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Threshold of harmfulness of weeds

Threshold of harmfulness of weeds – the level of weed infestation of crops that determines the economic damage of weeds in crops.

Three thresholds of harmfulness are distinguished:

phytocenotic;
critical;
economic.

Feasibility of extermination measures is determined by the equality of economic costs of these measures and the economic benefit obtained from the increase in yield, after the destruction of weeds.

Threshold of harmfulness of weeds
Schematic representation of thresholds of harmfulness

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Dependence of yield on weediness of crops

Quantitative assessment of the dependence of yield on weed infestation allows:

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

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

U = ae-bx + c,

where U is the yield of the main crop on the infested area, t/ha, %, a is the yield loss at maximum crop infestation; e is the base of natural logarithms (e or exp = 2.7183); b is the rate of yield reduction of the crop from weeds; x is weed abundance, pcs/m2, %; c is the yield at maximum crop infestation.

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

A simplified exponential relationship is presented in the table.

Table. Yield predictions of major crops as a function of weed abundance (% of yield in the absence of weeds), Tulikov, 1987.

Crop
Number of weeds per 1 m2
5
10
25
50
100
200
500
Winter wheat
98.1
96.4
91.4
84.2
72.9
59.0
46.5
Spring wheat
98.2
96.6
91.7
84.3
72.4
56.1
37.5
Barley
98.5
96.9
92.6
86.5
76.8
65.1
54.6
Buckwheat
97.0
94.2
86.8
77.2
65.6
56.7
53.6
Rice
98.4
96.2
92.5
85.8
74.7
59.1
40.2
Flax fiber
99.1
98.2
95.7
91.5
84.0
71.3
47.2
Corn for silage
97.1
94.3
86.4
74.8
56.9
34.7
14.8
Potatoes
97.6
95.3
89.1
80.6
68.8
57.0
50.9
Sugar beet
97.0
94.1
86.0
74.2
55.9
33.8
14.2
Soybean
93.4
87.7
74.2
60.9
50.5
47.0
46.8
Annual grasses
98.0
96.0
90.3
81.7
67.4
47.4
23.1
Perennial grasses
97.0
94.3
87.9
80.9
74.6
71.8
71.4
Sunflower
97.4
94.9
88.2
78.6
64.9
50.3
40.8

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

The dependence of yield on crop infestation allows us to solve a number of questions:

  • tactical issues, such as determining the possible yield reduction if the weed infestation is reduced by only 50%, establishing the yield increase if weeds are completely eradicated, etc;
  • strategic, for example, to determine the crop that will incur the least yield losses at a particular level of weed infestation of a given field; to determine the resistance of crops to weeds, etc.

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

Specific weed harmfulness is the value of crop yield losses per unit of weed abundance (per 1 g, per 1 plant, etc.).

Analysis of the relationship between crop yield and weed infestation shows that the higher the weed infestation of a crop, the lower is the specific harmfulness of weeds. That is, with the same number of weeds eliminated, the yield increase in a heavily infested field is much less than in a weakly infested field.

Seed bank

Phytocoenotic threshold of harmfulness (PTH) – the level of weed infestation at which the harm caused to crops is minimal, and the economic costs of weed control exceed the economic benefit obtained from the increase in yield.

Critical (statistical) threshold of harmfulness (CTH) is a level of weed infestation at which economic costs of weed control are approximately equal to the benefit received from the yield increase. Yield losses at this threshold do not exceed the statistical error of 3-6% of the actual yield, although sometimes can be felt by the enterprise.

Economic threshold of harmfulness (ETH) is the level of weed infestation at which the economic cost of weed control is less than the benefit derived from the yield increase resulting from the complete elimination of weeds. Thus, the money spent on extermination measures pays for itself by the additional yield increase obtained from a field cleared of weeds. The increase in yield is more than 5-7% of the actual yield. With low field yields or low production values, the economic threshold of pest damage is determined by a yield increase of more than 8-12%. Conversely, for some industrial crops, such as fiber flax, sugar beet, the economic threshold of harmfulness is reduced to 2-4%.

