Johnsongrass (Sorghum halepense (L.) Pers.) Interference, Control and Recovery under Different Management Practices and its Effects on the Grain Yield and Quality of Maize Crop

Maize is an important crop grown on significant acreage around the world, and a major constraint for its growth is weed interference. Thus, field studies were conducted to examine johnsongrass interference, control, and recovery under different management practices and its effects on maize. Our results indicated that the most johnsongrass aboveground biomass was recorded in the nontreated and weed-infested for 55 days after sowing (DAS) treatments, while the lowest values were in nicosulfuron treatments (48 and 60 g a.i./ha). Among the various herbicide treatments, the greatest johnsongrass aboveground biomass was recorded in the isoxaflutole (applied pre-emergence at 99 g a.i./ha) + 1 hoeing treatment. Johnsongrass aboveground biomass at 78–85 DAS was 1.4- to 6.0-fold greater than that at 55 DAS, revealing johnsongrass recovery after nicosulfuron treatments. Johnsongrass competition had a significant impact on maize growth and grain yield. The main crop parameters, such as aboveground biomass, grain yield, and protein content, were lowest in the nontreated and weed-infested for 55 DAS treatments, while the greatest values of these parameters were recorded in the weed-free and nicosulfuron treatments. In conclusion, our results indicated that timely and effective chemical control of johnsongrass is essential for improving grain yield and quality of maize.


Introduction
Maize is a cereal crop, grown in significant acreage around the world for grain and biomass production. In 2017, worldwide maize production was about 1.135 billion tonnes, while the cultivated area was about 197 million hectares [1]. In the last 40 years, an increase of 51.5% in the cultivated area was recorded, while the yield was increased by approximately 65%, mainly due to the development of high yielding hybrids and improved cultural practices. Although maize yield could be affected by several parameters, such as fertilization, pests, irrigation, cultivated hybrids, and environmental conditions [2][3][4][5], weed competition is the main limiting factor affecting both the growth and yield of this crop [6,7]. This crop should be kept weed-free for the first six to eight weeks after sowing to obtain maximum yield [8]. Lehoczky et al. [7] reported that the maize biomass was 64% lower in weedy plots due to the weed competition.
Among the most prominent weed species affecting growth and cultivation of maize is johnsongrass (Sorghum halepense (L.) Pers.). This perennial weed is commonly found in the regions where maize crops are cultivated [9][10][11]. Its control is mainly based on post-emergence application of sulfonylurea (e.g., foramsulfuron, nicosulfuron, and rimsulfuron) and triketones herbicides (e.g., tembotrione) [11][12][13]. In fields with low johnsongrass density, early detection and control of small patches must be a part of integrated johnsongrass management systems [9]. Johnsongrass is a very competitive weed with a high reproductive capacity [14,15] and can cause 17%-81% yield losses in maize crops [11,16]. Moreover, this weed species expands rapidly in the field [9,17], while plants originating from rhizomes are more competitive than seedling plants [12,18,19]. Therefore, rhizomatous johnsongrass management is critical to achieving a high yield in a maize crop.
Thus, the aim of the present study was to examine the effects of johnsongrass on maize growth, grain yield, and protein content under different management practices. Emphasis was also placed on examining johnsongrass control and recovery under different nicosulfuron and isoxaflutole treatments. Nicosulfuron was applied post-emergence, while isoxaflutole was applied preemergence (PRE) followed by hoeing or post-emergence application of nicosulfuron. The study of johnsongrass recovery under different management practices is important to minimize the rhizome growth of johnsongrass and enhance maize yield and grain quality.

Study Site
Two field experiments were established in 2017 and 2018 at the University of Thessaly farm. The soil was sandy clay loam in texture (clay: 26%, silt: 36%, and sand: 38%) with a pH of 7.4. Meteorological data for both the cultivation periods are presented in Figure 1. Total precipitation during the cultivation period was 289 and 294 mm in 2017 and 2018, respectively. The field was plowed in October and again in the spring, when two passes with a chisel plow and a rotary tiller were made to prepare a fine seedbed. Maize (Zea mays cv. P1547) was planted on 3 May 2017 and 24 April 2018 using a pneumatic sowing machine. The inorganic fertilizer 15-15-15 (5% S) was applied at the time of planting (200 kg ha −1 ) in a band near the seed row, while ammonium nitrate (34.5-0-0) was applied at 30 and 60 days after maize emergence in two equal doses (125 kg ha −1 ).
Isoxaflutole (Merlin Flexx L, Bayer Hellas A.E.B.E., Athens, Greece; safener: cyprosulfamide) was applied directly after sowing, while nicosulfuron (Milagro 240 SC, Syngenta Hellas A.E.B.E., Anthousa Attikis, Greece) was applied when the maize plants reached 4-6 true leaf stage (BBCHscale: growth stages 14-16) on 23 May 2017 and 16 May 2018. Herbicides were applied using a handheld field plot sprayer with six flat-fan nozzles calibrated to deliver 300 L ha −1 of spray solution at 250 kPa pressure. The field was irrigated with sprinklers after the isoxaflutole application to thoroughly wet the top 7 to 10 cm of soil. After crop emergence, the field was irrigated using a drip irrigation system. The irrigation intervals varied throughout the growing season and ranged from 7 to 10 days, depending on maize growth stage and daily temperature.

