Soil depletion
Maize crops drain nutrients from the soil, do not compete well with weeds after the seedling stage, and are subject to many diseases and pests. In consequence, the cultivation of high-producing maize requires high fertilizer levels as well as herbicides, pesticides and fungicides, which can be deleterious to biodiversity and soil conservation. Heavy fertilizer applications can cause nitrate leaching and soil erosion. Herbicides may contaminate the groundwater while pesticides alter biodiversity (European Commission, 2000). In developing countries, where fertilizers are too expensive for general use, the nutrient uptake is higher than when fertilizers are regularly used, thus reducing soil fertility and causing further soil degradation (CIMMYT, 2009).
Genetically modified (GM) maize
The debate about the environmental impact of GM crops is complex and a full discussion of the issue is beyond the scope of this datasheet. GM maize hybrids with insect or herbicide tolerance have been commercialised since 1996 and are highly popular in some countries (93% of planted maize in the USA in 2014; Ferrareto et al., 2015) because of their economic benefits, but their environmental impact has been the subject of considerable debate. Positive impacts have been reported for GM maize, notably a reduction in herbicide and pesticide use, with some benefits for wildlife (Edwards et al., 2009). However, potential negative effects have been alleged, including the alteration of the DNA of soil microflora, and deleterious impacts on useful insects such as butterflies and bees, or on non-GM plants (including maize) through genome transfer (Dale et al., 2002).
Life Cycle Assessment
Production of maize silage includes two major stages, chopping and ensiling. The Life Cycle Assessment (LCA) of maize silage production showed that the chopping stage was responsible for the major part of the environmental burden of maize silage (more than 88% for all the evaluated impact categories of the LCA), while ensiling is responsible for a smaller share of the overall impact (Bacenetti et al., 2015). Compared to maize silage stored in bunker silos, silage stored in silo bags showed lower environmental impact for all the evaluated impact categories (from -5% to -9%). Such a difference is mainly explained by lower DM losses occurring through the use of the bags (Bacenetti et al., 2015).
NO2 gas in the maize silo
Maize plants containing high amount of nitrates produce nitrogen dioxide (NO2) during the early stages of fermentation. This severely injures lung tissues and may cause sudden death. NO2 is heavier than air and thus tends to remain above the silo. It has a bleach-like odour and forms a reddish-brown haze. Maize with potential high nitrate content should not be harvested soon after rain ending a dry period. Ensiling in horizontal silos or in silo bags reduces the risk of NO2 intoxication (Bagg, 2012).