1. Introduction
The Central Valley of California is a highly productive agriculture region in the United States with a hot, semi-arid climate, where intensive tillage and irrigation is the normal practice in agronomic production systems [1]. In this region, silage maize (Zea mays) production is an important feed crop for the California dairy industry. In 2022, the area under silage maize production in California was approximately 135,570 ha acres with an average yield of 58.3 metric tons (mt) ha−1 [2]. However, in recent years, there have been issues of environmental sustainability of silage maize production systems in many parts of the country because of its reliance on intensive tillage, chemical inputs, irrigation [3,4], and degradation of agricultural lands especially in irrigated, arid agroecosystems [4]. California also has an arid climate where irrigated silage maize yield production generally occurs with conventional tillage, and thus is no exception to such concerns [1]. Several changes in the silage maize cropping system have been explored since the early 2000s in California to address some of these issues [1,5]. Among these, reduction in tillage and incorporation of cover crops are two major practices [6]. In recent years, some growers in California have adopted soil health management practices to mitigate the reduction in soil organic matter (SOM) and improve soil health [7]. One of these practices is the use of cover crops. It has been stated, as early as 1989, that the use of cover crops in California agricultural systems was the only practical means of improving and maintaining the SOM levels necessary for achieving high levels of crop productivity [8]. Although cover crops have been used since Roman times [9], their adoption is fairly low in California [10]. However, they are now being reintroduced into California for their multiple benefits and as a tool to increase soil health [7]. Furthermore, a survey reported that about one-third of the acreage planted with a cover crop in the US received a financial assistance payment from either Federal, State, or other programs to support cover crop adoption [7]. With the many benefits of cover cropping and added financial incentives, several people are adopting cover crops in their production systems. However, in California, adoption of conservation tillage and cover crops is less than 2% [6] and less than 5% [10], respectively, but in recent years there is renewed interest in these practices [5].
In annual cropping systems, cover crops must be terminated to prepare for planting the main crop. Several methods of cover crop termination exist such as discing, plowing, mowing, animal grazing, winter kill, herbicide applications [11,12], and in recent years, roller-crimping [12]. Each of these methods has its own limitations. For example, mowing shreds the cover crop, causing the residue to break down quickly, leaving less time with soil cover. Discing and plowing can be destructive for topsoil structure, release stored carbon (C), and increase dust, while herbicides can be an environmental concern [12,13,14]. Thus, the roller-crimper may have an advantage because it allows for the greatest amount of unshredded surface residue [12] and may require no herbicides [15].
Although cover crop termination by roller-crimpers was introduced more than two decades ago [15], it is a relatively new technique in California and has yet to be adequately explored in the arid, irrigated cropping systems of the state [16] and irrigated systems in other semi-arid areas [17]. Roller-crimpers come in variable sizes and are comprised of channels that form a rolling V shape described as a Chevron pattern, which allows for the crimping of plant material, and the drum can be filled with water for added weight to aid in rolling [18]. Cover crop roller-crimping timing is crucial: if performed too early, crops will stand back up and regrow, and if performed too late, the cover crops will set viable seeds that can germinate and compete against future crops [11,19]. Nevertheless, the benefit of using a roller-crimper to terminate cover crops is to preserve the intact biomass while gaining the other benefits of improved soil health and weed suppression.
A study evaluating the potential of cover crop termination with a roller-crimper, planting of strip-tilled silage maize into the cover crop residue, weed suppression by the cover crop residue, and effects on silage maize yield in an overhead irrigated system may be beneficial for the Central San Joaquin Valley of California. Therefore, a study was conducted to (i) estimate the percent kill of the cover crops with the roller-crimper, (ii) assess the dry biomass produced by the cover crop at termination, (iii) estimate the percent soil cover by the cover crop residue until maize canopy closure, (iv) estimate the percent weed cover, and (v) assess the maize silage yield.