Data Collection
Maize height, stem diameter, and aboveground biomass were measured at 36-42 DAS (BBCHscale: growth stages [16][17][18][19], 55 DAS (BBCH-scale: growth stages [19][20][21], and 78-85 DAS (BBCH-scale: growth stages 61-63). These measurements were recorded from five plants randomly selected from each plot. The aboveground biomass was determined after drying the samples at 60 °C for 96 h. The relative chlorophyll content (SPAD values) was also determined in the field at 36-42 DAS, 55 DAS, and 78-85 DAS, on fully expanded maize leaves using a handheld SPAD-502 chlorophyll meter (Konica Minolta Optics, Inc., Osaka, Japan). The chlorophyll content was measured on three leaves from three plants per plot. Moreover, johnsongrass aboveground biomass was measured at 36-42 DAS, 55 DAS, and 78-85 DAS. For aboveground biomass determination, a square quadrate (1 by 1 m) was used, and the johnsongrass plants were removed by cutting at ground level. Aboveground biomass was determined after drying these samples for 96 h at 60 °C.
For the determination of grain yield and yield components, plots were harvested in 17 October 2017 and 8 October 2018. Ten plants were randomly harvested to quantify ear length and grain yield. After harvest, 1000 grain-weight was measured by weighing three random 100-grain samples from each treatment [20]. Protein and starch content were determined in the dry grains by near-infrared reflectance (NIR) spectroscopy technique using the DA 7250 NIR analyzer (Perten Instruments, Hägersten, Sweden).

Statistical Analyses
Data from both years were separately subjected to statistical analyses (ANOVA), which was carried out using the SigmaPlot statistical package, v.12 (Systat Software, San Jose, CA, USA). When the ANOVA model was significant (p ≤ 0.05), the difference between means was evaluated using a Fisher's LSD test (p ≤ 0.05). Finally, a Pearson's correlation analysis was carried out to assess the relationship between johnsongrass aboveground biomass and maize parameters.

Johnsongrass Aboveground Biomass
At first measurement (36-42 DAS), johnsongrass aboveground biomass ranged from 0 to 2557.8 kg ha −1 in 2017 and from 0 to 2680 kg ha −1 in 2018 ( Figure 2). The most johnsongrass aboveground biomass was recorded in the nontreated and weed-infested for 55 DAS treatments, while the least values were recorded in the weed-free and nicosulfuron treatments. At the second measurement (55 DAS), the most johnsongrass aboveground biomass was also recorded in the nontreated and weedinfested for 55 DAS treatments. At the final measurement (78-85 DAS), johnsongrass aboveground biomass ranged from 111.1 to 8644.1 kg ha −1 in 2017 and from 85.8 to 7588.9 kg ha −1 in 2018, while there were statistically significant differences among the different management treatments. The most johnsongrass aboveground biomass was recorded in the nontreated plots, while there were no significant differences among the nicosulfuron treatments.

Maize Growth
Johnsongrass competition had a significant effect on the maize crop parameters, such as aboveground biomass, plant height, and stem diameter. Plant height ranged from 171.6 to 261.6 cm in 2017 and from 109.4 to 271.8 cm in 2018 for the final height measurements (78-85 DAS). In both the experiments, the lowest plant height ( Figure 3) and stem diameter ( Figure 4) were recorded in the nontreated and weed-infested for 55 DAS treatments. For these parameters, there were statistically significant differences among the herbicide treatments. The lowest plant height was recorded in the isoxaflutole + 1 hoeing treatment, while there were no significant differences among the four nicosulfuron treatments.  The maize aboveground biomass was also affected by johnsongrass competition, as the lowest maize aboveground biomass was recorded in the nontreated and weed-infested for 55 DAS treatments ( Figure 5). The aboveground biomass ranged from 6378 to 17,155 kg ha −1 and from 4453 to 22,592 kg ha −1 , in 2017 and 2018, respectively, for the final aboveground biomass measurements (78-85 DAS). Moreover, the lowest aboveground biomass was recorded in the nontreated, weedinfested for 55 DAS, and isoxaflutole + 1 hoeing treatments.