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

The threshold of economic feasibility of weed control (TEFC) is the level of weed infestation, at which the profitability of extermination measures is not less than 25-40%. That is economic benefit received from complete elimination of weeds by 25-40% exceeds economic costs of their extermination. The necessity of introducing this threshold is justified by the fact that the purpose of functioning of any enterprise is to make profit. An agricultural enterprise can be considered economically efficient if the profitability of production is not less than 25-40%. It is worth noting that technical efficiency of weed control measures usually does not exceed 70-90%, so the actual level of weed control at the threshold of economic feasibility should be higher by 1.1-1.4 times the theoretical one in order to ensure the established level of profitability (table).

Table. Thresholds of weed harmfulness in field crops, pcs/m2 (Tulikov, 1987).

Crop
Value intervals PTH, %
Critical thresholds, CTH, %
Economic thresholds, ETH, %
smallest
largest
smallest
largest
Winter wheat
4-7
12
20
14
26
Spring wheat
4-7
12
21
15
27
Barley
4-7
13
26
16
32
Buckwheat
4-6
7
10
8
14
Rice
4-6
11
20
16
27
Flax fiber
2-3
11
17
17
23
Corn for silage
4-6
6
11
8
14
Potatoes
3-5
6
11
8
13
Sugar beet
3-5
5
9
7
11
Sunflower
4-6
7
12
10
16
Soybean
4-6
3
5
4
7
Annual grasses
7-10
17
27
23
32
Perennial grasses
7-10
12
20
17
25

The diversity of cultivated crops and species composition of weeds, differences in soil and climatic, production and economic features of farms, as well as dissimilarity of other farming conditions indicate the diversity, dynamics and zoning of economic thresholds of weed harmfulness in agricultural crops.

Given that the harmfulness of weeds depends on a number of factors of culture (eg, in row crops is higher than in cereals and grasses), species composition of weeds, soil and climatic conditions, agricultural practices, geographic location, etc.), the calculation of economic threshold of weeds is conducted for

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

Herbacritical periods of crops

Herbacritical (critical) period of a crop is the growth phase or vegetation period of a cultivated plant during which it shows maximum sensitivity to the impact of weeds.

Herbicritical period of crops allows you to determine the most effective timing of extermination measures and minimize yield losses. The absence of weeds at the beginning of the herbacritical period and further maintenance of crop cleanliness, provides the maximum yield and product quality in specific conditions with minimal economic costs for weed control.

In practice, it is not always possible to carry out extermination measures in optimal terms according to agrotechnical conditions. For example, the treatment of grain crops with herbicides of phenoxyacetic acids group is carried out from the beginning of the phase of complete tillering until the emergence of a tube, when the crop is already in the herbacritical period within one or two weeks. Similar examples are observed in crops of some row crops (potatoes, corn, etc.) when control measures are transferred to the period of inter-row tillage, ignoring the techniques of weed control in pre- and post-emergent periods. Therefore, it is necessary to proceed from the general principle – after a crop enters the herbacritical period weed control measures give the less economic effect, the later they began to be implemented.

For most cultivated plants the beginning of the herbacritical period falls on the initial phases of growth. The period in which crop sprouts are practically insensitive to weeds is determined by agricultural techniques and biological features of cultivated plants. A number of scientists showed that the herbicidal period for beans occurs after 1 week, for oats – 1-1.5 weeks, sunflower – 2 weeks, soybeans – 2-3 weeks, sorghum – 3 weeks, and sugar beet – 3-4 weeks.

Winter wheat crops are most susceptible in the fall during the first four weeks after sowing. The influence of weeds decreases by 2-4 times in the spring, although it causes a 7% decrease in yield.

Appearance of mass sprouts of weeds in grain crops in the second half of the growing season no longer leads to a significant impact on the yield. Extermination measures during this period mainly improve harvesting conditions and prevent replenishment of the weed seed bank in the soil.

On the contrary, the long-flax, sugar beet, potato and vegetable crops are more sensitive to weeds in the second half of the growing season and their emergence into the upper tier, which leads to a decrease in yield due to the worsening of plant life and increasing losses during harvesting.

The following results were obtained in the Non-Black Soil Zone to study the influence of the duration of weed vegetation in winter rye crops. The presence of weeds within 15 days of the beginning of vegetation resulted in reduction of grain yield by 6,2%; 30 days – by 7,3; 73 days – by 15,2; 94 days – by 23,9 and within 110 days – by 33,7%.

Sources

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 с.