2. Materials and Methods
2.1. Study Site
The study was conducted at the California State University, Fresno, CA, farm (36°49′30″ N 119°45′06″ W). Prior to this study, the crop rotation at the site was silage maize and winter forage wheat (Triticum aestivum) with conventional tillage for several years. The soil type at the site was a Hanford series sandy loam (Coarse-loamy, mixed, super active, nonacid, thermic Typic Xerorthents). A subsection of a field irrigated with a center-pivot system was used for this study. The irrigation system was a Valley® Center Pivot that included four 45.7 m spans for a total of 182.9 m. The system was restricted to ¾ of a circle because of the presence of a water reservoir on the northeast side of the field.
2.2. Preliminary Study
A two-year (2018/2019 and 2019/2020) preliminary study was conducted to evaluate cover crop species suited for roller-crimping. The previous silage maize crop residue was tilled with a 4.6 m stubble disc in early October of each year. Two passes were made with the disc to ensure that the residue was completely tilled into the soil to make a suitable seedbed for the experiment. Eighteen 182.9 m long and 4.6 m wide plots were then marked out for planting six cover crop treatments in October of 2018 and 2019. The width of the plot was based on the width of the grain drill planter and the roller-crimper. The 18 plots were then subdivided into three blocks with each block containing six cover crop treatments arranged as a randomized complete block design. The six cover crop treatments and their seeding rate were as follows: (i) ‘High diversity mix’ (Green Cover Seed®, Bladen, NE, USA; mixture of 40 different plant species) @ 33.6 kg ha−1, (ii) a mix of ‘Merced’ Rye (Secale cereale), purple vetch (Vicia americana Muhl. Ex Willd.), and fava bean (Vicia faba) @ 44.8, 44.8, and 56 kg ha−1, respectively, (iii) a mix of ‘Lacy’ phacelia (Phacelia tanacetifolia), ‘Trophy’ rape (Brassica napus), and ‘Tillage’ radish (Raphanus sativus) @ 16.8 kg ha−1 each, (iv) a 50% mix of fava bean (Vicia faba) and ‘Magnus’ pea (Pisum sativum) at 140.1 kg ha−1, (v) ‘Multiplex cover crop mix’ (Lockwood Seed & Grain®, Chowchilla, CA, USA; mixture of bell beans, ‘dundale’ peas, ‘nitroplex’ peas, purple vetch, and barley (Hordeum vulgare) @ 140.1 kg ha−1, and (vi) a combination of ‘high diversity mix’ (33.6 kg ha−1) and ‘multiplex cover mix’ (112.1 kg ha−1). The seeding rates were based on the seed suppliers’ recommendations.
A John Deere® (JD) 515 grain drill attached to a 7820 JD tractor (John Deere and Co., Moline, IL, USA) equipped with a Global Positioning System (GPS) was used to map each treatment plot. The treatments were then planted using the JD (John Deere and Co., Moline, IL, USA) grain drill. Each treatment in three replications was planted and the grain drill was vacuumed and properly cleaned before planting the next treatment to avoid seed contamination. All seeds were sown at a depth of 2.54 cm. The treatments were planted on 14 December and 14 November, in 2018 and 2019, respectively.
2.3. Treatments and Experimental Design
Based on the experiences in the preliminary studies, treatments for the main experiment were selected. In the preliminary studies, difficulties were encountered in planting seeds of different sizes in the mixture with the same drill, which influenced the performance of the treatments. The small-seeded species were released more easily from the drill than the larger-seeded species, which resulted in patches of either the small-seeded or large-seeded species rather than an even stand. Some combination treatments and the rapeseed and the tillage radish treatments were dropped from the study because of the inability of the roller-crimper to kill these crops.
