THE EFFECT OF WEEDS UPON CROP PRODUCTION 
A. L. BAKKE AND E. H. PAMMEE 
Weeds have an important bearing on the extent and the char- 
acter of the crop produced. An examination of iany developing 
crop during the growing season brings the issue to the foreground. 
In the case of pasture land the efficiency is much reduced by al- 
lowing weeds to grow. One of us 1 has found that more than 
39 per cent of the pastures of Iowa contain the small ragweed 
( Ambrosia art e me sii folia) which not only prevents the growth of 
blue grass but also makes it difficult for cattle to graze. What has 
been specifically stated for pasture land is as pertinent to other 
crops in varying degrees. 
It has been pointed out by Livingston 2 that transpiration is practi- 
cally a simple function of the leaf surface and that the total 
transpiration is a measure of the growth of a plant whether it 
is one growing in a waste place or of economic importance. 
Hunt 3 makes the assertion that weeds are harmful because they 
exhaust the moisture from the soil. Corn plots on which weeds 
grew contained less moisture than plots kept cultivated. The 
yield was much greater where weeds were not allowed to start 
until June 27 instead of June 14. 
Kiesselback 5 makes it clear that weeds such as sunflowers give 
off more than three times as much water per plant as corn, while 
the water used per unit of dry matter was slightly more than 
double that of corn. In other words, a sunflower plant will con- 
sume as much water as a hill of corn. Brenchly 6 in a recent publi- 
cation states that weeds like mustard ( Brassica ) did better when 
they were associated with other plants than when they were sub- 
jected to the competition of their own species. Wheat is more effec- 
1 Pammel, E. H., Are the pastures of Iowa producing effciently la. Agricul- 
turist, 21 , 103-104; 1920. 
The weed flora of Iowa: Iowa Geological Survey, Bull. 4 ; 1913, Des Moines, 
Iowa. See p. 669. 
2 Livingston, B. E., Relation of transpiration to growth in Wheat: Bot. Gaz., 40 , 
178-195; 1905. 
3 Hunt, Thomas F., The importance of nitrogen in the growth of plants: Cornell 
Univ. Exp. Sta. Bull. 247; 1907. 
5 Kiesselback, T. A., Transpiration as a factor in crop production: Nebraska Agri. 
Exp. Sta. Res. Bull. 6; 1915. 
6 Brenchly, Winifred E-, The effect of weeds upon cereal crops: New Phytol. 16 , 
53-76; 1917. 
