Lecture 7 Factor affecting crop-weed competition

 

Factor Affecting Crop-Weed Competition

1. Period of weed growth

ü  Weeds interfere with crops at any time they are present in the crop. Thus, weeds that germinate along with crops are more competitive.

ü  Sugarcane takes about one month to complete its germination phase while weeds require very less time to complete its germination. By that time crop plants are usually smothered by the weeds completely.

ü  First 1/4 - 1/3 of the growing period of many crops is critical period.

ü  In direct sown rice more severe weed competition than transplanted rice. However, in a situation, where weeds germinate late, as in dry land wheat and sorghum, the late-stage weeding is more useful than their early weeding.

ü  In general, for most of the annual crops first 20-30 days weed free period is very important.

2.  Weeds / Crop density

ü  Increasing weed density decreasing the crop yields.

ü  In rice density of Joint vetch (Aschynomene virginica) and barnyard grass, if it is >10 plants/m2 rice yields were reduced by 20 and 11 q/ha respectively.

3. Crop density also effect the weed biomass production

ü  Increase in plant population decreases weed growth and reduce competition until they are self-competitive for soil moisture and other nutrients.

ü  In wheat reduced row spacing from 20 to 15 cm reduced the dry matter yield of Lolium and Phalaris spp by 11.8% and 18.3% respectively.

4. Plant species effects Weed species

ü  Weeds differ in their ability to compete with crops at similar density levels. This is because of differences in their growth habits and to some extent due to allelopathic effects.

ü  At early stage of growth, cocklebur (Xanthium stramonium) and wild mustard (Brassica spp.) are better competitor for crops than many grasses because of their fast-growing leaves that shade the ground heavily.

ü  In dry areas perennial weeds like Canada thistle (Cirsium arvense) and field bind weed (Convolvulus arvensis) were more competitive than annual weeds because of their deep roots and early heavy shoot growth.

5. Soil and Climate influence

A. Soil fertility

ü  Under limited nutrient conditions, competition exists between the crop and the weed. Soil type, soil fertility, soil moisture and soil reaction influence the crop weed competition.

ü  Elevated soil fertility usually stimulates weeds more than the crop, reducing thus crop yields.

ü  Method and time of application of fertilizers to crop determining whether added fertilizer will suppress or invigorate weed growth in fields.

ü  Application of fertilizers during early crop growth season when weed growth is negligible was more beneficial.

ü  Band application of fertilizers to the crop will be inaccessible to inter row weeds.

B. Soil moisture status

ü  Weeds differ in their response to available moisture in soil. Russian thistle Salsola kali showed similar growth in both dry soils and wet soils; where as large crab grass Digitaria sanguinalis produce more growth on wet soil.

ü  When fields are irrigated immediately after planting then weeds attain more competitive advantage over crops. If the weeds were already present at the time of irrigation, they would grow so luxuriantly as to completely over cover the crops.

ü  If the crop in irrigated after it has grown 15 cm or more in a weed free environment irrigation could hasten closing in of crop rows, thus suppressing weeds.

C. Soil reaction

ü  Abnormal soil reactions (very high or very low pH) often aggravate weed competition.

ü  Weeds offer intense competition to crops on abnormal pH soils than on normal pH soils.

ü  In acid soils Rumex acetosella and pteridium spp, saline alkaline soils Taraxacum stricta, Agropyron repens are the dominant weeds.

D. Climatic influences

ü  Adverse weather conditions per se drought, floods and extreme of temperature intensify weed-crop interference since most of our crop varieties are highly susceptible to such climatic influences whereas the weeds are tolerant to their stresses.

6. Cropping practices

A. Time of planting crops

ü  If the time of planting of a crop is such that its germination coincides with the emergence of first flush of weeds, it leads to intense weed-crop interference.

ü  Usually longer the interval between emergence of crop and weeds, lesser will be the weed crop interference.

B. Method of planting of crops may also affect the weed-crop competition

ü  Weed seeds germinate most readily from top 1.25 cm of soil, though it is considered up to 2.5 cm depth.

ü  Avena, barnyard grass, Xanthium and Vicia spp may germinate even from 15 cm depth.

ü  Therefore, planting method that dries up the top 3-5 cm of soil rapidly to deny weed seeds opportunity to absorb moisture for their germination and usually post pone weed emergence until first irrigation.

ü  By that time crop establishes well and competes with weeds.

C. Critical period of crop-weed competition and Yield reduction in some crops

Crop	Critical Period of crop weed competition	Reference
ü  Rice	ü  15-45 DAS	ü  Reddy and Reddy
ü  Wheat	ü  30-50 DAS	ü  Choudhary (2008)
ü  Maize	ü  15-35 DAS	ü  Reddy and Reddy
ü  Sorghum	ü  15-45 DAS	ü  Bharti (2009)
ü  Soybean	ü  14-45 DAS	ü  Surianto et al. (2017)
ü  Groundnut	ü  15-35 DAS	ü  Reddy and Reddy
ü  Sugarcane	ü  2-120 DAS	ü  Akanksha (2010)
ü  Pigeon pea	ü  30-60 DAS	ü  Ali M. (2017)
ü  Geen gram	ü  20-40 DAS	ü  Sheoran et al. (2008)
ü  Black gram	ü  30-45 DAS	ü  Vivek et al. (2008)
ü  Chickpea	ü  15-60 DAS	ü  Singh and Singh, (1992)
ü  Field Pea	ü  20-60 DAS	ü  Singh et al. (2016)
ü  Lentil	ü  20-60 DAS	ü  Jamin et al. (2012)

D. Crop density and rectangularity

ü  It determines the quantity and quality of crop environment available to the growth of weeds.

ü  Wide row spacing with simultaneous high intra row plant population may induce dense weed growth.

ü  But square method of planting is ideal to reduce intra row competition.