Relative Chlorophyll Content
At the final measurement, the SPAD values ranged from 38.37 to 57.77 in 2017 and from 32.07 to 59.97 in 2018. Similar to the maize biomass, the relative chlorophyll content of maize (SPAD values) was affected by johnsongrass competition, as the least values were observed in the nontreated and weed-infested for 55 DAS treatments, while there were no statistically significant differences among the nicosulfuron treatments for this parameter (Figure 6).

Maize Yield and Quality
Johnsongrass competition had a significant effect on the maize yield, ear length, and 1000 grainweight (Table 1). Grain yield ranged from 2841 to 13,949 kg ha −1 in 2017 and from 1743 to 12,788 kg ha −1 in 2018. In both the experiments, the least grain yield was recorded in the nontreated and weedinfested for 55 DAS treatments. Among the various herbicide treatments, the least grain yield was recorded in the isoxaflutole + 1 hoeing treatment, while the greatest values were recorded in nicosulfuron treatments. The ear length and 1000 grain-weight were also affected by johnsongrass competition, as the least values were recorded in the nontreated and weed-infested for 55 DAS treatments. The ear length ranged from 10.7 to 22.7 cm and from 11.9 to 22.2 cm, in 2017 and 2018, respectively. Moreover, the highest 1000 grain-weight and ear length were recorded in the weed-free and nicosulfuron treatments. Similar to the maize yield, johnsongrass competition had a significant effect on the maize quality ( Table 2). Protein content ranged from 7.12% to 8.19% in 2017 and from 6.70% to 7.83% in 2018. In both years, the least protein content was recorded in the nontreated and weed-infested for 55 DAS treatments. Moreover, there were statistically significant differences among the management treatments. Within the different management treatments, the least protein content was recorded in the untreated and weed-infested for 55 DAS treatments, while there were no statistically significant differences among the nicosulfuron treatments and weed-free treatment. Starch content ranged from 62.07% to 64.83% in 2017 and from 62.87% to 64.57% in 2018. In both years, the starch content was affected by johnsongrass competition, as the least values were recorded in the nontreated and weedinfested for 55 DAS treatments, while the greatest starch content was recorded in the weed-free and nicosulfuron treatments.

Johnsongrass Control
Greatest johnsongrass aboveground biomass was recorded in the nontreated and weed-infested for 55 DAS treatments. At the final measurement (78-85 DAS), among the herbicide treatments, the most aboveground biomass was recorded in the isoxaflutole + 1 hoeing treatment (Figure 2). Stephenson and Bond [21] reported that the herbicides thiencarbazone-methyl + isoxaflutole and isoxaflutole applied PRE, provided 74% to 76% rhizomatous johnsongrass control, while Ortiz et al. [22] observed that isoxaflutole provided excellent johnsongrass seedling control (100%).
Isoxaflutole retarded johnsongrass growth with plant foliage turning white for a short period, and thus, the johnsongrass plants were shorter at nicosulfuron application than those in the nontreated plots. Despite these effects, the pre-emergence application of isoxaflutole has not significantly increased the efficacy of nicosulfuron against johnsongrass, although it is well known that the growth stage of johnsongrass at the time of application affects herbicide efficacy. According to Eleftherohorinos and Kotoula-Syka [28] and Djurkić et al. [29], the post-emergence herbicides (e.g., nicosulfuron) provided the greatest efficacy when johnsongrass plants are 10-35 cm tall. In another study, Johnson and Norsworthy [30] reported that nicosulfuron efficacy decreased as johnsongrass size increased at application. Furthermore, in both years, johnsongrass aboveground biomass in the final measurement at 78-85 DAS was 1.4 to 6.0-fold greater than that in the second measurement at 55 DAS, revealing that the recovery from herbicide injury of johnsongrass plants was observed in nicosulfuron treatments.

