The cover crop termination choice to designing sustainable cropping systems

European Journal of Agronomy 114 (2020) 126000

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European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja

The cover crop termination choice to designing sustainable cropping systems

T

María Alonso-Ayusoa,b, José Luis Gabrielb,d, Chiquinquirá Hontoriaa, Miguel Ángel Ibáñezc, Miguel Quemadaa,b,* a

Dep. Agricultural Production, Universidad Politécnica de Madrid, Avda. Complutense S/N, Madrid, 28040, Spain CEIGRAM, Universidad Politécnica de Madrid, Avda. Complutense S/N, Madrid, 28040, Spain c Dep. Agricultural Economics, Statistics and Management, Universidad Politécnica de Madrid, Avda. Complutense S/N, Madrid, 28040, Spain d Departamento de Medio ambiente, Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA-INAGEA), Ctra. de la Coruña, Km., 7,5. Madrid, 28040, Spain b

A R T I C LE I N FO

A B S T R A C T

Keywords: Barley Vetch Glyphosate Sustainability Energy consumption Roller-crimper Corn

The use of winter cover crops (CCs) in annual rotations is a tool to increase agroecosystem sustainability. To optimize their benefits, the choice of how to terminate CCs in spring is crucial, because this decision may lead to differences in soil–plant variables and affect cash crop productivity. The use of a roller-crimper is an emerging option in both conservation and organic agriculture but its adoption in Mediterranean regions is still rare. In this work the objective was to identify best CC termination practices to maximize benefits and increase agro-ecosystem resilience. To this end a field experiment with a barley/vetch (Hordeum vulgare L. / Vicia villosa L.) CC mixture followed by an irrigated corn (Zea mays L.) was conducted in Central Spain in three consecutive years. Each year, three CC termination methods (roller-crimper, glyphosate + roller-crimper, and CC residue incorporation) and four post-emergence operations to reduce weed pressure in summer (post-emergence herbicide, inter-row cultivator, the combination of the two, or without operation) were compared. The effect on the spring soil water content and temperature, weed control, soil inorganic N and the corn grain yield and N content were evaluated. Energy and economic analyses were conducted. An ineffective CC termination by roller-crimper was overcome when using glyphosate or post-emergence herbicides. However, the roller-crimper was less dependent on post-emergence operations than residue incorporation to achieve proper weed control and attain good productivity. The results showed that in early corn growth stages, roller-crimper use enhanced the soil’s water conservation and decreased soil temperature compared to CC residue incorporation. Moreover, the energy cost was lower for roller-crimper termination. Our findings suggest that the roller-crimper increases the environmental sustainability in Mediterranean regions, but farmers may encounter economic risks. Further research is needed to find proper conditions that maximize the potential of this CC termination method under Mediterranean conditions.

1. Introduction

(Sainju and Singh, 2001) or be left on the soil surface as a mulch, common in no-till systems (Quemada et al., 1997). In these systems, the use of burndown herbicides such as glyphosate is a common practice (Kornecki et al., 2009). Despite its effectiveness for termination, this herbicide has been in the spotlight in the European Union since 2016 (Silva et al., 2018) and therefore nonchemical alternatives, which will also make the method suitable for organic farming systems, should be identified. An alternative is the use of the roller-crimper (Kornecki et al., 2006). Compared to mowing, residues are more uniformly extended after roll-crimping and lasts longer, so more benefits in terms of weed suppression and soil water conservation are expected (Creamer

The use of cover crops (CCs) has increased over the last few decades since they are considered a tool to increase sustainability in annual rotations when substituting winter fallow (Gabriel et al., 2012; Schipanski et al., 2014). Together with the species choice and the termination date, the CC termination method in spring is one of the most important management decisions that can lead to differences in soil moisture, weed suppression, N dynamics and cash crop productivity (Alonso-Ayuso et al., 2014; Wayman et al., 2015; Bavougian and Read, 2018). The CC residue can either be incorporated into the soil by tillage

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Corresponding author at: Dep. Agricultural Production, Universidad Politécnica de Madrid, Avda. Complutense S/N, Madrid, 28040, Spain. E-mail address: [email protected] (M. Quemada).

https://doi.org/10.1016/j.eja.2020.126000 Received 15 July 2019; Received in revised form 23 December 2019; Accepted 6 January 2020 1161-0301/ © 2020 Elsevier B.V. All rights reserved.

