The use of conservation tillage practices has been gaining in popularity in Wyoming to reduce energy requirements and to conserve and protect soil and water resources. Yields in dryland and irrigated conservation tillage systems have been comparable to those for conventional tillage systems; however, increased management has been required. The only significant differences observed due to tillage system in irrigated row crops were with sugarbeet plant populations and in dryland wheat with grain protein.

Weeds have been a problem in all cultural systems devised through the ages. Only the means of control and the specific weeds that are trouble some have changed. Weed management in conservation tillage cropping systems is highly dependent on herbicides; however, effective management programs must also manipulate weed-crop associations in favor of the crop at the expense of the weed. This paper will discuss a number factors influencing weed management decisions, including weed spectrum shifts, crop rotations, fertilization, cultivation, early preplant herbicide application, as well as herbicide input levels.

Reduced control of existing weed species is one problem associated with conservation tillage systems, but perhaps a more important consequence of reduced tillage is the potential for shifts in weed species composition. A change in weed species composition will result in the emergence of species that are tolerant of standard weed control practices. In other words, when a particular combination of weed control treatments is chosen, certain weeds will be favored. The most common observation relative to weed shifts is that perennial weeds tend to become more prevalent under conservation tillage systems.

Several factors contribute to changes in weed populations under conservation tillage. Changing tillage practices most immediately impacts the vertical distribution of seeds in the soil profile, which can affect weed plant density and species composition. The response to burial of individual weed species differs, which potentially gives a selective advantage to certain species. In addition, strong selective pressure placed on weed populations by herbicides can contribute to these species shifts.

The importance of a crop rotation in a weed control program has long been recognized. Other benefits reported for rotating crops include maintenance of soil fertility, better distribution of labor and higher crop yields. Crop and herbicide rotation can be an effective component of a weed management program.

The major effect of a crop rotation is to allow herbicide selection to match the weed problem. Since herbicide select ion to control all weed species is difficult, crop rotations will allow the use of different herbicides so that a weed species will not-go uncontrolled for two years in a row.

A vigorous, growing crop is often the most effective and economical weed control practice available. Fertility is one of the factors that can be managed and manipulated to optimize crop competitiveness. For example, broadcast nitrogen applications in a winter wheat fallow rotation have increased downy brome populations when compared to unfertilized treatments.

In addition, broadcast nitrogen has double root growth, tripled shoot growth, stimulated earlier spring growth and increased downy brome root penetration in soil. However, nitrogen placement can alter this competitive relationship between downy brome and winter wheat. Significantly fewer downy brome plants were observed in banded, compared to broadcast, nitrogen treatments, while wheat utilized more nitrogen and produced more grain with banded rather than broadcast treatments.

Although many think that row crop cultivation is not a part of reduced tillage systems, cultivation can be very effective for weed control, and yet maintain residue on the soil surface to prevent soil losses by erosion. In fact, cultivation, along with ditching, is essential in furrow-irrigated reduced tillage systems.

Very little soil erosion will occur from crop cultivation if cultivation is done during periods of reduced rainfall intensity and frequency, and when crops are providing a partial canopy. It is important that cultivators be equipped with sweeps that destroy weeds but leave a significant amount of residue on the soil surface.

Application of herbicides 30 to 45 days prior to crop planting has proven particularly effective as a weed management strategy in conservation tillage systems. Advantages of early preplant herbicide application include: prevention of weed establishment, elimination of the need for burndown treatments at planting (paraquat or, glyphosate) and reduced potential for herbicide carryover from one crop season to the next. Early preplant herbicide treatments in no-till systems have included Dual on sugarbeets, Dual plus 2,4-D on dry beans and Bicep or Extrazine on corn. Early preplant rates have been approximately 1-1/3 the rate used at planting.

As tillage is eliminated, greater demands are placed on other weed control methods to maintain adequate weed control. Higher herbicide input levels have been required to overcome the increased weed pressure in conservation, compared to conventional, tillage systems. Weed control obtained with low levels of herbicide input in conventional systems is often as good as high input levels in conservation tillage systems.