 tion and run-off, it is easy to see why crops often fail in areas 

 having an average annual rainfall of about 22 inches. 



(2) They deprive the crop of a considerable amount of 

 available plant food. 



(3) They smother crop plants. A young stand of lucerne 

 may be ruined in this way by species of Chenopodium. 



(4) Weeds are a constant source of expense, ns they in- 

 crease the cost of every operation — preparing of land, seeding, 

 cultivating, harvesting and marketing the cron. 



(5) The growing of certain crops often has to be aban- 

 doned on account of weeds, e.g., maize in iields badly infested 

 with Eooibloem or Witchweed (Striga lutea). 



(6) Some weeds are poisonous to livestock, e.g., Stink- 

 blaar (Datura stramonium) and Dubbeltje (Terrestris trihuUs). 



(7) Milk is often tainted when certain weeds are eaten 

 by stock, e.g.. Stinking Mayweed (Anthemis cotula). 



(8) The value of grain, lucerne and grass seed is much 

 decreased by the presence of weed seeds. The value of hay 

 containing weeds is lessened. Further, weeds lower the price 

 obtainable for land — a farmer will pay a higher price for clean 

 land than for weedy land. 



(9) Parasitic weeds rob their hosts of nourishment, e.g., 

 dodder on lucerne, and witchweed on maize. 



(10) Weeds often harbour or favour the development of 

 injurious insect pests and fungus diseases, e.g., early blight 

 of potatoes (Alternaria solani), also affects species of Datura 

 (Stinkblaar). 



(11) They may injure stock, as in the case of Jointed 

 Cactus (Opuntia auriantiacn) , or the products from stock, e.g., 

 the value of wool is much denreciated by the fruits of Bur- 

 Weed or Boetebosje iXanihium spinosum). 



Weeds are distributed naturally by wind, birds, and other 

 animals ; but their distribution is also attributable partly to 

 impurities in agricultural seed, to seeds contained in livestock 

 manures, to threshing machines and to tillage implements 

 which carry weed seed from one field to another. 