The previous wheat crop residue was tilled using a 4.6 m stubble disc in early October 2020 and 2021, respectively. Two passes were made with the disc to ensure that the residue was completely tilled into the soil to make a suitable seedbed for the experiment. Fifteen 182.9 m long and 4.6 m wide plots were then marked out for planting of the treatments. The 15 plots were then subdivided into three blocks to have five cover crop treatments randomized within each block to represent a randomized complete block design with three replications. The five cover crop treatments and the seeding rates were as follows: (i) ‘Merced’ rye alone @ 89.7 kg ha−1, (ii) ‘ultra-high diversity mix’ @ 67.3 kg ha−1, (iii) ‘multiplex cover crop mix’ @ 78.5 kg ha−1, (iv) a mixture of fava bean and ‘Lacy’ phacelia @ 33.6 and 22.4 kg ha−1, respectively, the two species were planted in separate passes because of the difference in their seed sizes, and (v) a mixture of ‘Merced’ rye, field pea, and purple vetch @ 33.6 kg ha−1 each. These seeding rates were based on the recommendation of the seed suppliers. The plots were mapped, and the treatments were planted as described earlier on 11 November and 22 October, in 2020 and 2021, respectively. A no cover crop control treatment was not included in the study because the main objective of this study was to evaluate the efficiency of the roller-crimper in terminating the cover crops. Moreover, the entire area (more than five hectares) surrounding the experimental plots had no cover crop silage maize planted using conventional methods and the average yield of this entire area could be used for non-statistical comparative purposes.
The cover crops were irrigated with 12.7 mm of water with the center pivot system in November 2020 to ensure seed germination because of inadequate precipitation; however, no irrigation was applied in 2021 because of adequate rainfall at planting (Figure 1). The average monthly maximum and minimum temperatures during the experimental period are displayed in Figure 2. The data were obtained from the California Irrigation Management Information System weather station located near the experimental plots. Except for the initial low precipitation during the onset of the experiment in 2020/2021, the precipitation and temperature patterns were similar between the two years of the study.
Cover crop emergence/ground coverage were visually assessed on a scale of 0 to 100% (where 0 = no emergence and 100 = complete ground cover by the cover crop) during the growing season. No data on actual plant densities and number of cover crop plants by species within an area were taken because of difficulty in identifying the actual cover crop species and the weed species during emergence. Data on such could have been taken at a later stage of plant growth but we wanted to avoid any form of destructive sampling of the plants before roller-crimping them. When the rye-alone plots reached the soft-dough stage, all the experimental treatments were terminated with a 4.6 m roller-crimper with a Chevron pattern produced by I & J Manufacturing® (Gordonville, PA, USA). At this time, the cover crops in the other treatment had reached flowering or seed set while the Phacelia plants were at complete maturity. The roller-crimper was rear-mounted on a Kubota 9000® tractor (Kubota Corp., Osaka, Japan) equipped with a GPS (Figure 3). The GPS program was used to ensure that the cover crop swath width was rolled in its entirety to ensure maize planting could follow behind in the exact plot placement. Cover crop was terminated on 30 April and 5 April, in 2021 and 2022, respectively. Cover crop termination was visually evaluated for two weeks before planting maize. Data on the percent kill of the cover crop were taken weekly until canopy closure of the maize crop. Percent cover crop kill was based on a scale of 0 to 100 (where 0 was no kill, complete regrowth, and all erect plants; 100 was complete kill, no regrowth, and no erect plants).
All the treatment plots were strip-tilled after the cover crop was evaluated and an acceptable termination rate was achieved. Acceptable termination rate was considered as less than 10% green tissue remaining on the plants on average in each plot. A 4.6 m, a 6-row Orthman® strip tiller (Lexington, NE, USA) was connected to the 7820 JD tractor, and the GPS was used to ensure the strips were in the exact place within the plot to allow for the maize planting to follow. The strip-tiller initially removed a section of residue approximately 6 in wide followed by an 8 inch deep ripping shank. A pre-plant application of urea ammonium nitrate (UAN 32 composed of 25% nitrate nitrogen (N), 25% ammonium N, and 50% urea) was made at 140.3 L ha−1, based on recommended standard practice, in all the plots through the center pivot overhead irrigation system, including the standard conventional plots in the rest of the field adjacent to the experimental plots. The strip-tilling was conducted on 25 May and 9 May, in 2021 and 2022, respectively.