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(CR23X, Campbell Scientific, Logan, Utah, USA). From 1985–2015, historical daily precipitation and temperature time series were taken from the Spain02 database (Herrera et al., 2016), which includes a large database of meteorological data for the Spanish territory (Fig. S1). The CC mixture of barley (Hordeum vulgare L., cv. Hispanic. P1000 seeds=47 g; seeding rate=48.9 kg ha−1) and hairy vetch (Vicia villosa L. cv Vereda. P1000 seeds=31 g; seeding rate=48.4 kg ha−1) was sown in early October. The seedbed preparation consisted in plowing once with a cultivator (∼20 cm) and a roller pass after sowing. Due to scarce rainfall, 20 mm of irrigation water was applied to ensure CC establishment before sowing in 2017. Cover crops did not receive fertilizer or irrigation during their growing period. The termination method and post-emergence weed operations were the factors studied, using a split-plot randomized block design with four replications. In spring, the experiment was divided into 12 plots (12 m × 12 m) where the main factor – the termination method – was located. After corn seeding, each of the 12 plots was split into four subplots (6 m × 6 m), in which the post-emergence operation was tested, so the total number of subplots studied was 48. By mid-April, when the barley was at the middle of the heading stage (GS-55, decimal scale; Lancashire et al., 1991) and the vetch at the elongation stem stage (GS-35), depending on the treatment, CCs were terminated by: a) roller-crimper pass (ROL), b) glyphosate application + roller-crimper (RGL) or c) mowing and soil incorporation (INC). Glyphosate was applied at a rate of 1.07 kg a.e. ha−1 1 week before the other operations, in early April. The roller-crimper drum, with a 0.55-m external diameter, 2.5 m wide and filled with water, weighed 1.4 Mg. An additional 0.5 Mg was added to the roller in the form of iron counterweights, making the total weight of the equipment 1.9 Mg. Cover crops were roll-crimped with two passes. For the INC treatment, the CC was chopped with a hammer mower and then incorporated into the soil with a disc harrow to a 20 cm depth. A few days after the CC termination, around April 20th, the corn was direct sowed (FAO 700 cycle; seeding rate = 80,000 plants ha−1). From mid-May, irrigation water was delivered by sprinklers until the first half of September. At the end of April 2017 and 2018, a single irrigation was needed to break the soil crust that jeopardized corn emergence, present in the plots where the residue was incorporated. The irrigation schedule and doses were estimated from the daily values of crop evapotranspiration (Allen et al., 1998) using daily local data. The post-emergence treatments established in the subplots were: i) post-emergence herbicide (h), ii) inter-row cultivator (c), iii) postemergence herbicide + inter-row cultivator (hc) y, iv) without a postemergence treatment (0). The post-emergence herbicide (nicosulfuron, 1.5 L ha−1 + mesotrione 1 L ha−1) was applied (in h and hc) to the corn (GS-15) in the second half of May. Later, in the first half of June, inter-row cultivator tillage was performed (in the c and hc subplots). Ammonium nitrosulphate was hand-broadcasted for the entire experiment, split into two applications when the corn had three and eight fully extended leaves. The N rate (160, 150 and 200 kg N ha−1 in 2016, 2017 and 2018, respectively) was calculated by correcting the expected corn N uptake (∼280 kg N ha−1, Quemada et al., 2014) by the soil inorganic N content up to 0.6 m present prior to corn sowing, the estimated soil N mineralization in nonfertilized plots (70 kg N ha−1, Gabriel and Quemada, 2011) and CC residue mineralization, that was estimated as half of the N content in the CC aboveground biomass at termination (Schomberg and Endale, 2004). Available P (Olsen) and K (extracted with acetate) were determined each year, and in 2016 all the plots received 30 kg P ha−1 and 100 kg K ha−1 before sowing to ensure availability.

and Dabney, 2002; Davis, 2010; Navarro-Miró et al., 2019). However, a number of challenges related to the roller-crimper use have been identified as the termination effectiveness, which is very dependent on the species and the phenological stage, and incomplete termination may lead to cash crop yield losses (Mirsky et al., 2011; Peigné et al., 2015). To ensure proper termination, an advanced CC stage is required, but this can mean a thick residue mulch, a decrease in the soil temperature and therefore the delay of cash crop establishment (Luna et al., 2012). Moreover, in semiarid regions, a late termination could lead to pre-emptive competition with the cash crop (Alonso-Ayuso et al., 2014). The glyphosate + roller-crimper combination was proposed to overcome some of these challenges (Kornecki et al., 2009), but our understanding the overall roller-crimper effect needs to be improved to decide if it is worth implementing in a particular agroecosystem. Replacing fallows by CCs and the management practices selected for their establishment and termination could lead to differing energy consumption because the soil disturbance operations and inputs applied will vary (Canali et al., 2013). Furthermore, it is essential to evaluate the economic sustainability of management practices, as costs are considered one of the major barriers to CC adoption (Arbuckle and Roesch-McNally, 2015) and the inclusion of post-emergence operations as additional tillage or herbicide application, may have an effect that should be considered. Therefore, energy and economic analyses of rotations including CCs are needed to identify the best management practices. Roller-crimper adoption in the Mediterranean region is still rare. In Italy, diverse experiments were conducted to evaluate the termination effectiveness of different methods (Frasconi et al., 2019) or to study their impact on the yield and quality of organic vegetables, N dynamics and weed control (Canali et al., 2015; Ciaccia et al., 2016). Some studies included comparisons in energy consumption (Canali et al., 2013; Diacono et al., 2017). However, in Mediterranean regions, more field experiments assessing CC termination methods are needed given the high year-to-year weather variability, and the variety of soils involved. In addition, pressure is rising for agriculture less dependent on chemical inputs and the roller-crimper’s effectiveness remains a challenge (Peigné et al., 2015). Moreover, the interaction of CC termination with different post-emergence strategies, either chemical or mechanical, to reduce weed pressure has not been studied yet. To identify best CC termination practices to maximize benefits and increase agro-ecosystem resilience, a field experiment was conducted in three consecutive years in central Spain, with irrigated corn as the cash crop. The effect of different termination methods (i.e., residue incorporation vs. roller-crimper vs. glyphosate + roller-crimper) combined with different post-emergence operations (inter-row cultivator pass and/or post-emergence herbicide) was evaluated on corn grain yield and quality, weed control, soil N, soil temperature and water content in the soil–plant system. Energy consumption, costs and net benefits were also compared to broaden sustainability evaluation. 2. Material and methods 2.1. Experimental design The field experiment was conducted in three consecutive years (2015/2016; 2016/2017; 2017/2018) at the “La Chimenea” research site (Aranjuez, Central Spain: 40°04′03.9″N and 3°32′26.3″W, 550 m.a.s.l.). The experiments were conducted in adjacent fields separated by < 50 m and located over the same soil type. Prior to the experiments, the soil was fallow for more than 1 year. The soil is classified as Typic Calcixerept (Soil Survey Staff, 2014), with a loam topsoil, highly calcareous and with moderate soil organic matter content (pH ∼8.2; organic C: 1.1 g kg−1). Further detail on the soil characteristics can be found in Gabriel et al. (2010). The climate in the area is cold semi-arid (BSk), according to the Köppen classification. Weather variables were recorded throughout the experimental period with a weather station

