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Demographics of glyphosate-resistant and susceptible Italian ryegrass populations from Paraná

Italian ryegrass (Lolium multiflorum Lam.) is an annual plant and a native of Southern Europe and currently distributed in temperate and humid subtropical climate regions worldwide (Terrel, 1968). Italian ryegrass has high biomass accumulation; thus, it is used as forage production in integrated systems and as soil cover in conservation ones. As a weed, it is highly competitive, especially with winter cereals, and can reduce the wheat grain yield by up to 90% (Trusler et al., 2007). Italian ryegrass presents a very high fecundity reaching 44,000 seeds plant-1, providing natural seedbank replenishment (Bararpour et al., 2017). Unfortunately, Italian ryegrass control is mainly based on herbicide applications. Successive applications of the herbicide with the same mode of action (MOA) increase resistant plants’ selection pressure and promote the spread of resistance. Nowadays, ryegrass is resistant to herbicides from eight different modes of action in 12 countries (Heap, 2020). In Brazil, Italian ryegrass has evolved resistance to herbicide from three modes of action, including enolpyruvylshikimate-phosphate synthase (EPSPS), acetolactate synthase (ALS), and acetyl-CoA carboxylase (ACCase) inhibitor herbicides (Weed Science, 2020). Furthermore, and there are also cases of multiple resistances involving them. Italian ryegrass has evolved resistance to herbicides commonly used in winter cereal crops in Brazil, which increases the complexity of weed management in such crops. Therefore, a better understanding of weed biology can help identify the most vulnerable stages of the plant life cycle, thus adopting more efficient management strategies. Knowledge of weed biology is essential for improving integrated weed management. While there is considerable information on the biology of congeneric Lolium rigidum (Gonzalez-Andujar, Fernandez-Quintanilla, 2004), little is known about the demography of L. multiflorum, most of these studies have been conducted in greenhouses, evaluating biomass accumulation and seed production (Vargas et al., 2005; Fernández-Moreno et al., 2017). Moreover, understanding weed demographic parameters enables studies of population dynamics to make projections of Italian ryegrass populations (Gonzalez-Andujar et al., 2010). Weed demographic studies have been conducted with other species, such as Conyza bonariensis and Salsoa australis (Borger et al., 2009; Zambrano-Navea et al., 2018). These authors observed behavioral differences of plants according to the season of establishment on some demographic parameters, such as emergence, survival, and seed production. Abstract: Italian ryegrass (Lolium multiflorum L.) has become notorious due to glyphosate resistance evolution. To study the demography of susceptible and glyphosate-resistant Italian ryegrass populations, several experiments were conducted in the grower’s field to evaluate seedling emergence, survival and reproductive traits patterns in three cohorts, and the effect of density on seed production. Results showed that seedling emergence is fast, and under suitable environmental conditions, it can happen within 30-days. Lower plant survival was observed in the first two cohorts (2% and 3%, respectively) compared to the last (13%). A larger number of spikes (spk) per plant (pl) was observed in plants of the first two cohorts (50.34 and 47.02 spk pl-1, respectively) in comparison to the last one (26 spk pl-1); however, the third cohort presented a higher potential for flower (flwrs) production per spikelets (spklt) (13.5 flwrs spklt-1) in detriment to the first two (11.9 and 12.12 flwrs spklt-1). Fecundity was 20,300 and 13,830 seeds pl-1 for susceptible and glyphosate-resistant plants, respectively, and presented density-dependent regulation. A knowledge of Italian ryegrass’s demographic parameters provides valuable information for identifying particularly vulnerable life-history processes and developing population dynamics models that can contribute to designing more rational control strategies.