The average cover crop biomass in each plot was estimated by taking four 0.25 m2 quadrat samples per plot on 21 May and 13 April, in 2021 and 2022, respectively. The quadrat was tossed randomly in each plot and the vegetation inside the quadrat was clipped at the soil surface and placed into paper bags. The samples were then dried in a forced-air drying oven at 60 °C for 72 h to a constant weight, weighed, and data were recorded. The biomass estimates were taken after and not prior to roller-crimping as we were more interested to know the amount of residue lying flat on the plots prior to strip-tilling and not actually the biomass of the erect plants prior to roller-crimping.
Liberty link® maize variety ‘P2089AML’ was planted on 25 May and 9 May, in 2021 and 2022, respectively. This is a glyphosate- and glufosinate-resistant variety from Pioneer Seeds®. The planting was conducted with a 6-row, 76.2-cm bed, JD 7300 Max Emerge 2 vacuum planter (John Deere and Co., Moline, IL, USA), using sprocket combo 35/27 and plate A50617 at a pressure of 10 psi to obtain a planting density of 78,455 seeds ha−1/ac (Figure 4). The seeds were planted approximately 6.35 cm deep and 17.5 cm apart. The planter had electronic sensors to monitor seed planting and an alert system for clogging of the planter. The planting was performed in the direction of the rolled cover crop to avoid the cover crop residue impeding and clogging the planter.
All fertilizer inputs and maize planting dates were the same for all the treatment plots and the adjacent conventional field. This comprised one application of 56.8 L at pre-plant, one application of 113.6 L as side dressing, and three fertigations through the center pivot system during the maize growing season at 93.5 L ha−1 for a total of 701.5 L ha−1 N in the entire season in the whole experimental area. These amounts were based on soil test recommendations.
All irrigations during the maize growing season were scheduled using crop evapotranspiration coefficients developed for maize by the University of California Cooperative Extension. Irrigation was routinely monitored to ensure accurate coverage and uniformity. Visual inspections were made to ensure adequate growth and crop stress levels.
One application of glyphosate (Roundup Powermax®) and glufosinate (Liberty®) at 3.22 and 3.95 L ha−1, respectively, were made on the 10 June and 14 June, in 2021 and 2022, respectively, to control weeds. The cover crop had already died at the time of herbicide application, so the herbicide had no effect on the percent kill of the cover crops by the roller-crimper. Evidence of this is provided in the results section describing cover crop kill. No other herbicide applications were made thereafter. The soil covered by the cover crop residues and presence of weeds in each plot were monitored bi-monthly until the maize canopy closed. The area of soil covered by the cover crop residue was visually evaluated on a scale of 0 to 100% (where 0 was bare soil with no cover crop residue in the entire plot; 100 was complete soil covered by the cover crop residue with no visible bare soil in the entire plot). Similarly, weed populations in the plots were also visually evaluated on a scale of 0 to 100% (where 0 was no weed control and complete cover of the plots by weeds; 100 was complete weed control with no visible weed presence in the entire plot). The percent cover visual estimates method was used because of the large size of each plot (182.9 × 4.6 m). Initial attempts of taking data estimates by random quadrat samples showed that the data could be very skewed because of the bare spots without soil cover and spots with presence of weeds being extremely patchy in the fields in both years. Each time the quadrat was thrown randomly in each plot, no bare spot or weed patches were caught in the sample because the areas not covered by the cover crop and areas with weed growth were very limited in each plot (Figure 5). However, two 0.25 m2 quadrat samples in each treatment plot were taken in areas that had weeds to assess the types of weed species present in the plots. Data on soil and weed cover were taken up to maize canopy closure only because it was difficult to enter the plots after canopy closure.