2.2. Measurements in CC: ground cover, cumulated biomass and N content Each year, the CC ground cover was monitored from October to April. Every 2–3 weeks, images were acquired from a nadir perspective at a 1.5 m height in eight points, randomly selected at the beginning of 2

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dried and weighed to determine the dry matter grain yield and to determine N concentration. For each subplot, the N content of each plant component was calculated as the product of the dry biomass times the N concentration. The total N content in the corn was obtained by adding the N content of both plant components.

the season; and the ground cover was determined by digital image analysis techniques (Ramírez-García et al., 2012). Before CC termination, the aboveground biomass was determined by hand harvesting the biomass in eight randomized 0.5 m x 0.5 m squares. In the laboratory, barley and vetch biomass were separated, oven-dried (60 °C, 48 h) and weighed. The C and N concentration in barley and vetch subsamples was determined using the Dumas combustion method (TruMac CN, Leco Instruments, St. Joseph, MI, USA). The total N content in the CC aboveground biomass was calculated by multiplying the dry matter collected in the sampled squares by the corresponding N concentration, and converted to kg per ha bases. In September, the CC residue remaining on the surface was determined by collecting the residue biomass (0.5 m x 0.5 m subplot−1) in ROL and RGL treatments. The residue was oven-dried and weighed.

2.6. Energy consumption To compare the energy consumption between treatments, the fuel consumed in the CC termination and post-emergence operations was considered, but the energy consumed in common operations (CC establishment, corn sowing, irrigation, fertilization and corn harvest) was excluded. As indirect inputs, the energy required for the manufacturing of herbicides was included, but the energy required to manufacture common inputs (fertilizers and seeds) was excluded. Likewise, the embedded energy in farm machinery (manufacturing and transport) and the human labor energy were not considered. The summed energy values were compared between CC treatments, and the different proportions of the fuel consumption and the proportion of the energy derived from herbicide manufacturing were noted. In addition, the energy productivity was calculated as the ratio between the corn grain yield at harvest (3 years x 4 replicates = 12 values per combined CC treatment) and the energy consumption for each CC treatment (1 value per combined treatment). Calculations were made using energy reference values (Table S1).

2.3. Spring soil water conservation, soil temperature and weed control benefits At late April in 2016 and 2017, and early May in 2018, the volumetric soil water content (SWC) and soil temperature were measured by inserting in situ a hand-held probe (Decagon 5TM®, Decagon Devices, Pullman, WA, USA) in the topsoil layer (7 cm deep), previously calibrated in the laboratory. Four measurements per block were taken and the average value was considered. At the end of June, weed density was determined to assess the impact of the CC treatments. In two 0.5 m × 0.5 m squares per plot, weeds were counted and identified, and the mean value was calculated for each subplot.

2.7. Economic analysis

2.4. Soil inorganic N

An economic analysis was performed to assess the farm profitability of CC treatments. First, specific costs were compared between treatments. Therefore, similar to the energy analysis, only costs derived from CC termination, post-emergence operations and herbicide inputs were considered. These costs were summed and compared between CC treatments. Second, stochastic net benefits (€ ha−1) were calculated following the expression:

Each year, the soil inorganic N (ammonium + nitrate) was determined in April before CC termination, and in October after the corn harvest. Soil samples were taken with a helicoidal auger. In April, six samples were randomly taken from the experimental field 60 cm deep, at 20-cm intervals. After corn harvest, the sampling was performed for the main treatments (ROL, RGL and INC) in the subplots corresponding to the post-emergence herbicide (h) and herbicide + cultivator (hc) in order to assess differences in the soil inorganic N between the cultivator pass tillage and without it. In this case, two soil cores per subplot were taken 60 cm deep (at 20-cm intervals) and combined per depth. In the field, samples were placed in a cooler and within the 2–3 days in the laboratory a subsample was extracted with 1 M KCl, centrifuged and decanted. The supernatant volume was stored at −20 °C until analysis. Nitrate concentration was determined using the Griess-Illosvay method after reduction of NO3− to NO2− with a Cd column (Keeney and Nelson, 1982), and NH4+ using the salicylate-hypochlorite method (Crooke and Simpson, 1971).

