Pak. J. Weed Sci. Res. 10(3-4):175-184, 2004

Poaceae: The Role Of Grass Weeds In Cereal

Agro-Ecosystems Of Pakistan

Shahida Khalid^[1]

Abstract

Weeds of Poaceae are important competitors in crops.  55 grass weeds of 40 genera are reported from various crops in Pakistan. The success of control measures depends on the proper identification of grass weeds. Understanding the classification of grasses is important for chemical control. Panicoideae is the most important of Poaceae as far as grassy weeds are concerned. 47% of the total grass weeds belong to this subfamily. Several members of subfamily Panicoideae are able to partially detoxify atrazine especially the tribe Paniceae, in which 31% of the noxious grass weeds are present. In Andropogoneae all the genera except Rottboelia are perennials. Chloridoideae represents 20% of the total grass weeds with phenotypic plasticity responsible for their widespread distribution. Eragrostideae is an important tribe of Chloridoideae represented by 8 weedy genera.  Pooideae has 29% of the grass weeds, although Aveneae is the most abundant tribe, the importance of the other tribes must be seriously considered. The Pooideae is increasing in wheat where a weed alert is necessary on most of its members especially the newly emerging Bromus and Lolium.  Seed heterogeneity, subsequently seed germination and noxiousness are directly related to the flowering period in grass weeds. 

Key words:    Poaceae Andropogoneae Panicoideae Chloridoideae flowering.

Introduction

The grasses are the most widely distributed of all flowering plants and form prominent future of the flora of every continent (Bor, 1968). The grasses comprise some 9,000 species grouped into about 650 genera. Although not the largest, the family is ecologically the most dominant and economically the most important in the world (Heywood, 1993).

Many grassy species are among the most destructive weeds. Their slender and apparently delicate form belie their ability to compete with most other plants. They occur in every major agricultural area in the warmer regions of the world. As rotation becomes shorter in traditional tropical farming systems due to population pressure, the nutrients become depleted in the soil and as chemical control is beyond the means of the producers, grassy weeds become an impediment to sustainability (Dekker, 1991).

The grassy weeds are highly conspicuous and often significant components of the flora of virtually every cropping system in the world.  They are found in nearly every crop during the respective seasons, rabi and kharif.

Adaptability is an important character shown by grasses enabling them to thrive in difficult and demanding habitats. However some species have special ecological requirements, such as need of high moisture or good fertility, low tolerance of poorly drained soils, which limit their extension.  Grasses like Imperata cylindrica are associated with favorable moisture.  Many Chloridoideae are well adapted to sandy soils and are drought tolerant.  Grasses give a definite response to fertilizers.  Eragrostis and Digitaria do well in unfertilized soils Brachiaria and Eleusine indica are known to respond to high levels of fertilizer.

Although, they are simple in their structure, grasses are able to compete successfully with most types of plants. Perennial grasses, once established, may grow into stout tufts or tussocks or form large spreading plants with rhizomes or stolons. Producing new plants at intervals along their length. Annual grasses, which generally grow into simple tufts, rely on their high seed production, not only for survival but also as a means of enduring seasonally adverse conditions.

Grasses have the unique features of an intercalary meristem and a leaf meristem which, combined with the growth from the base of the plant and the leaf-sheath, enables them to survive burning, trampling, grazing and mowing.

Photoperiod sensitivity

Many grasses exhibit a range in photoperiod sensitivity, subsequently affecting the growth pattern. In any one population of Avena fatua there is a range in photoperiod sensitivity so seed will be produced throughout the growing season. The changing photoperiod experienced by the plant through the season may influence the plant habit and seed production, but in majority of cases, the duration of primary seed dormancy.

Phenotypic plasticity

The high phenotypic plasticity of many Chloridoideae, such as Cynodon dactylon, Eleusine indica, Dactyloctenium aegyptium or Eragrostis ciliaris, has allowed them to colonize most annual crops of tropical regions under a wide range of edapho-climatic conditions, they are now considered as cosmopolitan weeds (Maillet, 1991).