Maize was harvested on 1 September and 30 August in 2021 and 2022, respectively. Estimates of maize silage yield were made by taking maize ear and remaining entire plant weight in each plot in each replication. Since each plot had six rows of maize, samples were taken in the four inner rows with one maize row on either side as buffer rows. A random spot within each plot towards the inner section of the 182.9 m row was selected to take yield estimates from. This was performed to avoid variability in maize growth towards the edges of the plots. A 1/1000th of an acre (5.3 m long and 0.76 m wide, containing four rows of maize) area was harvested in each plot for yield estimation. The yield estimation was conducted on acre basis according to local grower practices using English units but converted to metric units for presentation. Stand counts were also taken in this area. Then, five random maize stalks from the selected area were cut at the soil surface for yield estimates. The sampled plants were labeled and tagged and brought outside the plots for weighing. The maize ear and stalks were put into a large plastic bucket and their fresh weights were recorded on a digital scale. A similar data collection operation was also conducted in four random areas of the adjacent conventional standard field for non-statistical comparative purposes, as mentioned earlier.
2.4. Data Analysis
Data were analyzed using SAS v. 9.2 (SAS Institute Inc., Cary, NC, USA). Assumptions of analysis of variance (ANOVA) were tested by Shapiro Wilk’s test for normality and Bartlett’s test for homogeneity of variance at a 0.05 level of significance. Data for all the parameters met the assumptions of ANOVA and therefore needed no transformations. The data were then analyzed by using the general linear model (PROC GLM) of SAS. An alpha level of 0.05 was set to determine significant differences. Replications and years were considered as random effects and cover crop treatments as fixed effect, and interactions between years and treatments were assessed. Since there was no interaction between year and treatments, the data from the two seasons were combined and reanalyzed. When the ANOVA indicated significant differences between treatments, the means were separated using Fisher’s Least Significant Difference (LSD) test at a 0.05 level of significance.
3. Results and Discussion
3.1. Findings of the Preliminary Studies
The preliminary study showed that all of the treatments, except the mixture of ‘Trophy’ rape, ‘Tillage radish’, and fava beans, had more than 95% kill with the roller-crimper. The ‘Trophy’ rape, ‘Tillage radish’, and Phacelia mixture had less than 60% kill at mid-July. The two Brassica species (‘Trophy’ radish and ‘Tillage radish’) continued to grow and thus were not suitable for roller-crimping. The other species mixtures did not have regrowth and were mostly terminated by the roller-crimper. As mentioned earlier, the other issue was the patchiness and uneven stands of the species in the plots that had a mixture of small- and large-seeded cover crop species that caused unevenness in the release of the seeds from the planter during the seeding process. Figure 6 shows the comparison between the treatment plots, which shows the Brassica plots still alive while the other treatments were not. Based on these results and experiences, the cover crop treatments (mentioned in the materials and methods section) for the main experiments were selected. Hereafter, all the results are from the two-year main experiment.
3.2. Cover Crop Stand Establishment
In both years, as mentioned before, data on cover crop establishment were difficult to collect because of the high diversity of species in the mixed species treatments and the challenge of recognizing the species. Therefore, only visual estimates were taken that showed excellent seedling emergence and stand establishment in all the plots. The rye-alone treatment had excellent (>90% of visually estimated ground cover) stands that were maintained until the soft-dough stage at roller-crimping. Although the ultra-high diversity mix contained 40 different plant species, only five or six species (these included peas, cereal grasses, beans, vetches, and clovers) dominated the plots. The emergence and stands of all five species in the multiplex cover crop mix was also excellent (>90% of visually estimated combined ground cover by the species until roller-crimping). The fava bean + phacelia treatment was the slowest to emerge and cover the ground. However, after emergence, the growth of both the species was rapid and resulted in excellent stands (>90% of visually estimated ground cover) that was maintained until the time of roller-crimping. In the rye, field pea, and purple vetch treatment plots, the emergence, stand establishment, and ground cover by all three species was also excellent (>90% of visually estimated combined ground cover). Although data were not taken, among the three species, the density of rye seemed more than the other two species, probably because of the erect and competitive nature of this species compared to the legumes (field pea and purple vetch). The statements on excellent stands that were obtained and maintained throughout the growing season in this study can be corroborated by the picture after the roller-crimping process (Figure 7).