π = grain yield (1) × corn price (2) − cost (3) where: (1) is the stochastic corn grain yield, (2) the grain price paid to the farmer and (3) the total cost for each treatment. For each treatment, the 12 grain yield values (3 years × 4 replicates) were adjusted to a distribution function (@Risk®, Palisade Corporation, v. 7.6). The adjustment to the Gamma function was selected based on the χ2 criteria and a truncation at 0 kg ha−1 as the absolute minimum yield. Then Monte-Carlo simulations were applied to obtain variation probability functions. The corn grain price used was the average of prices observed in Spain from January 2017 to June 2018 (174€ Mg−1, CV < 3.5 %, Spanish Ministry of Agriculture, 2019). To calculate the stochastic net benefits, the average cost of the three years was used for each treatment. Here, the total cost includes machinery use (fuel consumption) of all operations, but does not include irrigation (water and power), labor wages or costs derived from transport operations. The cost of seeds, fertilizer and herbicides applied were considered. In the calculation, the average cost between the 3 years was used for each treatment. In Table S1, the different costs and corresponding references are summarized. A generalized lineal model with a Gamma distribution was performed and values were adjusted to one distribution function, assuming the same dispersion parameter for all treatments, and thus treatment means were compared.

2.5. Corn N status, grain yield and grain N content The corn N status at the flowering phenological stage (GS-65) was evaluated with the leaf clip sensor Dualex® Scientific (Force-A, Orsay, France), which measures chlorophyll activity (Chl). In each subplot, the main value of Chl was obtained as the average of measurements made in seven random plants per plot. The harvest index (= grain/ (grain + rest of the aboveground biomass)) was calculated two days before the corn harvest. A 1-m strip next to the central row in one subplot of each treatment was handharvested and separated into plant components (grain vs. rest of aboveground biomass). The rest of aboveground biomass (leaves and stalks) was grinded with a bio waste shredder machine. A subsample of each component was oven-dried and weighed. Total N concentration was determined by Dumas method in a subsample of the rest of aboveground biomass. At harvest, two 4-m strips of the central rows in each subplot were harvested with an experimental combine and the fresh grain yield was weighed in the field. A grain subsample was oven-

2.8. Statistical analysis Each of the variables studied were estimated at a different time in the experiment, as follows: 1) prior to the establishment of the main factor plots (CC biomass and N content; determined in early April), 2) prior to the subplot establishment corresponding to the post-emergence 3

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operations factor (SWC and soil temperature; determined in late April or early May), and 3) after the establishment of the subplots (weed density, chlorophyll activity, corn grain yield and N content, soil inorganic N, net benefit and energy productivity; determined after June). Therefore, three statistical models were used in this study. For the first case, we used a linear model to determine differences between the two CC species in the mixture prior to termination. In this analysis, the CC species (barley / vetch) was considered a fixed effect, while the environmental variability was the only random effect. For the second case, a linear mixed model was used. The termination method and the block were considered fixed effects (Dixon, 2016), and the year and the interaction between the termination method and the year were considered random effects. For the third case, we used a linear mixed model in which the termination method, the post-emergence operations and their interaction were considered fixed effects, while the subplot, the year, and the interaction between the year and the factors were considered random effects. In the second and third cases, years were analyzed separately when an interaction between the year and the termination method factor was significant. In that case, only the subplot was considered a random effect. Data were transformed when necessary to meet normality and homoscedasticity assumptions. Tukey post-hoc tests were used to separate means (P≤0.05). To estimate the net benefit means, a generalized linear mixed model adjusted to a family function Gamma was used. Analyses were performed with the software R version 3.5.1 (R Core Team, 2018).

Fig. 1. Volumetric soil water content (bars, m3 m−3) and soil temperature (black dots, °C) measured in late April 2016 and 2017, and early May 2018, before the start of the corn irrigation, for each of the termination methods treatments: ROL = roller-crimper; RGL = glyphosate + roller-crimper; INC = residue incorporation. The SE is indicated over each bar and dot. For each year, uppercase letters indicate significant differences in soil water content between termination treatments, and lowercase letters indicate significant differences in soil temperature (Tukey test, P ≤ 0.05).

led to a higher N content than the vetch. Five months after CC termination, the residue biomass remaining at the soil surface was ∼45 % of the amount in spring 3.4 Mg ha−1). 3.3. Spring soil water conservation, soil temperature and weed control benefits

