The use of herbicides on crop fields has created shifts in weed populations and the emergence of herbicide resistance. Because of this phenomenon, there is interest in the exploitation of allelopathic activity of crop plants. This was assessed in field experiments at the Tygerhoek Research Farm (19°54″ E, 34°08″ S), South Africa, to determine whether significant weed control could be achieved via allelopathy. This locality contains weakly developed residual soils (pH 5.1) of Mispah (Entisol) type containing 22% clay and 1.6% carbon. Uniform scattering of a quantity of plant residues preceded drilling. The rotational plant species planted into the plant residues consisted of barley (Hordeum vulgare L. variety Clipper), canola (Brassica napus L. variety ATR Hyden), wheat (Triticum aestivum variety SST 88), lupines (Lupinus albus L. variety Tanjil), alfalfa (Medicago sativa L. variety SA standard), medic (Medicago truncatula Gaertn. variety Parabinga), and ryegrass (Lolium multiflorum Lam. variety Energa). These crops are used in crop rotation in this grain production area. Six plant species were used in the second experiment, planted at 90° angle across the first after the latter was harvested, namely barley, wheat (variety SST 027), lupines (variety Tanjil and variety Quilinock), ryegrass and ryegrass weed type (L. multiflorum × perenne). Broadleaf weeds comprised 88.5% of total weed spectrum. Medic suppressed ryegrass weed type while lupines suppressed grass weeds. An acceptable production practice using allelopathic crops for weed control will likely consist of combining continued limited amounts of herbicides with leguminous crop residues.

Information on the temperature requirements for germination of weed seeds can be used to predict the soil temperature required for weed emergence. The germination behaviour of the two types of mature C. bengha/ensis seeds, ie. subterranean and aerial, were studied to establish whether differences in seed germination provide these plants with an apparent competitive advantage over other weed species. Seeds of aerial and subterranean types were placed separately on germination towels and put inside plastic bags. The subterranean seeds germinated significantly faster than the aerial seeds, indicating greater thermal sensitivity of the former type of seed. The dormancy displayed by aerial seeds could inter alia be attributed to delayed germination and not to loss of viability. The optimum germination temperature for subterranean seeds was 21/28°C, and for aerial seeds 18/25°C. This broad adaptability may require continuous C. bengha/ensis management efforts and supports the need for an effective residual herbicide, or illustrates the need of timely and probably multiple postemergence herbicide applications.Inligting oor temperatuurvereistes vir die ontkieming van onkruidsade kan gebruik word om die grondtemperatuur wat vir onkruidopkoms vereis word, te voorspel. Die ontkieming van die twee tipes ryp C. bengha/ensis saad, wat ondergronds en bogronds geproduseer word, is bestudeer om te bepaal of verskille in saadontkieming die plante van 'n oenskynlike kompeterende voordeel, oor ander onkruidspesies voorsien. Sade van ondergrondse en bogrondse saad, is afsonderlik op ontkiemingsrolle en toe binne plastieksakkies, geplaas. Die ondergrondse saad het meer hitte-sensitiwiteit getoon deurdat dit betekenisvol vinniger as die bogrondse saad ontkiem het. Die dormansie van bogrondse saad kon, onder andere, aan vertraagde ontkieming en nie aan 'n verlies in kiemkragtigheid, toegeskryf word nie. Die optimale ontkiemingstemperatuur van ondergrondse en bogrondse saad was onderskeidelik 21/28°C en 18/25°C. Hierdie wye aanpasbaarheid van C. bengha/ensis mag deurlopende bestuur vereis en ondersteun die behoefte vir 'n effektiewe residuele onkruiddoder of iIIustreer die noodsaaklikheid van tydige en moontlik veelvuldige na-opkoms onkruiddodertoedienings.

Weed resistance to herbicides present one of the greatest current economic challenges to agriculture. Herbicide resistant ryegrass (Lolium spp.) is a serious problem in Western Cape grain producing areas. Morphological and pathogenic analyses were performed on ryegrass samples. Morphologically, 50% of specimens were classified as rigid ryegrass, 48% as a hybrid, namely L. multiflorum x L. perenne and 2% as perennial ryegrass. Fusarium pseudograminearum (cause of Fusarium crown rot) was isolated from six localities. Pathogencity tests confirmed that F. pseudograminearum isolates obtained from ryegrass and wheat are pathogenic on both crops, indicating that ryegrass can act as an alternative host and a source of inoculum of this important soilborne pathogen. Grass weed infestation can favour the disease, and grass weed control is therefore recommended as part of an integrated strategy to manage crown rot. Knowledge on morphological differences among ryegrass may be important to guide differential weed management of ryegrass. Smother cropping as part of conservation farming, should receive more prominence to suppress herbicide resistant ryegrass and simultaneously reduce the occurrence of crown rot.

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