In the Southeastern US, alfalfa is increasingly grown as a companion crop inbermuda grass pastures. It offers producers a significant profit potential through selling hay, or by grazing or conserving the surplus as haylage. Most of these growers were attracted to alfalfa because they have seen enormous improvement in the quality of their hay, and they were able to save on the application of nitrogen fertilizer. Studies evaluating the sustainability of production of alfalfa inter seeded in Bermuda grass showed that alfalfa competes well with Bermuda grass,even in drought conditions . Another part of the forage system in the region is cereal rye that is grown for late fall and early winter grazing when Bermuda grass goes dormant. The allelopathic effect of rye has been recognized since the early 1980’s, and a number of studies have illustrated the extent and severity of rye allelopathy over other species .
Rye reduced the density and biomass of weed species such as red root pig weed and common lambs quarters,mobile vertical rack when used as cover crop mulch in soybean, and enhanced soybean yield .Utilization of winter rye cover crop reduced corn biomass yield by 10% to 15%and the effect was more severe under no-till system . The role of cereal rye to suppress weeds was described extensively. To control weeds in other crops, rye can be utilized as cover or mulch. However, to control weeds, highly allelopathic rye cultivars are required . The influence of rye residue on germination of different plant species such as lettuce , proso millet, and corn was described . Barnes and Putnam reported the effect of two phytotoxic compounds from rye:on germination of various dicots and monocots such as, lettuce , tomato ,barnyard grass , cressetc.. Another phytotoxic compound extracted from rye is hydroxamic acid, which is exuded from plant roots and can reduce weed biomass by up to80% compared to other cover crops . Further, the Hx released from rye also inhibits the growth of other species such as Avena fatua .
The term “allele pathy” refers mostly to detrimental interactions between plant species, and is triggered by allele chemicals produced by one of the plants . In addition to producing allele chemicals, allelopathic crops and their residues also produce microbial toxins at the time of decomposition, which jointly create aphytotoxic environment . Allelo chemicals are released from plant roots,leaves, and stems and affect other crops within the same area . Allelopathy has been exploited for controlling weeds either by growing allelopathic crops prior to planting main crops or by isolating and applying the active natural compounds from the allelopathic crop as natural herbicides. Crops such as rice , wheat , barley , oat , cereal rye , soybeanetc. have been recognized for their allelopathic activities . Similarly, allelopathy of some legumes such as black gram, mung bean etc. have also been reported . Allelo pathy has been used to manage weeds and is considered as natural herbicides that are safer to the environment compared to synthetic herbicides . For instance, phytotoxic products from sorghum and Sudan grass hybrids were effective in suppressing alfalfa, Johnson grass, annual ryegrass , common lambs quarters and large crabgrass .
Similarly,rice hull incorporated alfalfa-rice-byproducts were reported as very effective allelopathic materials to inhibit weed emergence in paddy fields . Alfalfa pellets were effective in suppressing the emergence of Eleo-charis acicularis and Rotalaindica when applied at the dose of three tons ha−1. Additionally, alfal fade composed roots acted as allele pathic agents on blady grass and reduced seed germination by 50% . Alfalfa is also recognized forits own auto toxicity and researchers have recommended not planting alfalfa seed in an established alfalfa stand .In recent years, scientists have acknowledged allele pathy as a biotechnological tool to manage weeds targeting either the transfer of allelopathic genes from donor plants to the crop of interest through breeding or genetic engineering, or by enhancing the crop itself to produce phytotoxins via cell cultures . In rice,scientists have explored the variability in allelopathy among different cultivars and identified quantitative trait loci associated with rice allelopathy .