3. Results 3.1. Weather conditions

The interaction between the termination method and the year was significant for the soil water content (SWC) and soil temperature variables. Therefore, years were analyzed separately (Fig. 1). Prior to corn irrigation, differences in topsoil SWC and temperature were observed between termination treatments. Each year, the treatments with the CC residue on the soil surface (ROL and RGL) showed a greater SWC when compared to INC treatment. No differences were observed between the roller-crimper treatment including glyphosate and the treatment without it. Regarding soil temperature, INC showed a higher surface temperature than ROL and RGL in 2016 and 2017. No differences in soil temperature were observed between treatments in 2018. From 2016–2018, up to 27 weed species were identified at the sampling date in June (Table S2). The species more representative at this time (each one had a relative abundance > 15 %, at least 2 of the 3 years) were Amaranthus sp., Convolvulus arvensis L., Polygonum aviculare L. and Portulaca oleracea L. For the June weed density, the interaction between the termination method and the year was significant, so years were analyzed separately. For each year, the interaction between both factors was significant (Fig. 2). In 2016, within the ROL termination group, the combination of cultivator and herbicide (ROL_hc) resulted in a lower weed density than the ROL_h (Fig. 2A). In RGL plots, post-emergence treatments including herbicide had a lower weed density than the other treatments. Within the INC group, the combination of herbicide and cultivator had a lower density than the herbicide and cultivator treatments independently, which were in turn lower than INC_0. In 2017 and 2018, no differences were observed

During the first two experimental years the annual rainfall was lower (325 and 303 mm, respectively) than the average rainfall of the 1985–2015 period at the same location (376 mm), while the third year (2017–2018) the annual rainfall (407 mm) was higher (Fig. S1, Supplementary Material). In addition, the rainfall distribution varied between years: the 1 st year was characterized by a dry autumn (38 mm in October–December) and a wet spring (187 mm in March–May), while conversely the 2nd year had a rainy autumn (159 mm) but an extremely dry spring (44 mm). The third experimental year, autumn rainfall was within the average (59 mm), but spring was rainy (194 mm, with > 100 mm in March). Regarding spring temperatures, the first and third year had lower temperatures (i.e. 16.1 °C and 16.4 °C in May 2016 and 2018 respectively) than the mean of the 1985–2015 period (18.0 °C), whereas the 2nd year, spring was warmer (i.e. 20 °C in May 2017). 3.2. Measurements in CC: ground cover, cumulated biomass and N content 30 % of ground cover occurred 107, 30 and 71 days after CC sowing in 2016, 2017 and 2018, respectively. Prior to termination, ground cover by the CC was > 80 % (Table 1). Barley accumulated a greater biomass than the vetch in the CC mixture, each year: barley biomass accounted for 92 %, 94 % and 86 % of the CC mixture at termination, in 2016, 2017 and 2018 respectively. The N concentration was higher in the legume than in the grass. Greater biomass accumulation by barley

Table 1 Cover crop growth metrics for each year: number of days after CC sowing at which 30 % ground cover was achieved; percent ground cover by CC prior to termination; aboveground biomass (kg DM ha−1); N concentration (%) and N content (kg N ha−1) of each CC species in the mixture at the termination date. For each year and variable, different letters indicate significant differences between CC species in the mixture (Tukey test, P≤0.05). Days after sowing: 30 % of the soil covered year 2016 2017 2018

107 30 71

Ground cover

Biomass (kg DM ha−1)

N concentration (%)

N content (kg N ha−1)

(%) prior to termination

barley

vetch

barley

vetch

barley

vetch

84.5 95.5 97.4

4369.6 b 7111.2 b 6139.8 b

398.4 a 467.0 a 1298.4 a

2.98 a 2.90 a 1.68 a

3.77 b 3.54 b 3.94 b

124.0 b 209.3 b 102.7 b

15.2 a 16.3 a 50.6 a

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Fig. 2. Weed density (plants m−2) observed in June for the treatments resulting from the termination methods (ROL = roller-crimper; RGL = glyphosate + rollercrimper; INC = residue incorporation) through post-emergence operations (h = post-emergence herbicide; c = inter-row cultivator; hc = herbicide + cultivator; 0 = with no post-emergence operation). The SE is indicated over each bar. For each year and termination factor, different letters indicate significant differences between post-emergence treatments (Tukey test, P ≤ 0.05).

was not significant, so data were analyzed together (Fig. S2). Chlorophyll activity was similar for the different post-emergence treatments within the ROL and RGL termination methods; whereas for the INC termination method all the treatments that received a postemergence operation had a higher chlorophyll value than the treatment without a post-emergence operation (Fig. S2). For most of the corn variables determined at harvest, a significant interaction between the termination factor and the year was observed, and therefore years were analyzed separately. Besides, for two of the three years (2016 and 2017), a significant interaction between the two factors was observed (Fig. 4 and Fig. S3). Within the roller-crimper without glyphosate group, ROL, the treatment with the combined postemergence operations (ROL_hc) had the highest values than the other treatments in 2016 (Fig. 4A, D), while in 2017 and 2018 no differences were found between treatments (Fig. 4B, C, E and F). Regarding the RGL group, in 2016 and 2018, no differences in grain yield were observed between post-emergence treatments (Fig. 4A and C), while in 2017 RGL_c achieved a lower grain yield compared to treatments that received post-emergence herbicide (Fig. 4B). Within the INC group, INC_hc achieved a greater grain yield than the control (INC_0) in 2016 and 2018. And in 2017, INC_0 had the lowest grain yield values. The grain N content results are presented in Fig. 4D–F and were in

between post-emergence treatments within the ROL and RGL groups, while in the residue-incorporated group, INC_0 had greater density than INC_c and INC_hc in 2017 and 2018 (Fig. 2B, C). 3.4. Soil inorganic N Prior to corn sowing, the soil inorganic N in the 60-cm soil profile was 69, 16 and 13 kg N ha−1 in 2016, 2017 and 2018, respectively (Fig. 3A– C). After corn harvest, no differences were observed in soil inorganic N between termination methods. Regarding the cultivator pass, in 2016 and 2018, no differences were found between treatments (Fig. 3D–F). In October 2017 (Fig. 3E), significant differences in two of the three soil layers studied were observed between post-emergence treatments that included a cultivator pass (hc) and those without it (h), so treatments were differentiated:the treatment that received a cultivator pass in summer had higher soil inorganic N content than treatments that did not receive a cultivator pass (Fig. 3E). 3.5. Corn N status, grain yield and grain N content At corn flowering, the interaction between the termination factor and the years for chlorophyll activity assessed with the optical sensor