Dormancy and grass seeds

There are many ways in which weeds exploit dormancy to ensure their survival. Successful management of weeds often requires knowledge of this attribute, even if there is an incomplete understanding of dormancy mechanisms (Terry, 1991).  Seed production, soil seed bank and longevity of the seed in the soil all determine the noxious characters of the weed.  Re-infestation continues by high seed production, which continuously adds to the soil seed bank.  The seed bank is the function of the longevity of seeds in soil and is determined by the inherent viability of seed and dormancy mechanisms.  It is encouraging to note that grass seeds are not noted for their longevity. Even the most persistent of grass seeds are not viable beyond 6-9 years. This gives scope for control, but the important point remains the control of seed production for a few years (Terry, 1991). Grassy weeds may have either one flush of germination or successional germination throughout the growing season controlled by dormancy and seed heterogeneity.

Intensive agriculture and grass weeds

Grass weeds seem to be much more harmful under intensive agriculture. Fertilizer and herbicides may induce shifts in weedy communities in favor of some troublesome Poaceae (Maillet, 1991).

Mechanism of herbicide detoxification

Weeds and crops may have similar physiological properties, such as mechanisms of herbicide detoxification. A large number of Panicoideae are able to partially detoxify atrazine by non-enzymatic hydrolysis to hydroxy-atrazine and by enzymatic conjugation of atrazine to glutathione (Bhowid & Nandihalli, 1988).  On many large-scale farms and several experimental stations increased abundance of Digitaria, Panicum, Brachiaria, Setaria and Rottboelia species in maize and sorghum fields is induced by continuous use of triazine herbicides.

C₃ and C₄ Grasses

Many grasses possess an additional chemical pathway known as the Hatch/Slack pathway. In grasses the basic leaf-blade anatomy can be used to find out if a grass has C₃ or C₄ photosynthetic patterns of leaf anatomy. The two basic types of leaf-blade anatomy are the Kranz and non-Kranz patterns. The Hatch-Slack pathway operates in the radially arranged cells of the chlorenchyma and releases CO₂ into the outer bundle sheath, where it is absorbed by the Calvin-Bensen cycle. This anatomical structure (Kranz anatomy) shows a special significance when viewed from the physiological and ecological points of view that the grasses with Kranz-type leaf-blade anatomy follow the C₄ photosynthetic pathway and the grasses with the non-Kranz type of leaf-blade anatomy follow the C₃ photosynthetic pathway (Chaudhary, 1989). In addition to the regulation of Co₂ concentrating mechanism in Kranz cells of the C₄ subtypes, a rapid translocation of photosynthate from bundle sheath cells to the phloem in grass with C_4, Mestome Sheath (MS) type (NADP dependent malic enzyme) may contribute to maintain their high performance in photosynthesis and water use efficiency through shorter interveinal distances (Kawamitus, et al. 2002). According to Duvall, et al (2001), Paniceae demonstrates unique variability of photosynthetic physiology and anatomy including both non-Kranz and Kranz species and all subtypes of the latter. Reasons for the success of grass are due to vegetative propagation, high seed production, perennating habit, protected vegetative shoot meristum, ability to produce new tillers and variations in their root system. Certain fibrous-rooted and creeping grasses are valuable for soil retention and land reclamation. Similarly, marsh grasses such as reeds torpedo grass prevent the muddy soil on the edge of marshes and canals from being washed away (Bor, 1968).

Materials and Methods

Grass weeds occurring in different grain fields collected over the last 10 years have been included in this study.  The plants have been grouped into sub-families, tribes and genera. Seed production period of the important weeds determined from field studies and floras. The seed production period of the grass weed was determined on this basis. The grass weeds are tabulated according to the sub-families. The common names and habitat in which they occur is also provided. This will be useful in determining the diverse role of Poaceous weeds in agro-ecosystems of Pakistan.  

Results And Discussion

Diversity in grass weeds

In this study 40 genera of grasses representing more than 55 species are reported as weeds in Pakistan (Table-1). The most important, at present, are Avena fatua and Phalaris minor in wheat. The cropping systems particularly the rice-wheat and maize-wheat are suitable for the proliferation of these weeds. Next come Echinochloa, Imperata, Brachiaria, Urochloa and Cenchrus. The biological and ecological diversity of grass weeds allows them to appear in most agro-ecosystems. Most of the grass weeds belong to the subfamilies Chloridoideae, Panicoideae and Pooideae (Table-2). Out of the 7 tribes of Chloridoideae present in the flora of Pakistan (Cope, 1981) 4 have weedy genera with Eragrostideae as the most important tribe (Table-3).

