**9. Conclusions**

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

had the maximum oxalate concentration (2.92).

**7. Nutritional comparison of weeds and forage crops**

**8. Limitations and implications**

digestibility leading to lower palatability of weeds. Tick weed and nutsedge recorded similar saponins (0.22%), while button weed contained higher saponins (0.35%). Phytates produce phytic acid which acts as a chelator of various macro-minerals (calcium and magnesium) and trace mineral (iron and zinc) leading to a severe deficiency of these minerals. Button weed with 1.18% phytates remained superior to nut-sedge and asthma plants. Oxalate is another important anti-nutritional factor which binds with calcium to form calcium oxalate leading to calcium unavailability. Asthma plant recorded the lowest oxalate concentration (2.36%) while button weed

It was suggested that weeds such as Arizona sunflower (*Tithonia tubiformis*), wood sorrel (*Oxalis divergens*) and bush sunflower (*Simsia amplexicaulis*) commonly found in America, Mexico, Argentina and Chile contained tannins in safer limits and might be utilized to feed dairy animals. In contrast, a fatty acid called malvalic acid was isolated from cheese weed (*Malva parviflora* L.) which caused deaths of dairy animals. Similarly, different phenolic compounds were reported to be the major reason behind low palatability of many weed species. In addition, the presence of phytochemicals and free oxygen metabolites in weeds contributed to mastitis and ultimately led to udder edema along with deteriorating the reproductive performance of cattle. Similarly, spotted knapweed (*Centaurea stoebe ssp. micranthos*) in North American rangelands contained an allelo-chemical named cnicin (a sesquiterpene lactone compound) which reduced its palatability by imparting bitter taste and deterred grazing. In contrast, knapweed was readily consumed by small ruminants' preferably at rosette and bolting stages compared to flowering and seed-set phonological stages. This preference was associated with higher protein and lower fiber content at rosette stage in comparison to flowering or seed-set stages without any link between cnicin content and knapweed palatability [28, 49–51].

To the best of our knowledge, no comprehensive studies have been reported pertaining to qualitative analyses of different native and exotic weeds with forage crops. Some of the weeds such as Canada thistle, spotted knapweed, white-top, Russian knapweed and pigweeds contain protein in the range of 15–22% while typical grasses has only 2–11% protein, thus have the potential to become cost-free source of plant protein. In addition, higher leaf-stem ratio in weeds impart them superiority over grasses in terms of higher digestibility. Moreover, weeds provide nutrients rumen microbes which enable dairy animals to digest lower quality forages and thereby reducing overall feed cost. Weeds are always available even during periods of drought in arid areas and their utilization can help to obtain sustainable supplies of milk throughout the years with minimum cost. Dairy animals being fed on protein rich weeds tend to gain weight more rapidly and that too with no additional cost.

The studies regarding nutritional quality, presence of anti-nutritional or toxic substances and palatability of most of the native weed species is lacking. However, one of the most important limitations in utilizing tropical weeds for dairy animals is the presence of anti-nutritional factors such as tannins which are harmful and toxic to ruminants [13, 38, 51]. Animals being fed on weeds having high concentration of tannins witnessed digestibility depression. Weeds having exceptionally higher lignin content caused a sharp decline in feed intake leading to serious malnutrition in dairy

**120**

The exceptional resistance to drought, higher biomass production under unfavorable pedo-climatic conditions, rapid regeneration capacity, and acceptable nutritional quality at all phonological stages suggests that there are opportunities to utilize weeds as forage for all types of ruminants. Weeds availability throughout the year warrants their potential to fulfill essential dietary needs of animals and favors their inclusion as supplementary forage especially during extreme weather conditions. However, controlled field investigations for determining the appropriate growth timings, nutritional quality, anti-nutritional factors and biomass production potential of different native weeds must be done in rangelands while maintaining a balanced and healthy rangeland ecosystem.