Fig. 3. Soil inorganic N (kg N ha−1) present in the 0-0.2, 0.2-0.4 and 0.4-0.6 m soil layers prior to corn sowing in April (A, B, C) and after corn harvest, in October (D, E, F). The segment indicates the SE. For each soil layer, in October 2017 (E), different letters indicate significant differences between post-emergence treatments at each soil layer (Tukey test, P ≤ 0.05).

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Fig. 4. Corn grain yield (A, B, C, Mg DM ha−1) and grain N content (D, E, F, kg N ha−1) for the treatments resulting from the termination methods (ROL = rollercrimper; RGL = glyphosate + roller-crimper; INC = residue incorporation) through post-emergence operations (h = post-emergence herbicide; c = inter-row cultivator; hc = herbicide + cultivator; 0 = with no post-emergence operation). The SE is indicated over each bar. For each variable, year and termination factor, different letters indicate significant differences between post-emergence treatments (Tukey test, P ≤ 0.05).

line with the grain yield results. Similar results were found for other variables determined at harvest such as total corn biomass and total N uptake that are presented in the Supplementary Material (Fig. S3). 3.6. Energy consumption analysis The roller-crimper termination without glyphosate support (ROL) had the lowest energy values and saved more than 1000 MJ ha−1 when compared with the most intensive treatment, INC_hc (Fig. 5A). For the roller-crimper + glyphosate termination method, the herbicide manufacturing accounted for 40–60 % of the energy consumption. In the treatments in which the CC was incorporated, the consumption for the termination and post-emergence operations exceeded 1200 MJ ha−1 and was mainly due to fuel consumption. Among the post-emergence operations, the use of an inter-row cultivator consumed the most energy: 380 MJ ha−1 compared to the homologous treatments without a cultivator. The energy productivity results, i.e. the kg of corn grain produced per MJ−1 consumed for each treatment, are presented in Fig. 6. In 2016, few differences were observed between treatments within each termination group (Fig. 6A). In 2017 and 2018, few differences were observed between post-emergence treatments within the RGL and INC groups. For the ROL, it was notable that the ROL_0, closely followed by ROL_h, showed a larger energy productivity than all treatments with a cultivator pass (Fig. 6B and C). Both values from ROL_0 and ROL_h were much larger than the other treatments (> 30 kg grain MJ−1)

Fig. 5. Energy consumption (A, MJ ha−1) and cost (B, € ha−1) of the CC termination and post-emergence operations, for the treatments resulting from the termination methods (ROL = roller-crimper; RGL = glyphosate + rollercrimper; INC = residue incorporation) by the post-emergence operations (h = post-emergence herbicide; c = inter-row cultivator; hc = herbicide + cultivator; 0 = with no post-emergence operation). The economic and energy cost of common operations (CC sowing, corn sowing, fertilization and corn harvest) and common inputs (seeds, fertilizers) are excluded. The different colors in the bars indicate the contribution of the herbicide input (manufacture) and fuel consumption in the cost.

3.7. Economic analysis In Fig. 5B, the costs of the CC termination and summer post-emergence operations are presented for each treatment. The cost differed by 53 € ha−1 between the most intensive CC management (INC_hc) and the most economical treatment (ROL_0). For the residue incorporation group the main economic cost was from the fuel consumption. Adding a post-emergence herbicide application increased cost across all termination method groups. The generalized linear model showed that the highest mean net benefits were attained by INC_hc and RGL_h treatments, significantly greater than INC_0 and ROL_0 (Fig. 7). The cumulated probability function showed that the maximum potential stochastic net benefits would be larger for INC_hc, whereas the RGL_h treatment stabilized the

net benefits since the difference between maximum and minimum values was lower than INC_hc (i.e., the tails observed in the adjusted function were shorter). At the same time, ROL_0 and INC_0 had the lowest stochastic net benefits (the distribution curves overlapped), and their average values were significantly lower than INC_hc and RGL_h. 6

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Fig. 6. Energy productivity (kg corn grain yield MJ consumed−1) calculated for the treatments resulting from the termination methods (ROL = roller-crimper; RGL = glyphosate + roller-crimper; INC = residue incorporation) by the post-emergence operations (h = post-emergence herbicide; c = inter-row cultivator; hc = herbicide + cultivator; 0 = with no post-emergence operation). The SE is indicated over each bar. For each year and termination factor, different letters indicate significant differences between post-emergence treatments (Tukey test, P ≤ 0.05).