3.3. Cover Crop Termination by the Roller-Crimper
One pass of the roller-crimper was successful in terminating the cover crops in all the plots with almost no regrowth at all in both years of the study. No herbicides or any other interventions were needed. Data are presented as late May, early June, early July, and late July instead of the exact evaluation dates because of a few days’ difference in the evaluation days between the two years. The last evaluation taken prior to maize canopy closure in mid-July of both years showed a greater than 90% kill of the cover crops in both years of the study (Figure 8). The only difference between the treatments was in late May, in which the ultra-high diversity mix treatments plots showed some upright plants, which were mostly the Brassica species. However, no differences in the cover crop kill were noticed thereafter. The success in the termination of the cover crops with no regrowth can also be observed (Figure 9). Brassica species cover crops were difficult to terminate in a preliminary study as well. Although one pass of the roller-crimper was sufficient for cover crop termination, other studies have shown variable results with one pass. For example, a rye + crimson clover cover (Trifolium incarnatum) cover crop had to be rolled three times [20]. The importance of the timing of the rolling has been emphasized to avoid cover crop regrowth. For example, the appropriate time for successful rolling of a cereal rye cover crop is reported to be between 50% anthesis and early milk stages (corresponding to 10.1–10.3 on the Feekes’ scale) [14]. However, determining the appropriate time to roller-crimp a multiple-species cover crop system can be difficult. In this study, most of the species in the mixed cover crop treatments were at flowering or seeding stage, and roller-crimping at this stage was successful in both years. Since all the treatments were roller-crimped at the same time, this corresponded to the soft-dough stage of the rye-alone treatment. A study in California reported failure of the main crop due to continued growth and competition from the roller-crimped cover crops, but the study did not provide information on the growth stage of the cover crop at the time of the roller-crimping [16]. Moreover, various cover crop species were used in that experiment. A study in Italy reported that flaming in combination with rolling provided better cover crop kill than roller-crimping alone [21]. Flaming in a semi-arid environment in Fresno would perhaps not be suitable because of fire hazards. In another study in Spain, it was reported that roller-crimping supplemented with postemergence herbicide provided better cover crop kill than roller-crimping alone [22]. However, in the current study, roller-crimping alone was sufficient for the successful termination of the cover crops.
3.4. Percent Soil Cover by the Cover Crop Residue
The cover crop biomass provided 30 to 70% soil cover until mid-July in both years of the study (Figure 10). Similar to the cover crop kill estimation, data were not taken after maize canopy closure. Furthermore, data are presented as late May, early June, early July, and late July instead of the exact evaluation dates, as mentioned earlier. The percentage of soil covered by the cover crop residue was greatest in the rye plots and least in the fava bean + phacelia plots in mid-July, indicating that residues in the latter treatment disintegrated rapidly (Figure 9 and Figure 10). At this evaluation time, soil cover by the rye residue was greater than 90%. The percent soil cover from the residues in the other cover crop treatments ranged from 70 to 80% while it was approximately 30% in the fava bean + phacelia plots.
3.5. Cover Crop Biomass
All the treatments, except for the ultra-high diversity mix, had similar aboveground dry biomass (Figure 11). The aboveground biomass in the four treatments ranged from 9.4 to 14.8 mt ha−1, whereas that of the ultra-high diversity mix was about 7.2 mt/ac. Therefore, rye-alone had double the dry biomass compared to the ultra-high diversity mix. Data from other northern US states show a lower amount of cover crop rye aboveground biomass. For example, rye dry biomass yield of approximately 4 mt ha−1 was reported in a study in southern Illinois [23], approximately 2.9 mt ha−1 in Nebraska [24], and up to 9.9 mt ha−1 in Minnesota, USA [25]. However, a study in California [26] reported that winter rye cover crop produced up to 17.3 mt ha−1 of shoot biomass. There are multiple benefits of having high cover crop biomass because it provides an estimate of the potential amount of C material that can benefit soil health [27].
