Frequently Asked Questions

In China current weed control strategies are based on spraying soil herbicides shortly after sowing. MLHD is different. MLHD makes use of contact herbicides (eg. bentazone), which are applied in low doses in an early stage of weed growth. When done properly, there is a considerable reduction of herbicide use, better weed control and less risk of crop damage. Some important elements of the MLHD method are discussed here.

What is MLHD

MLHD means Minimum Lethal Herbicide Dosage. MLHD is a new concept within chemical weed control. The main objective is a high level of weed control with precisely the right amount of herbicide to kill the weeds (the lethal dose). Roughly the MLHD method is composed of three parts:

Calculating the required herbicide dose, based on the type of weed(s), the growing stage and size of the weeds and the efficacy of the chosen herbicides.
Keeping track of the weather conditions to determine the optimal moment of spraying
Measuring the effect of the spraying with a PPM, shortly after spraying in order to determine if the dose was sufficient. The PPM measures photosynthetic activity and can determine if the weed will die, long before a visual effect can be seen on the leaves.

The power of the method lies in the combination. Determination of the sufficient ammount of herbicide is done in an early stage (small weeds). Shortly after spraying it is possible to determine with the PPM if the treatment was effective enough. In this way a timely decision can be made if an additional treatment would be needed and in what dosage. Without a PPM there is a considerable risk involved in spraying with very low dosages. Before it is possible to visually access if the treatment was effective, already a whole week may have passed. If the effect of the treatment wasn't enough, it means that the weeds have grown larger and are more difficult to control. Because of this risk, dosages are usually chosen with a considerable safety margin. The possibility to determine efficacy almost immediately after treatment, makes it possible to actually use the lowest possible dose without any risk!

Dose depends on species and stage

For most contact herbicides, there is a relation between stage and minimum effective dose. The larger the weeds, the more herbicide is required to have sufficient weed control. Also some weed species are more sensitive than others. After a weed species inventory, tables of weed sensitivity are used to select the preferential herbicide. Then other tables are used to read the recommended minimum lethal dose in dependence of weed stage. In maize, a good moment of spraying is the 4 leaf stage. If spraying is done earlier, less herbicide can be used but a second treatment may be required. If spraying is done later, a higher dose must be applied.

Seedling | 2 leaf stage | 4 leaf stage | 6 leaf stage | 8 leaf stage (too late!)

Improved spraying technique

Spraying technique gets due attention. The spraying solution is prepared carefully using a measuring cup. To obtain an even distribution of spraying liquid, the lance sprayer is equipped with a pressure equalizer and a special nozzle. Finally spraying is not done by sweeping the lance, but by running parallel tracks at constant speed with the lance in fixed position. Spraying between the crop rows may further reduce an effect on the crop.

The right weather conditions

Choosing the right time of spraying will further contribute to good results. The weather conditions should preferably be such that they are optimal for herbicide retention on and uptake into the weeds. Usually conditions with low windspeed, moderate radiation and high air humidity are most suitable.

Measurement of herbicide efficacy

Because spraying and transport of herbicides into the plant have significant uncertainty, there is always some risk of insufficient weed control. Measuring the weeds with the Plant Photosynthesis Meter (PPM) 2 - 3 days after spraying, predicts the effect and eliminates this risk. Similar measurements on the crop give information on possible crop damage.

Sound research basis

Chinese weed control experts of ICAMA, HAAFS, CABET and the ICA’s of Shandong, Hebei, Heilongjian and Beijing, have co-operated with Dutch experts during 4 years to develop and adapt the new weed control approach for Chinise circumstances. Greenhouse experiments and field trials have been carried out. The method is demonstrated to extension workers and farmers.

Good results

The MLHD method has been tested in maize, wheat and soybeans. In Maize the method is ready for large scale implementation in practice. The MLHD dosing tables have appeared to be adequate to advise the minimum spraying dose. The Plant Photosynthesis Meter appeared very useful for checking spraying efficacy and possible damage to the crop. Large reductions in sprayed herbicide volume have been realized. In maize a reduction of 50-70% was demonstrated in 20 villages in Shandong and Hebei.

Role of the extension officer

Extension officers have played an essential role in the successful execution of the demonstration experiments in Shandong, Hebei, Heilongjian and Beijing. They have the necessary modern knowledge in weed science and weed control to understand the MLHD method and to apply this method in the practice. It is therefore their role to inform and demonstrate this new method of weed control to the farmers, and to guide them in implementing this method. ICAMA and the provincial ICA’s will help and advise. To this end, please register yourself as MLHD-supporter. We will keep you informed.