Threshold of weed harmfulness

Dependence of yield on weed infestation of crops

Quantitative assessment of the dependence of yield on weed infestation of crops allows one to:

• build a strategy for weed control;
• forecast the dynamics of the dependence of yield on the infestation level;
• determine a control strategy for future periods.

The scientific substantiation of the dependence of crop yield on weed abundance was first established in his works by I. N. Shevelev and was developed in a number of works by other scientists who proposed various mathematical approaches to determining the “weeds — yield” relationship. A. M. Tulikov summarized the accumulated experimental data and determined an exponential relationship between yield and weed infestation of crops at a confidence probability of 95%:

where

U

— yield of the main crop in a weed-infested plot, t/ha, %;

a

— yield loss at maximum weed infestation of crops;

e

— base of natural logarithms (e = 2.7183);

b

— intensity of crop yield reduction due to weeds;

x

— weed abundance, plants/m², %;

c

— yield at maximum weed infestation of crops.

The coefficients a, b, and c are determined for each specific crop. The yield of the main crop U can be expressed in t/ha or in %, in which case weed numbers are taken equal to the average number per 1 m² or in percent, with 1000 weeds per 1 m² taken as 100%. In the latter case, 100% yield will equal the maximum yield in t/ha in crops completely free of weeds.

The simplified exponential relationship is presented in the table.

For example, the use of herbicides in potato crops reduced weed infestation from 200 to 25 weeds per 1 m², then the yield increase will be 32.1% (89.1 — 57.0 = 32.1%). At a yield in the treated potato plot of 22.0 t/ha (89.1%), the actual increase will be 7.9 t/ha (32.1%).

The relationship between yield and weed infestation of crops allows solving a number of issues:

• tactical issues, for example, determining the possible yield reduction if infestation is reduced by only 50%, establishing the amount of yield increase with complete weed eradication, etc.;
• strategic issues, for example, determining the crop that will suffer the least yield losses at a specific infestation level of a given field; determining crop resistance to weeds, etc.

A distinction is made between general and specific harmfulness of weeds.

Specific harmfulness of weeds — the amount of crop yield loss per unit of weed abundance (per 1 g, per 1 plant, etc.).

Analysis of the dependence of yield on weed infestation of crops shows that the higher the crop infestation, the lower the specific harmfulness of weeds. That is, with the same number of destroyed weeds, the yield increase on a heavily infested field is significantly less than on a lightly infested one.

Thresholds of weed harmfulness

Phytocenotic threshold of harmfulness (PTH) of weeds — the level of crop infestation at which the damage caused to cultivated plants is minimal, and the economic costs of weed control exceed the economic benefit obtained from the yield increase.

Critical (statistical) threshold of harmfulness (CTH) — the level of crop infestation at which the economic costs of weed control are approximately equal to the benefit obtained from the yield increase. Yield losses at such a harmfulness threshold do not exceed the statistical error of 3–6% of the actual yield, although they may sometimes be felt by the enterprise.

Economic threshold of harmfulness (ETH) — the level of infestation at which the economic costs of weed control are less than the benefit obtained from the yield increase resulting from complete destruction of weeds. Thus, the funds spent on control measures are recouped by the additional yield increase obtained from a weed-free field. The yield increase is more than 5–7% of the actual yield. At low field yield or low product value, the economic threshold of weed harmfulness is determined by a yield increase of more than 8–12%. Conversely, for some industrial crops, such as fiber flax and sugar beet, it decreases to 2–4%.

V. A. Zakharenko proposed introducing the concept of the threshold of economic feasibility of control.

Threshold of economic feasibility of control (TEFC) of weeds — the level of infestation at which the profitability of control measures is ensured at not less than 25–40%. That is, the economic benefit obtained from complete destruction of weeds exceeds the economic costs of their destruction by 25–40%. The need to introduce this threshold is justified by the fact that the goal of any enterprise is profit. An agricultural enterprise can be considered economically efficient if the profitability of production is not less than 25–40%. It should be noted that the technical efficiency of weed destruction measures, as a rule, does not exceed 70–90%, therefore the actual infestation level at the threshold of economic feasibility of control should be 1.1–1.4 times higher than the theoretical one in order to ensure the established profitability level.

The diversity of cultivated crops and weed species composition, differences in soil-climatic and production-economic characteristics of farms, as well as dissimilarity of other farming conditions, indicate the diversity, dynamism, and zonality of economic thresholds of weed harmfulness in agricultural crop stands. Considering that the harmfulness of weeds depends on a number of factors: crop (for example, in row crops harmfulness is higher than in cereals and grasses), weed species composition, soil-climatic conditions, agricultural technology, geographic location, etc., the economic threshold of harmfulness is calculated for:

• each crop;
• each field (plot, territory);
• each herbicide, etc.

Herbicritical period

The herbicritical (critical) period of a crop — growth phases or periods of vegetation of a cultivated plant during which it exhibits maximum sensitivity to the impact of weeds. The herbicritical period of crops allows determining the most effective timing of control measures and minimizing yield losses. The absence of weeds at the beginning of the herbicritical period and further maintenance of crop cleanliness ensures maximum yield and product quality under specific conditions with minimal economic costs for weed control.

In practice, it is not always possible to carry out control measures at optimal times due to agronomic conditions. For example, treatment of cereal crops with herbicides of the phenoxyacetic acid group is carried out in the period from the beginning of the full tillering phase to stem elongation, when the crop has already been in the herbicritical period for one to two weeks. Similar examples occur in stands of some row crops (potato, maize, etc.), when control measures are postponed to the period of inter-row cultivation, ignoring methods of weed destruction in the pre- and post-emergence periods. Therefore, one should proceed from the general principle: after a crop enters the herbicritical period, weed control measures give the smaller economic effect the later they are implemented.

For most cultivated plants, the beginning of the herbicritical period falls on the initial growth phases. The period during which crop seedlings are practically insensitive to weeds is determined by agricultural technology and the biological characteristics of the cultivated plants. A number of scientists have shown that the herbicritical period in bean occurs after 1 week, in oats — 1–1.5 weeks, sunflower — 2 weeks, soybean — 2–3 weeks, sorghum — 3 weeks, and sugar beet after 3–4 weeks.

References

• Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. (2000). Agriculture. Textbook for universities. Moscow: Kolos Publishing House.