3.6. Weed Cover
The cover crop residue provided excellent weed control. Although, in late May, the fava bean + phacelia treatment plots had a higher weed cover than the other treatments, there was no difference in weed cover between the treatments thereafter (Figure 12). In late May, approximately 18% of the fava bean + phacelia plots were covered with weeds compared to less than 5% in the other treatments. This was probably because of less soil cover in the fava bean + phacelia plots in late May. The fava bean + phacelia residues disintegrated more rapidly than the other treatments in both years. In most treatments, weed cover was less than 5% until mid-July. The herbicide applications made in early June in both years also helped in suppressing the weeds and volunteer cover crop plants. Although data were not taken, the reseeding seemed to be of rye and barley. Evaluations taken in December of both years showed that the major weed species, other than volunteer cover crops, were burning nettle (Utrica urens), henbit (Lamium amplexicaule), shepherd’s purse (Capsella bursa-pastoris), common mallow (Malva parviflora), annual sowthistle (Sonchus oleraceus), common groundsel (Senecio vulgaris), and lesser swinecress (Lepidium didymum). These are all winter annual weed species common to the area. Their presence was less than 3 on average per square yard.
A study in California showed that cover crops followed by a cash crop had less than 10% weeds while the no cover crop had more than 40% [28]. A study in Spain reported that roller-crimping compared to shredding of the cover crops reduced bermudagrass (Cynodon dactylon) cover in the treatment plots in a vineyard [17]. In an organic vegetable production study in Denmark, roller-crimping of cover crops reduced weed growth by 63% compared to that by full incorporation of the residue by tillage [29]. The types of weed species present and the amount of cover crop residue generated could also influence the success of the roller-crimping in controlling weeds [30,31]. The high amount of cover crop biomass produced in this study and the soil cover provided by the roller-crimped cover crop may be the reason for low numbers of weed emergence and weed cover.
3.7. Silage Maize Yield
Average (2020/21 and 2021/22) silage maize yield was similar between all the treatments and ranged from approximately 60.5 to 71.7 mt ha−1 (Figure 13). Although statistical comparisons cannot be made with the silage maize yield grown with standard practices in the adjacent field as it was not a part of the experimental design, data collected from several areas of the standard plots showed that the cover plots had similar, if not higher, yields than the standard plots. The stand establishment was also observed to be excellent. In contrast, a study in Nebraska reported that maize grain yield was 78% lower in roller-crimped cover plots than in standard tillage plots with no cover crops [32]. Similarly, studies also reported lower grain maize yield [22] and sweet maize yield [33] in roller-crimped cover crop plots compared to other treatments and they attributed the reason to ineffective cover crop kill with the roller-crimper. A study suggested that while there were consistent yields of no-till planting soybean (Glycine max) into roller-crimped rye cover crops, maize yield was reduced because of constraints of fertility management [19]. However, in this present study, fertilizers were applied by ground and fertigation through the overhead irrigation system.
4. Conclusions
This study showed that roller-crimped cover crops could be used successfully in strip-till silage maize systems under overhead irrigation in the Central Valley of California. The cover crops were successfully terminated by one pass of the roller-crimper when rolled at the soft-dough stage of the rye cover crop. No cover crop regrowth occurred. The cover crop residue biomass provided more than 90% soil cover that resulted in more than 90% weed control until maize canopy closure. Although a postemergence herbicide application was made in early June, the weed pressure was very low at herbicide application. Maize silage yields were similar in all the treatments and comparable to a standard maize silage production system. Therefore, this winter cover crop roller-crimper terminated system has the potential to add organic matter to the soil in the form of cover crop residue, provide good weed control, and reduce herbicide need. Although comparisons were not made, this system may provide benefits such as reduced labor savings, tractor equipment repair savings, C emission reductions, less dust, storage of more C in the soil, and soil health improvement, which need to be explored in future studies.
