Agronomic Practices and Oryza sativa Infestation Level

The farms that adopted plowing to prepare their rice seedbeds mainly showed low O. sativa infestations (44%), but 39% of the farms showed medium infestations (Figure 2C). Most of the farms that performed minimum tillage had a medium O. sativa infestation (47%), while 40% of the farms recorded a low infestation. The farms that adopted both techniques showed a prevalence of low infestations in rice fields (67%) (Figure 2C). The plowing of rice fields is a common preparatory tillage practice in many areas in the world, as was also demonstrated in a similar survey on rice cultivation techniques adopted in the United States (Kanapeckas et al. 2018). Plowing moves the shed O. sativa seeds deep into the soil, which results in a lower weed emergence, while shallow tillage generally concentrates O. sativa seeds close to the soil surface, thus favoring their emergence (Zhang et al. 2019).

The preponderance of the farms recorded a low O. sativa infestation level when either dry seeding or water seeding was performed, while a dominance of medium O. sativa infestation was shown for the farms that adopted both practices. Nevertheless, about 25% of the farms that adopted dry seeding reported a high infestation level of O. sativa (Figure 2D). According to the literature, the dry seeding of rice favors O. sativa infestations, and farmers who apply this seeding method tend to compensate for this disadvantage by performing crop rotation (Gealy et al. 2000; Wang et al. 2017). In our study, only a few farmers applied both dry seeding and crop rotation, as monocropping is more traditional for rice cultivation in the area.

A small prevalence of low O. sativa infestation (about 45%) was found on farms that cultivated rice with continuous flooding, while 38% had a medium O. sativa infestation (Figure 2E). Intermittent irrigation and the application of both continuous flooding and intermittent irrigation showed a prevalence of medium infestations, although this result cannot be considered robust, as fewer than five farms applied these practices. Permanent flooding is largely adopted in many areas of the world, as it can reduce weed emergence and growth (Chauhan 2013; Wang et al. 2017).

In this study, almost 88% of the farms purchased certified rice seeds, and almost an equal number (about 40%) of them recorded either low or medium O. sativa infestation levels (Figure 2F). Most of the farms that self-produced their rice seeds or used both certified and self-produced seeds also showed low infestations, but only about 10 farms fell within to these categories.

The majority of the rice farms (about 57%) that adopt crop rotation showed a medium O. sativa infestation, while among farms that practiced cultivated rice monocropping, most (about 46%) showed a low infestation, and 37% had a medium infestation (Figure 2G). Crop rotation is considered a practice useful to reduce weed dominance, while monocropping has the opposite effect (Chauhan 2013). The medium infestation shown in our study by most of the farms that adopted crop rotation, together with the fact that only about 14% of the farms adopted rotation (Table 1), would appear to be an indication that rotation is introduced as a last resort when O. sativa infestation is quite significant. However, high numbers of O. sativa infestations were recorded in almost 17% of the farms that cultivated rice by means of monocropping and in only about 7% of the farms that rotated rice with other crops (Figure 2G). The introduction of at least another crop in the rotation was found to reduce O. sativa infestation in a study conducted in Italy in which rice was alternated with soybean, where a 97% reduction of the O. sativa seeds present in the first 10 cm of the seedbank was observed (Ferrero and Vidotto 1997).

aMixed seed was omitted, as all the farms had low infestations and thus no variability emerged.

Farms that used a stale seedbed (about 50% of the total) mainly reported medium O. sativa infestation levels, while the farms that did not adopt this practice more frequently reported low infestation levels (about 56% of the farms) (Figure 2H). This result can be explained by considering that the stale seedbed practice is specifically performed to control O. sativa; it is likely that the farms that had more O. sativa infestation problems adopted this technique and thus had higher weed infestation levels. On the other hand, the farms that did not use a stale seedbed had fewer initial O. sativa problems.

A similar result was observed on the farms that used imazamox for O. sativa control, a practice that is linked to the adoption of Clearfieldº varieties. The prevalent level of infestation on these farms was medium (about 47%), while a prevalence of low O. sativa infestations was observed on the farms where imazamox was not applied (Figure 2I). This technique was specifically developed for O. sativa control, and it is therefore also conceivable that the farms with higher amounts of O. sativa infestations decided to cultivate Clearfieldº rice varieties (Milan et al. 2017; Singh et al. 2017).

The spread of imazamox resistance has been linked to the use of Clearfieldº varieties; again, in this case, more than 57% of the farmers who had resistance problems declared they had a medium level of infestation, while those who did not have resistance problems and did not apply imazamox reported low prevalence of O. sativa infestation (Figure 2J).