**2. Materials and methods**

*Invasive Species - Introduction Pathways, Economic Impact, and Possible Management Options*

In this way, some of the weed species can increase their establishment and dominance in indigenous agro-ecosystems along with invading adjacent as well as far flung terrestrial ecosystems. Thus contrary to notion that weeds are menace and agro-ecological systems must be kept free of indigenous and invasive weeds, their utilization as forage for dairy animals has the potential to become the most feasible

Globally, large ruminant's performance is directly influenced by the nutritional value of feed which accounts for over 50% of total expenditures. It deserves mentioning that dairy animals confront forage shortage owing to temperature extremes leading to drastic fall in milk production especially in developing countries. The shortage of forage and rising population of dairy animals has necessitated identifying and evaluating alternate feed resources which are cheap and can also fulfill animal's dietary needs. It has been established that weeds can inflict drastic influence on crops productivity and use of chemical herbicides for keeping them below threshold level, has led to serious concerns pertaining to their residual persistence in crops, soil and environment. Weeds utilization as animal feed holds potential because these are cheap owing to their abundance on field paths and water channels. Weeds harvested from cropped and non-cropped area may constitute an effective and biologically viable approach to keep weeds below the threshold level. Weeds utilization for feeding animals can also reduce herbicides use in agricultural lands which has the potential to curb environmental pollution. Many weeds have been reported to be resistant and better adapt to dynamic environmental conditions, and thus making them less prone to drastic impacts of climate change. In addition, it was reported that animals preferred naturally grown mixtures of weeds over crop residues and roughage during dry season. Furthermore, rapid regeneration favors many weed species for their inclusion as a source of vegetable protein in animal's diet [4–10]. Weeds such as Bermuda grass (*Cynodon dactylon*), Johnson grass (*Sorghum halepense*), canary grass (*Phalaris minor*), nut sedge (*Cyperus rotundus*), cheat-grass or drooping brome (*Bromus tectorum*), burr clover (*Medicago polymorpha*) and pigweed (*Amaranthus viridis*) contained organic matter over 90% indicating that these weeds can fulfill the dry matter requirement of animals. In addition, spotted knapweed (*Centaurea stoebe ssp. micranthos*), a weed of rangelands in Northern America was reported to displace local plant species, degraded wildlife habitats, altered biogeochemistry of soil and triggered soil erosion and thus its control through grazing was found to be biologically and economically viable. Similarly, broom snakeweed and medusa-head (*Taeniatherum caput-medusae*) were effectively controlled through controlled grazing with reasonably good palatability [11–17]. Along with substantial quantity, nutritional quality of weeds is of the utmost importance for dairy animals in order to produce milk on sustainable basis. Field bindweed (*Convolvulus arvensis*) and yellow duck (*Rumex crispus*) were reported to have significantly higher protein content (27 and 22% respectively) [11] which were greater than all cereal forages and most of the legumes, while button weed (*Diodia scandens*) contained 7.7% protein [18]. Although, a number of species belonging to *Commelinaceae* family such as climbing dayflower (*Commelina diffusa* L.), tropical spiderwort/wandering jew (*Commelina benghalensis* L.), Asiatic dayflower (*Commelina communis* L.), African dayflower (*Commelina africana* L.), white mouth dayflower/slender dayflower (*Commelina erecta*) are considered weed but constituted a major chunk of animal feed in Tanzania [19], rural regions of Mauritius [20],

USA [21] and in Kenya owing to reasonably good palatability [14, 18, 22, 23].

It is pertinent to mention that anti-nutritional factors (saponins, tannins, oxalates, etc.) of weeds constitute as the most crucial concern as far as animal nutrition is concerned. Tick weed (*Cleomea viscose*) recorded safer limits of anti-nutritional factors such as condensed tannins (0.0491%), saponins (0.23%), phytates (1.2%)

and pro-environment strategy [1–3].

**114**

In order to synthesize published findings pertaining to nutritional quality of weeds, search was performed on Google Scholar (http://scholar.google.com) and PubMed (http://www.PubMed.gov) using the below mentioned search strings:

1.Weeds and forages.

2.Weeds nutritional value.

3.Weeds, animal feed.

4.Anti-nutritional factors in weeds.

The search was time-restricted to 2000–2019, however owing to limitation of published findings; it was later on relaxed to 1990–2019.

The research studies were screened based on following criteria;


The screening process resulted in 55 studies which fully fit in the objectives and selection criteria.

### **3. Weeds under changing climate**

The rising temperature and carbon dioxide level along with the rapidly altering dynamics of rainfall and evaporation are the most important factors for determining management and utilization of weeds under changing climate. Weeds have been reported to have a greater genetic diversity compared to crops and thus can respond positively to agro-environmental changes. Owing to CO2 enrichment of atmosphere and rising temperature, some of the weed species can invade new

geographical localities while making the existing weed management strategies ineffective. In addition, weeds can have superiority over crop plants by virtue of better plant architecture and incorporating nitrogen and carbon in seeds. Rag weed (*Ambrosia artemissifolia*) developed more number of branches and leaf area along with producing greater number of pollens under increased temperature. Similarly, comparatively higher production of spines by Canada thistle (*Cirsium arvense*) in response to elevated CO2 level was reported [10, 24–27].

In addition, biomass production of bitter vine/American rope (*mikania mikrantha*), creeping oxeye (*Wedelia trilobata*) and Cairo morning glory (*Ipomea cairica*) was enhanced with increasing CO2 level [28]. Spurred anoda (*Anoda cristata*) gave the highest green biomass at CO2 fertilization up to 700 ppm and 32°C temperature, while barnyard grass (*Echinochloa crusgalli*) and Indian goose-grass/wire grass/ crowfoot grass (*Eleusine indica*) remained non-responsive to elevated temperature and CO2 concentration. It was concluded that elevated CO2 effectively enhanced the photosynthetic process even under water limited conditions indicating higher water use efficiency of weeds under drought stress which led to higher to biomass production. However, weeds response to elevated CO2 and temperature under well watered conditions continues to remain an unexplored aspect which demands further research to determine the physiological plasticity of different weed species [25, 26].

The temperature elevation as a result of global warming is feared to trigger weeds migration. Cogon grass *(Imperata cylindrica*) prickly acacia (*Acacia nilotica*) and witch weed (*Striga asiatica*) were reported to invade cooler areas of Europe owing to global warming, while some of the invasive weed species such as mesquite (*Prosopis juliflora*) can become more hardy and difficult to control owing to greater portioning of assimilates to roots under elevated temperature particularly under agro-ecological conditions of Indo-Pak subcontinent [27, 29–32].
