Numerous investigations indicate that the composition of the fertilizer, the time of application, and the placement in relationship to the crop seeds are critical factors in weed control in crops grown for seeds. Fertilization, irrigation, and other cultural practices should be employed in such a manner as to insure maximum benefits to the crop plants and minimum benefits to the weed population.

WEEDS are a total farm liability and all crops on the farm are subject to their competition.

The cost of controlling weeds in individual crops undoubtedly appears high because only the current crop is considered rather than the total weed problem in all crops grown on the farm.

Too often producers of crop seeds approach the problem of controlling weeds in crops only when the weeds are present in the current crop. In order to obtain a better balanced and more efficient weed-control program, farmers must be encouraged to use chemical weed-control methods in all crops. Greater emphasis must be placed on the necessity of supplementing this program by the rotational use of herbicides on all tolerant crops throughout the rotation in combination with various cultural and mechanical weed-control practices. Farmers should rotate different herbicides on the same crops grown continuously and learn more about the efficiency of rotating different herbicides on different crops throughout the rotation.

The rotation of herbicides on the same crop and on different crops throughout the rotation enables the grower to prevent undesirable ecological shifts in the populations of weeds. Using herbicides throughout the rotation illustrates the principle of keeping maximum pressure on the weed population by the use of a series of herbicides which differ in their effectiveness in controlling various weed spectrums. This procedure reduces the chance of a species that is tolerant to a specific herbicide from becoming dominant and spreading.

THE ROTATION of herbicides and the use of herbicides on all crops in the rotation also reduces the chance of an accumulation of herbicide residues in the soil. Residual-type herbicides can be rotated with those possessing little or no residual problems.

The potentialities for more efficient weed control in crops grown for seeds seem almost unlimited if herbicides could be developed for use in all crops and the weed problem is considered a total liability to the entire farm.

Herbicides must also be used in mixtures and in combination with mechanical and cultural practices for greater efficiency.

For example, herbicides are not available which will selectively control Canada thistle in growing tomatoes. However, a herbicide such as 4-(2,4- DB) can be effectively used to control Canada thistle in alfalfa and 2,4-D can be used to control it in corn. While herbicides are not available for the control of all weeds in all crops without causing any injury to the crop, herbicides are available which can be used for a wide variety of weed problems if the herbicides are used on all crops in a rotation which will tolerate herbicides.

FARMERS know that thorough preparation of the seedbed, followed by clean, efficient, shallow, timely cultivation, has an important place in weed control. It has been established, too, that there are no substitutes for adapted varieties that are properly fertilized and managed.

However, knowledge is needed of the value of the rotational use of herbicides, the use of mixtures of herbicides, the use of herbicides in combination with effective cultural and mechanical practices, and the multiple uses of herbicides in crop rotations, just as they have learned the value of rotating crops, the use of good varieties, clean seeds, and sound fertilizer practices.

Surveys indicated that in 1959 farmers used more than 100 million pounds of herbicides on approximately 50 million acres of agricultural land at an estimated cost of 150 million dollars. The herbicides applied on this acreage were developed for commercial use since 1950.

W. C. SHAW is Leader, Weed Investigations, Agronomic Crops, Crops Research Division, Agricultural Research Service.

L. L. DANIELSON is Leader, Weed Investigations, Horticultural Crops, Crops Research Division, Agricultural Research Service.