residues into the soil. This effect was already detected at corn flowering with the Dualex® optical sensor. In Italy, no differences in zucchini’s nutritional status were observed between two CC termination methods that included tillage and roller-crimper, but post-emergence operations were not evaluated (Montemurro et al., 2013). Canali et al. (2013) also observed a greater cash crop yield when barley CC was roll-crimped compared to residue incorporation. As observed in the experiment, the roller-crimper with glyphosate or the post-emergence herbicide application compensated the rollercrimper termination ineffectiveness (Kornecki et al., 2009). The combination of roller-crimper and a later cultivator pass has no practical interest for farming (Luna et al., 2012). After the roller-crimper, the CC root system remains anchored to the soil and makes it difficult to pass the cultivator, increasing the risk of cash crop damage. In this experiment, the lower grain yields observed in 2017 for the glyphosate + roller-crimper treatment with cultivator were attributed to corn damage. Therefore, the termination method and post-emergence operations could decrease the uncertainty of the CC effect on cash crop productivity, which has been identified as one of the main barriers for CC adoption (Roesch-McNally et al., 2018). The CC management including tillage may lead to differences in the soil N mineralization rates that could explain differences in the cash crop yield and N uptake (Reberg-Horton et al., 2012; Wells et al., 2014). In an experiment carried out in central Italy, differences in soil inorganic N between CC termination methods were observed at different cropping stages of the cash crop, but not at harvest (Ciaccia et al., 2015). In the current data, no clear differences were observed in the soil inorganic N after harvest. In our opinion, the effect on the soil inorganic N was not the main driver for the differences observed in the cash crop grain yield and quality, although it could have been responsible along with weed control, CC termination efficiency or the effect on spring soil temperatures. Further research with more soil inorganic N determinations throughout the cash crop period would contribute to understanding the implications of CC management in N dynamics (Kristensen and Thorup‐Kristensen, 2007). Otherwise, the large amount of remaining residues in October (> 3 Mg ha−1) was remarkable. Therefore, when using termination methods that leave the CC residue as mulch, one should consider another subsequent crop in autumn to reduce the risk of losing the N released from the residue by leaching. Ciaccia et al. (2015) warned that winter CCs – regardless of termination method – can leave high soil inorganic N, compared with a conventional rotation that does not include winter CCs. Despite the interannual differences in CC development, explained by weather conditions, each year the mixture covered the ground homogeneously, and led to a high biomass accumulation, and therefore environmental benefits may be expected during the CC period such as an increase in soil quality (García-González et al., 2018), nutrient recycling enhancement (Gabriel and Quemada, 2011) or weed control (Teasdale et al., 2007). But further CC benefits may be obtained after termination, during the cash crop period. Here, the roll-crimped

Fig. 7. Cumulated probability of the stochastic net benefit (€ ha−1) calculated for the treatments resulting from the termination methods (ROL = rollercrimper; RGL = glyphosate + roller-crimper; INC = residue incorporation) by the post-emergence operation (h = post-emergence herbicide; c = inter-row cultivator; hc = herbicide + cultivator; 0 = with no post-emergence operation). Irrigation (water and electricity) costs and wages were not considered. The mean net benefit calculated for each treatment obtained with a generalized linear model and adjusted to a Gamma function is presented next to the figure. Different letters indicate significant differences between treatments (Tukey test, P≤0.05).

4. Discussion In this study under Mediterranean conditions, the roller-crimper termination effectiveness varied from year to year. The first year, the roller-crimper without glyphosate did not terminate vetch in the mixture properly, and its regrowth hindered corn establishment and initial development. However, the following years, the roller-crimper termination was effective. Different studies indicated that the roller-crimper performance is successful when CC cereals are flowering (GS-61) and CC legumes already have visible pods (Mirsky et al., 2011). In this experiment, the roller-crimper termination was successful in earlier CC phenological stages, so effectiveness was associated more with weather conditions. The results indicate that wet springs may make it difficult to terminate CCs with the roller-crimper alone. A delay in the termination date could have ensured the effectiveness, but due to the risk of preemptive competition and the overlap with the corn sowing date, early or medium dates are recommended (Luna et al., 2012; Alonso-Ayuso et al., 2014). The ineffective roller-crimper termination led to penalties in the corn yield in 2016, in agreement with other studies (Mischler et al., 2010; Peigné et al., 2015). When the termination succeeded, yields were competitive in the current experiment and in the literature (Davis, 2010; Ciaccia et al., 2016). In addition, in treatments where the rollercrimper was used, yields were more stable and therefore less dependent on a post-emergence operation such as a cultivator pass or post-emergence herbicide applications than the method that incorporated CC 7

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glyphosate + roller-crimper for termination, with post-emergence herbicides, was demonstrated to be a solution to obtain favorable net benefits and stabilize them against other treatments, but this did not solve the herbicide reduction issue. Overall, the fallow substitution by winter CC is a useful practice to diversify agroecosystems in arable crop rotations in Mediterranean areas (Tosti et al., 2012; Ramírez-García et al., 2015), and this study contributes to increasing the understanding of the implications of the CC termination methods in the cropping system. These findings suggest that the roller-crimper can increase environmental sustainability, but further research is needed to design strategies that will maximize the success and termination effectiveness of roller-crimper termination in semiarid Mediterranean areas. Selection of CC species that are easy to terminate with the roller-crimper and with low regrowth (i.e., faba beans) may be an area of research that will lead to reducing the herbicide needs and increase economic sustainability.