Maize Growth
Johnsongrass competition had a significant effect on the maize growth and relative chlorophyll content. Plant height, stem diameter, aboveground biomass, and relative chlorophyll content were affected by johnsongrass competition, as the least values were recorded in the nontreated and weedinfested for 55 DAS treatments. It is important to point out that the experimental field in the present study was highly infested by rhizomatous johnsongrass that strongly competed with the maize plants for root space, nutrients, water, and light. Lehoczky et al. [7] also reported that maize and weeds strongly compete for nutrients. These researchers also found that the nitrogen content in weedy plots was 4.73 times lower than in the weed-free plots. In this regard, the application of nicosulfuron in maize improved plant growth due to the reduction of weed competition and thereby the increase of chlorophyll content. In comparison to the weed-free treatment, johnsongrass competition for 55 DAS reduced maize aboveground biomass by 47% to 71% in 2017 and 2018, respectively, while johnsongrass competition throughout the season (nontreated) reduced maize biomass by 62% to 80% in 2017 and 2018, respectively. These findings are confirmed by the results of the correlation analysis (Figure 7), since maize aboveground biomass showed a negative correlation with johnsongrass aboveground biomass (r = −0.741, p < 0.05 and r = −0.837, p < 0.01 in 2017 and 2018, respectively).
Among the various herbicide treatments, the least maize aboveground biomass was observed in the isoxaflutole + 1 hoeing treatment, while the greatest values were recorded in the nicosulfuron (60 g a.i. ha −1 ) and isoxaflutole + nicosulfuron (60 g a.i. ha −1 ) treatments. Maize biomass was increased by 2.5 to 4.9 times in nicosulfuron plots than that of weedy plots, slightly greater than that reported by Gubbiga et al. [31], probably due to the high field infestation by johnsongrass in our study.

Maize Yield and Quality
Johnsongrass competition had a significant effect on the maize yield and yield components. The ear length, 1000 grain-weight, and grain yield were affected by johnsongrass competition, as the lowest values were recorded for the nontreated and weed-infested for 55 DAS treatments. In both years, johnsongrass competition for 55 DAS reduced the maize grain yield by 58-72%, while in the nontreated plots, the grain yield was reduced by 79-86%. Similarly, Mitskas et al. [19] reported that johnsongrass competition reduced grain yield by 83%. The correlation analysis also revealed that grain yield showed a negative correlation with johnsongrass aboveground biomass (r = −0.716, p < 0.05 and r = −0.858, p < 0.01 in 2017 and 2018, respectively).
Among the various herbicide treatments, the least grain yield was observed in the isoxaflutole + 1 hoeing treatment, while its greatest values were recorded in the nicosulfuron (60 g a.i. ha −1 ) and isoxaflutole + nicosulfuron (60 g a.i. ha −1 ) treatments. In comparison to the weed-free treatment, johnsongrass competition throughout the season (nontreated) reduced ear length by 45% to 52% in 2018 and 2017, respectively, while 1000 grain-weight reduced by 24% to 32% in 2018 and 2017, respectively. Similarly, Mitskas et al. [19] also observed that the reduction of ear length in weedy plots was greater than that for 1000 grain-weight.
In general, the maize grain was of excellent quality. Starch and protein content ranged from 62.07% to 64.83% and from 6.70% to 8.19%, respectively. Labuschagne et al. [32] reported that the starch content at harvest in several maize genotypes ranged between 50.06% and 62.65%, while Silva et al. [33] observed that the protein content fluctuated between 6.9% and 10.5%; these value ranges are in agreement with our findings. Johnsongrass competition had a significant effect on the maize quality. In both the experiments, the lowest protein and starch content were recorded in the nontreated and weed-infested for 55 DAS treatments. In contrast, the greatest protein content was recorded in the weed-free and nicosulfuron treatments. In comparison to the weed-free treatment, johnsongrass competition for 55 DAS reduced the protein content by 11-13%. These findings are confirmed by the results of the correlation analysis since protein content showed a negative correlation with johnsongrass aboveground biomass (r = −0.789, p < 0.05 and r = −0.850, p < 0.01 in 2017 and 2018, respectively). Similarly, Mohammadi et al. [6] reported that the grain protein content in maize was 9.7% lower in weedy plots due to the weed competition. Reduction of the grain quality (e.g., gluten content) due to weed interference was also demonstrated in durum wheat by Karkanis et al. [34].

Conclusions
The results of this study indicated that johnsongrass competition had a strong impact on the growth, yield, and quality of maize. Johnsongrass competition for 55 DAS reduced the maize grain yield by 58-72%, while in the nontreated plots, the grain yield was reduced by 79-86%. Reduction in the grain quality (e.g., protein and starch content) due to johnsongrass competition was also observed. Among the various management treatments, the least maize aboveground biomass, grain yield, and protein content were observed in the nontreated and weed-infested for 55 DAS treatments, while its greatest values were recorded in nicosulfuron treatments. Additionally, the results of this study indicate that the pre-emergence application of isoxaflutole has not significantly increased the efficacy of nicosulfuron against johnsongrass. Finally, johnsongrass aboveground biomass at the final measurement (78-85 DAS) was 1.4 to 6.0-fold greater than that in the second measurement (55 DAS), revealing the recovery of johnsongrass plants in nicosulfuron treatments.