Crop lodging can lead to the physical collapse of the plant canopy and can happen spontaneously due to mechanical instability of the plant structure, through external forces such as wind, or both. Maize lodging can occur at both the stalk and root. Stalk lodging occurs when stalks are broken at or below the ear-bearing node, whereas root lodging refers to plants that lean at an angle greater than a certain threshold (typically 30 or 45°) from the vertical [1, 2]. Stalk lodging causes greater grain losses than root lodging [3]. When stalk lodging occurs before maturity, stalk breakage halts grain filling in the entire plant due to the death of the plant above the breakage site, resulting in yield reduction or even the failure of the entire crop [3–5]. In addition to grain loss, lodging during the dehydration period after physiological maturity (PM) reduces the grain quality and increases harvest costs [6, 7]. Our previous study reported that in mechanical grain harvesting, the maize ear loss increased by 0.15–0.59% for each 1% increase in the lodging rate. Additionally, it was found that the mechanical grain harvesting speed decreased exponentially with increasing lodging rate [8].
The accurate evaluation of the maize lodging resistance in the field can assist in the development of lodging-resistant varieties, the regulation of cultivation measures, and the selection of optimum planting environments. Previous studies on maize stalk lodging focused on aspects of plant morphology, stalk mechanical characteristics, stalk anatomical structure, carbohydrate accumulation and distribution, pests and diseases, planting density, water and fertilizer management, and plant growth regulators [9]. Studies on stalk morphology have shown that maize plants with long basal internodes have a higher ear position and center of gravity than plants with shorter basal internodes, which increases the risk of lodging [10]. In contrast, maize plants with short and thick basal internodes display greater stalk-lodging resistance [11]. About 50 to 80% of the strength of a maize stalk comes from its outer structure, the rind [12]. Several studies have indicated that the rind penetration strength (RPS), crushing strength (CS), and bending strength (three-point bending flexural tests) are all significantly negatively correlated with the stalk lodging rate [13]. Stalk strength is significantly positively correlated with the contents of cellulose, hemicellulose, and lignin [8, 14]. Furthermore, corn borers significantly increase the rate of stalk lodging by drilling into stalks [15], whereas maize stem rot weakens stalk tissue, which greatly increases the risk of stalk lodging [16]. Moreover, as plant density increases, the length of the basal internode significantly increases and the diameter significantly decreases, the contents of cellulose, hemicellulose, and lignin, and the stalk mechanical strength decrease, and the risk of lodging increases [14]. Reasonable water and fertilizer management and the application of plant growth regulators can reduce the internode elongation rate, the ratio of length to diameter, the plant height, and the ear height, promote structural carbohydrate accumulation, and increase stalk mechanical strength and lodging resistance [17, 18]. However, most of these studies were based on the resistance of the plant itself, and less consideration was given to the impact of the external environment on the plant, such as wind. Wind is the primary environmental factor responsible for crop stalk lodging. Stalk lodging occurs when plants are subjected to wind forces greater than the maximum force that the stalk can withstand before breaking. Therefore, the critical wind speed of lodging, which is the synthesized result of wind, leaf area, ear weight, ear height and mechanical properties of main stem internode etc, is needed to evaluate the lodging resistance of plants under different varieties and cultivation practices.
Mechanical grain harvesting is the developing direction of maize production in China [19]. Unlike traditional manual harvesting and mechanical ear harvesting, the mechanical harvesting of maize grain requires grain moisture contents lower than 25% [20]. In mechanical grain harvesting, maize is generally harvested 2–4 weeks after physiological maturity [21]. During maize grain dehydration via plant standing in the field after PM, the risk of lodging increases due to stalk senescence or stalk rot [22, 23]. Nolte et al. estimated that in Ohio, USA, stalk lodging increases by about 5% per week after 15 October and that the ear loss in bushels per acre is equal to an average of about one-third of the percentage of stalk lodging [24]. Additionally, Allen et al. reported that maize harvested late at 15% grain moisture had a 30% lower yield and a 42% higher lodging rate than maize harvested early at 25% grain moisture [25]. The Chinese national standard for mechanical maize grain harvesting (GB/T-21962-2008) suggests that the lodging rate should be less than 5% for such harvesting. In the past, maize harvesting in China was mainly performed by hand and via mechanical ear harvesting, and therefore research on maize lodging has mostly focused on the growth stage before physiological maturity [9, 14, 17, 26, 27]. After PM, the decomposition of stalk carbohydrate and the decrease of stalk moisture content causes the stalk mechanical strength to decrease. Additionally, at this stage, the leaves senesce and fall off, thus decreasing the windward area and wind force. However, little is known about the critical wind speed of stalk breaking before and after physiological maturity.
Based on previous studies, this study developed a new type of measurement device to determine maize lodging resistance. The critical wind speed of stalk breaking, the stalk mechanical strength, and the natural stalk lodging rate were investigated in different maize cultivars in order to identify the parameters as evaluate lodging resistance during the later growth stage of maize. Furthermore, the relationship with the stalk mechanical strength, critical wind speed of stalk breaking, and natural lodging rate in the field were analyzed to clarify the factors affecting the critical wind speed of stalk breaking during the late growth stage of maize. The results will help crop breeders develop lodging-resistant maize cultivars.