Panicoideae has 4 tribes of which 2 have most of the grass weeds present in summer crops. Paniceae has the largest number of weeds mostly occurring in kharif with Panicum and Cenchrus as rabi weeds and two important weeds having two flowering seasons, Paspalum paspaloides and Cenchrus biflorus (Table-3). Broadly, Paniceae were found to be paraphyletic with Andropogoneae, Panicum as found to be polyphyletic (Duvall, et al 2001). All the genera of Andropogoneae are represented by at least one perennial species except Rottboelia that is an annual and Sorghum with all perennial species.

The Panicoideae is represented by 17 weedy genera (Table-3). This is 47% of the weeds of Poaceae in Pakistan with weeds found in several kharif crops. The Chloridoideae represented by 11 genera with Cynodon, Dactyloctenium, Eleusine and Desmostachya as some of the important weeds again mostly in kharif, however Eleusine also found in wheat.

The Pooideae have 11 weedy genera belonging to 5 tribes infesting wheat and other Rabi crops Avena fatua and Phalaris minor are on top of this list with Lolium and Bromus needing a weed alert.  All the genera have annual plants except Brachypodium and Poa.  Poa annua is a tufted annual or short-lived perennial according to the ecological conditions. 

Correlating with taxonomical groups, the sub-families Arundinoideae, Bambusoideae and Pooideae have the non-Kranz (C₃) types of leaf anatomy, while the sub-family Panicoideae possesses genera with Kranz (C₄) and non-Kranz types of leaf anatomy (Clayton and Renvoize, 1986). Most of the C₄ plants are aggressive competitors in the crop environment during summer.

Seed production is the most important characteristic of the grass weeds (Table-4).  With a mixed grass flora no season is without grass weeds and seed influx to the soil bank.  It is evident from the flowering and fruiting period of the weeds, determined in field studies and reported by Cope, 1982, that 6.0% of the weeds produce seeds throughout the year, 29.0% for 5-6 months, 15% from 7-10 months and about 4.0% have two seasons of seed production.  While the remaining 46.0% mature in less than five months.  This is perfectly suited to the crop production system, where most of the weeds mature before the crop is harvested.

Grassy weeds in the future

Weeds are dynamic, changing with time, practices and introduction to new fields. It is important to control the spread to new areas and prevent build up and increase of already existing species. This can be prevented by restricting influx and movement of weed seeds from one area to the other in crop seed. Any changing cropping pattern will have a changed weed flora with the subsequent development of difficult to manage weeds. Trained manpower is the only solution to meet the challenges of the future. Grasses and sedges will become more important as they are similar and not distinguished from grass crop at the early stages. Herbicide resistance is likely to develop in the near future. Detailed knowledge of the application methods for specific problems will be required.

There is no one attribute physiological or otherwise that is associated with weediness.  It is more likely to be a combination of attributes which in the case of weedy tropical grasses include the highly plastic responses to environmental factors in relation to seed dormancy and flowering in particular, together with the existence of intraspecific variations, plasticity in breeding systems and the possession of certain morphological attributes, such as rhizome development (Groves, 1991).

Among annual Echinochloa colona, Eleusine indica, Dactyloctenium aegyptium are likely to increase while Imperata cylindrica and Cenchrus ciliaris are important perennial weeds of the future. Land preparation at different times of the year influences weed emergence, degree of infestation, and the dominance of a weed species in a community.  Deep ploughing can reduce the viability in some grasses while in others it will be helpful in maintaining viability.

Therefore in order to develop a control strategy for grasses there is a need for studies related to weed biology, the basic of course being the grass weed identification. 

Table-1.           Weeds of Family Poaceae /Gramineae recorded in Pakistan

(Concluded…..)

Habitat: Wc water channel; Fb field borders; S sugarcane; Vs/Vw summer/winter vegetables; R rice; L lawns; W wheat; O orchards; Oc other cereals; Co cotton; Ch chickpea; B rape seed; P potato; So soybean; Sf sunflower; G groundnut.

Table-2. Tribes of Family Poaceae with weedy members recorded in Pakistan

* Tribes with weedy members

Table-3.           Weedy genera present in the various tribes of Poaceae

*           Genera with some perennial species

**          All species perennial

***        Annual/Perennial genera with the same species acting as A or P according to the ecological conditions

Others all members annual.

Table-4.           Seed Production Period of important Grass Weeds

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