Cynodon dactylon (Bermuda grass).

Genetics: The chromosome number reported for C. dactylon varies from 2n = 18 to 2n = 36 with diploid and polyploid populations (Cook et al., 2005). Ramakrishan and Singh (1966) and Sarandon (1991) have found differences in total biomass and biomass partition according to the origin of the population. Sarandon (1991) points out that characters are highly heritable, which means that high genetic variability for biomass production and variable architecture allows an ample base for selection, which in most cases is induced by herbicides, mechanical control or forage production. Reproductive Biology: C. dactylon is wind-pollinated and generally self-incompatible, suffering from inbreeding depression when genotypes are self-pollinated. Quantitative traits such as seed yield and forage yield can be dramatically negatively affected by inbreeding depression (Cook et al., 2005). In diploid populations, caryopses are formed after zygote formation. In polyploids, which are sterile, caryopses may be apomictic. Physiology: This C4 plant (Kissmann, 1991) has high rates of accumulation under adequate irradiance, water and nutrient supply and may consume 75 kg of N, 20 kg of P and more than 1,500,000 litres of water for 5000 kg/ha of biomass dry matter (Fernandez, 1991). In the south of Santa Fe province, Argentina, a maximum biomass of 8000 kg/ha may be generated under a summer crop of maize or sunflower with >75% located in the first 10 cm of the soil profile (Lombardo, 1973), whereas in Balcarce (Argentina) about 5000 kg/ha is commonly found in maize or sunflower stubble. Phenology: A photoperiod of 13 hours induces flowering. Low night temperatures coupled with high diurnal temperatures induces blooming (Nir and Koller, 1976). A reduction in irradiance drastically decreases inflorescence production (Moreira, 1975). In North America, annual plants reproduce during spring and perennial plants reproduce all year long (USDA-NRCS, 2014). Longevity: C. dactylon grows as both an annual and perennial grass. The annual growth-form becomes dormant and turns brown when nighttime temperatures fall below freezing or average daytime temperatures are below 10°C (Cook et al., 2005). Activity Patterns: Seeds may be the route of invasion in weed-free fields through the faeces of cows (Rodriguez, personal communication). Rhizome biomass exhibits an annual cyclic pattern and, as with any perennial weed, low temperatures reduce biomass and viability is lost as a consequence of the consumption of materials due to respiration and maintenance. The digestibility of stocked material is severely decreased, implying a loss in forage quality (Vaz Martins, 1989). This is a character that has largely improved in cultivated varieties. Each node has a physiological self-governing structure in relation to the apex, but is highly dependent on substances from other plant parts. The mother plant determines the runner growth pattern on the soil surface according to the sugar-gibberellin balance (Montaldi 1970). Node disconnection may be caused by natural decay and cultivation and produces damage in the breakdown zone and changes in hormone and nutrient relationships. It is widely demonstrated that rhizome or runner fragmentation induces the activation of buds. The proportion of activated buds increases as the number of buds per segment decreases (Moreira, 1980; Kigel and Koller, 1985; Fernandez and Bedmar, 1992). The cultivation method is mainly responsible for vegetative propagation fragmentation. The higher the cultivation intensity, the smaller the segments produced (Kigel and Koller, 1985). Population Size and Structure This weed produces an enormous number of small seeds (0.25-0.30 mg), the viability and dormancy of which are highly variable according to genotype and the conditions when formed. The seed is important because it confers high genetic variability on the population. Perez et al. (1995) recorded a very low germination rate. Uygur et al. (1985) obtained up to 15% germination at constant temperatures of 35-40°C, and 50% at temperatures alternating between 20 and 30°C. Moreira (1975) obtained up to 80% germination with the help of nitrate, chilling and alternating temperatures, and Elias (1986) recorded up to 96% germination from heavier samples of seed. Seeds remain viable in the soil for at least 2 years (Caixinhas et al., 1988). As a rule, cultivars have relatively high viability. Osmo-conditioning of Bermuda grass seeds with PEG followed by immediate sowing improved seed germination and seedling growth under saline conditions (Al-Humaid 2002). The probability of emergence and successful establishment of C. dactylon decreases with the depth of the fragment, but increases with the weight of the node and internode (Perez et al., 1998). Growth from plants originated from a runner may exhibit a different biomass partition than that from plants originated from a rhizome (Fernandez, 1986). From sprouting onwards, weed growth is controlled mainly by temperature (optimum 25-30°C) and radiation, but also by humidity and soil fertility. The efficiency of carbohydrate reserve usage during sprout growth is highly dependent on temperature and the type of vegetative structure; it is maximum at 20°C and is higher for rhizomes than for stolons (Satorre et al., 1996). Runners and rhizome growth begins 30 days after growth but only if soil temperature is >15°C. Rates of 15 g/g/day have been recorded in Argentina (Lescano de Ríos, 1982).