Conceptualization, R.W.; Methodology, J.P.M., and A.S.; Formal analysis, A.S.; Investigation, R.W., J.V.-H., J.P.M., and A.S.; Resources, R.W. and J.P.M.; Data curation, R.W. and A.S.; Writing—original draft, R.W.; Writing—review and editing, A.S.; Visualization, J.P.M.; Supervision, A.S.; Project administration, R.W.; Funding acquisition, J.P.M. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.
We thank the Fresno State agricultural laboratory staff for the facilities and technical assistance. We also thank Lynn Sosnoskie, Cornell University, for her assistance with the project during the preliminary studies.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Average monthly precipitation (in mm) during the experimental period in 2020/2021 and 2021/2022.
Enlarge this image.Figure 2. Average monthly maximum and minimum temperatures (in degree Celsius) during the experimental period in 2020/2021 and 2021/2022.
Enlarge this image.Figure 3. Termination of the cover crops in the treatment plots with a 15 ft wide rear-mounted roller-crimper.
Enlarge this image.Figure 4. Maize planting in the strip-tilled rows with a GPS equipped John Deere 7300 Max Emerge 2 vacuum planter® (John Deere and Co., Moline, IL, USA).
Enlarge this image.Figure 5. View of the treatment plots showing the very patchy bare spots and weed populations in the inter-row spaces.
Enlarge this image.Figure 6. Treatment plots in mid-June in the preliminary study. ‘Trophy’ rape + ‘Tillage’ radish + phacelia (top left); bell bean + pea (top right); rye + purple vetch + bell bean (bottom left); multiplex mix (bottom right). (Photo: Lynn Sosnoskie).
Enlarge this image.Figure 7. A picture of the plots taken on 4 April 2021 immediately after roller-crimping.
Enlarge this image.Figure 8. Percent kill of the cover crops in the treatments after termination with the roller-crimper (average of 2020/21 and 2021/22) in different times of the season. Bars with the same letters at each evaluation time are not significantly different according to the Fisher’s Least Significant Difference (LSD) test at 0.05 level.
Enlarge this image.Figure 9. Visuals of the kill of the cover crops after termination with the roller-crimper. Picture taken on 6 June 2021 (left) and on 29 June 2021 (right).
Enlarge this image.Figure 10. Percent soil cover from the cover crop residues in the treatments after termination with the roller-crimper (average of 2020/21 and 2021/22) in different times of the season. Bars with the same letters for each evaluation time are not significantly different according to the Fisher’s Least Significant Difference (LSD) test at 0.05 level.
Enlarge this image.Figure 11. Average (2020/21 and 2021/22) aboveground dry biomass (±SE) in the treatments after cover crop termination. Bars with the same letters for each evaluation time are not significantly different according to the Fisher’s Least Significant Difference (LSD) test at 0.05 level.
Enlarge this image.Figure 12. Percent weed in the treatment plots after termination with the roller-crimper (average of 2020/21 and 2021/22) in different times of the season. Bars with the same letters for each evaluation time are not significantly different according to the Fisher’s Least Significant Difference (LSD) test at 0.05 level.
Enlarge this image.Figure 13. Silage maize yield (average of 2020/21 and 2021/22 in metric tons per hectare) in the different treatments. Silage maize yield from four random areas of the adjacent standard conventional field is also presented for comparative purposes. There were no differences between the treatments at a 0.05 level of significance.
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1 Department of Plant Science, California State University, 2415 E. San Ramon Ave. M/S AS72, Fresno, CA 93740, USA
2 Department of Plant Sciences, University of California, Davis, CA 95616, USA