residues contributed to soil water conservation in spring, even in drought conditions, in agreement with other studies (Canali et al., 2013; Wells et al., 2014). This may contribute to proper corn establishment in rainfed cropping systems or when irrigation water is only available in late spring or summer. Soil temperature was already reported to be lower with the roller-crimper termination than with the CC incorporation method during the early cash crop period (Canali et al., 2013; Jokela and Nair, 2016). Some authors have pointed out that in no-tillage systems lower soil temperatures may reduce yield (Delate et al., 2012). Here, the first experimental year was characterized by a wetter and cooler spring than the average (Fig. S1), and higher soil temperature associated with the residue incorporation could have increased corn emergence and growth in the first stages, whereas the lower soil temperature under the CC mulch delayed corn emergence and growth, but no clear differences were observed later in terms of corn productivity. In 2017 – drier and warmer – differences in soil temperature between treatments were also observed, but not in corn emergence, early growth or later productivity. Therefore, in agreement with other authors, the impact of soil temperature on crop productivity may be lower under Mediterranean conditions than in continental climates (Altieri et al., 2011; Canali et al., 2013). Furthermore, CC residue mulch protected the soil from crusting, which may hinder corn emergence (Kornecki et al., 2009), and the roller-crimper termination methods kept the soil covered throughout the crop rotation. In the current study, the experimental site is flat and little soil loss is expected. However, when there is slope, the residue mulch can greatly contribute to reducing soil erosion, which is one of the main environmental problems in the Mediterranean region (GarcíaRuiz (2010)). In addition, the results showed the potential of the rollercrimper for weed suppression, in agreement with other authors (Reberg-Horton et al., 2012; Wells et al., 2014). Williams et al. (2018) related weed suppression enhancement to lower plant-available N. In an experiment conducted in Pennsylvania, US, the glyphosate applied as a post-emergence herbicide after the roller-crimper termination contributed to greater weed suppression (Nord et al., 2012). In the current experiment, the post-emergence herbicide enhanced weed control mainly in treatments in which the residue was incorporated into the soil, in line with yield results. However, in the treatments terminated with the roller-crimper few differences were found between treatments whether or not they received post-emergence herbicide. Therefore, the roller-crimper could contribute to the reduction of herbicide application in crop rotations. Energy consumption is a useful indicator of environmental sustainability (Alluvione et al., 2011). The roller-crimper reduced energy consumption compared with the residue incorporation treatment, as in other studies (Canali et al., 2013; Diacono et al., 2017). Additionally, when the energy was related to productivity, the roller-crimper without glyphosate or post-emergence treatments increased the energy use efficiency, as long as the CC termination was effective. In the current analysis, the N fertilizer was excluded from the calculations because it was equal in all treatments. However, when comparing with systems that did not include CC the N fertilizer must be taken into account, since N fertilizers represent the highest indirect energy consumption source in rotations (Alluvione et al., 2011; Diacono et al., 2017). In a comparison with rotation without winter CC use, the fertilizer savings due to CC residue mineralization may reduce energy consumption, particularly when legumes are used as CC (Guardia et al., 2016). In terms of costs, the roller-crimper was less costly than the mowing and residue incorporation method, in agreement with Goddard et al. (2008). The difference between treatments depended mainly on herbicide application, given that they made up a large proportion of the rotation cost. However, the variable effectiveness of the roller-crimper termination was reflected by the stochastic analysis, which pointed out that the roller-crimper without herbicide support is economically riskier, as was the method that incorporated residues and did not receive any post-emergence operation. The combination of

5. Conclusions Our study confirmed the advantages of the roller-crimper for terminating winter cover crops as it increased the system’s resilience when compared with the method that incorporated the residues into the soil. The mulch associated with roll-crimping contributed to weed suppression, enhanced the soil’s water conservation in the first corn stages, and reduced energy consumption. The variable roller-crimper termination effectiveness remains one of its main drawbacks, and it may compromise the farmer profitability. However, the CC re-growth was controlled using glyphosate before implementing the roller-crimper or with a post-emergence herbicide. Besides, the roller-crimper termination method was less dependent on the summer post-emergence operation than the incorporation of CC residues to control weeds and maintain yields. Therefore, the rollercrimper showed great potential to increase sustainability in agricultural systems, but further research is needed to identify strategies that will maximize the potential and termination effectiveness in Mediterranean areas.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement María Alonso-Ayuso: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization. José Luis Gabriel: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. Chiquinquirá Hontoria: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. Miguel Ángel Ibáñez: Methodology, Formal analysis. Miguel Quemada: Conceptualization, Methodology, Investigation, Writing - review & editing, Project administration, Funding acquisition, Supervision.

Acknowledgments We are grateful to the Spanish Ministry of Economy and Competitiveness (AGL2017-83283-C2-1/2-R), the Comunidad de Madrid, Spain (AGRISOST-CM S2018/BAA-4330 project) and Structural Funds2014-2020 (ERDF and ESF) for financial support. We thank the staff from La Chimenea field station (IMIDRA) for their technical support. 8

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Appendix A. Supplementary data

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