1. Form selectivity: The shape, erection degree, surface structure, location of growth points of the plant leaves directly affect the adhesion and absorption of the liquid medicine, thereby affecting the degree of damage to the plant by the medicine. For example, the leaves of monocots are erect and narrow, the surface cuticle is thick, the surface area is small, the leaves and stems are erect, the liquid medicine is easy to roll off, the apical buds are surrounded and protected by heavy leaf sheaths, and the contact herbicide is not easy to damage the meristem And so on, make it strong in resistance. Dicotyledonous plants have flat and stretched leaves with a large area. The cuticle on the leaf surface is thin. The liquid medicine is easy to deposit on the leaves. The buds are bare and there is no leaf protection. Contact poisons can directly damage meristems and are sensitive to medicines. For example, Bolan can kill most broad-leaved plants and is relatively safe for monocotyledonous plants. For example, burning grass is safe for ungerminated woody plants and is effective for most young weeds.

2. Physiological selectivity: the selectivity produced by the differences in the absorption and conduction of herbicides by plant stems, leaves or roots. A herbicide that is easily absorbed and transmitted by plants, and plants are sensitive to it. Whereas plants cannot absorb and conduct herbicides, plants appear to be safe. For example, grass lawns can use "Geer" to eliminate broadleaf weeds, and broadleaf seedlings can use "Junhedi" to eliminate grass weeds.

3. Biochemical selectivity: the biochemical reactions of herbicides in different plants are different, or activation increases the herbicidal activity of the herbicide, or degradation makes the herbicide inactive. For example, "Haorui" is safe to bluegrass, tall fescue, and ryegrass turfgrass of the gramineous family, but not safe to goosegrass, crabgrass, etc. of the gramineous family. "Qiaosi" is safe to grasses such as tall fescue, ryegrass and Poa turfgrass, but is not safe to grasses such as setaria, goosegrass, crabgrass, etc.

4. Man-made selectivity: According to the nature and biological activity of weeds, soil conditions, and the differences in biological characteristics of crops and weeds, select appropriate agents and methods of use. ①Position difference selectivity: use the position and distribution differences of crops and weeds to avoid weeds by removing weeds from crops. For example, use "One Change Static + Burning Grass" in the seedlings to weed the low weeds by directional spray, and the safety of the seedlings can be ensured by not spraying the seedlings. ② Time difference selectivity: use the difference between the germination and emergence time of weeds and crops, or the difference between application and sowing time to form selectivity. For example, the dormant period of lawn in winter can be closed by weeding with "burning grass + Yichangjing + Yanhua". ③The selectivity of the difference in plant resistance: different plants or different parts of the same plant have different levels of resistance, which results in selectivity. For example, "Yibianjing" is not safe for the buds and leaves of some seedlings but is safe for old leaves and stems. "Geer" can kill bentgrass under high temperature and high dosage, but it is safe for Poa pratensis. ④Using protective substances or safeners to improve the selectivity of herbicide safety: Some herbicides are not highly selective and prone to phytotoxicity. The selectivity can be improved by using protective substances.

The selectivity of herbicides is not absolute, but relative. Selective herbicides do not have no effect on the protected plants at all, but selectivity under a certain target, dosage, time, method and environmental conditions.