Introduction
Milk production by dairy cows is highly dependent on diets formulated with high-quality forages. Historically, corn (Zea mays L.) silage and alfalfa (Medicago sativa L.) have served as the primary sources of forage for dairy cows in the U.S. Midwest (Gillespie Reference Gillespie2023; Kellogg et al. Reference Kellogg, Pennington, Johnson and Panivivat2001). In Wisconsin, corn silage and alfalfa were the top two forage crops harvested in 2023, occupying 25% of the state’s cropland (1.6 and 1.3 million ha, respectively) (USDA-NASS 2024). Fields are often rotated between alfalfa and corn to provide multiple production benefits such as increased forage yield of both crops, reduced pest populations, improved soil health and nutrient retention, and reduced reliance on nitrogen fertilizers (Huggins et al. Reference Huggins, Randall and Russelle2001; Kanwar et al. Reference Kanwar, Cruse, Ghaffarzadeh, Bakhsh, Karlen, Bailey and Kanwar2005; Olmstead and Brummer Reference Olmstead and Brummer2008; Russelle Reference Russelle2014; Sanford et al. Reference Sanford, Jackson, Booth, Hedtcke and Picasso2021). Inclusion of alfalfa in corn silage–based diets also benefits cattle health and productivity (Brito and Broderick Reference Brito and Broderick2006; Lopes et al. Reference Lopes, Cook and Combs2015).
Despite these benefits, land devoted to alfalfa production has decreased within the United States. Nationally, the area of alfalfa harvested (hay and haylage) has decreased by 32% over the past 15 yr, whereas harvested corn silage increased by 15% (USDA-NASS 2024). Reductions in alfalfa harvested in Wisconsin were even greater than the national average, declining by 44% between 2009 and 2023 (USDA-NASS 2024). Alfalfa plantings have typically been replaced with less diverse production systems based on annual row crops such as corn silage, corn grown for grain, or soybean [Glycine max (L.) Merr.] (Blum Reference Blum2020; Zulauf Reference Zulauf2018). Increasing the area where alfalfa is grown would improve the sustainability of crop and dairy production, which is aligned with national and Wisconsin priorities (Innovation Center of U.S. Dairy 2023; Wisconsin DATCP and UW System 2019).
Establishment of alfalfa by interseeding into corn silage is a novel cropping system that could be used to increase alfalfa usage on farms while providing multiple environmental benefits. In this system, alfalfa is established under a corn silage companion crop that is capable of producing up to 4.4-fold more high-quality forage than conventionally spring-seeded alfalfa. During its establishment, interseeded alfalfa serves as a cover crop before and after corn silage harvest and then is harvested in subsequent years, producing forage of comparable yield as alfalfa established by conventional methods (Grabber Reference Grabber2016; Grabber et al. Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Bjorneberg and Rogers2024). As a result, the interseeding system bypasses the low-yielding establishment year typical of conventionally spring-seeded alfalfa. In addition to improved yield, this interseeding approach has been shown to improve the profitability of alfalfa–corn silage rotations (Berti et al. Reference Berti, Lukaschewsky and Samarappuli2021; Osterholz et al. Reference Osterholz, Renz and Grabber2020). Environmental benefits from this approach include reduced soil erosion, nutrient runoff (Osterholz et al. Reference Osterholz, Renz, Jokela and Grabber2019), and nitrate leaching (Osterholz et al. Reference Osterholz, Ruark, Renz and Grabber2021b) compared with corn silage monocultures. Because of these benefits, there are multiple efforts underway aiming to increase adoption of this interseeding system in the U.S. Midwest.
Although this intercropping system provides economic and environmental benefits, stands of interseeded alfalfa can fail due to excessive competition from the corn silage companion crop and from defoliation and necrosis caused by disease and insect pests. These issues can be managed by planting corn silage at moderate populations, by early seeding of well-adapted alfalfa varieties, and by application of prohexadione-calcium growth retardant and foliar fungicide and insecticide (Grabber et al. Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Dias and Renz2023; Osterholz et al. Reference Osterholz, Renz, Lauer and Grabber2018). Observations across multiple studies by our group, however, suggest high populations of annual grasses such as yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.] might also contribute to poor establishment of interseeded alfalfa. Poor alfalfa establishment due to competition with high populations of annual grasses such as S. pumila has been reported as a concern by others (Norris and Ayres Reference Norris and Ayres1991). Maintaining control of weeds during establishment is important, as Chu et al. (Reference Chu, Cassida, Singh and Burns2022) found a weed-free period of 394 growing degree days (GDD) was needed to maximize alfalfa establishment and productivity. While herbicides (e.g., acetochlor) have been identified to control early-emerging weeds, including annual grasses, and resulted in the successful establishment of alfalfa (Osterholz et al. Reference Osterholz, Dias, Grabber and Renz2021a), they are applied near planting and are not effective in controlling annual grasses that emerge later in the season. Roundup Ready® (RR) corn and alfalfa hybrids used in conjunction with glyphosate are an effective tool to provide annual grass control in the alfalfa–corn interseeding system when applied postemergence. However, concern exists surrounding reliance on the repeated use of a single active ingredient, as it increases the risk of selecting for herbicide-resistant populations. Additionally, many of the alfalfa cultivars that are best adapted to the interseeding system are conventional, for which postemergence glyphosate is not an option (Grabber et al. Reference Grabber, Osterholz, Riday, Cassida, Williamson and Renz2021a).
Weed densities are frequently used as decision support tools for weed management (Larson et al. Reference Larson, Renz and Stoltenberg2016). While the effects of annual grasses on the establishment of interseeded alfalfa is poorly understood, its impact is likely density dependent, and identifying its threshold is crucial to inform management decisions. In this study, the effect of annual grass weed density on the establishment of alfalfa interseeded into corn silage was evaluated during 2023 at two locations in southern Wisconsin. Setaria pumila was selected as the model annual grass, as this late-emerging species is abundant in agronomic fields in Wisconsin (Fickett et al. Reference Fickett, Boerboom and Stoltenberg2013a, Reference Fickett, Boerboom and Stoltenberg2013b). Specifically, the impact S. pumila density in June had on summer ground cover, fall plant density, and fall biomass of interseeded alfalfa during the establishment year was evaluated.
Materials and Methods
Site Description
Field experiments were implemented in southern Wisconsin in 2023 at the Lancaster Agricultural Research Station (LARS; 42.82°N, 90.78°W) and the U.S. Department of Agriculture Dairy Forage Research Center at Prairie du Sac (PDS; 43.35°N, 89.76°W). In the 2 yr before experiment initiation, sites were planted with corn (LARS) or soybean followed by corn (PDS). Weed community composition included giant foxtail [Setaria faberi Herrm.], common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), dandelion (Taraxacum officinale F.H. Wigg.), and large crabgrass [Digitaria sanguinalis (L.) Scop.] at LARS. At the PDS site, C. album and redroot pigweed (Amaranthus retroflexus L.) were the only common species.
Soil types were Fayette (fine-silty, mixed, superactive, mesic Typic Hapludalfs) at LARS and Richwood (fine-silty, mixed, superactive, mesic Typic Argiudolls) at PDS. Before experiment establishment, soil samples were collected for characterization. Soil at LARS had 6.6 pH, 2.1% organic matter, 25 mg P kg −1, 106 mg K kg−1, and 0.99 g cm−3 soil bulk density. Soil at PDS had 6.4 pH, 2.8% organic matter, 33 mg P kg−1, 173 mg K kg−1, and 0.94 g cm−3 soil bulk density. Total rainfall and average temperatures for 2023 were obtained from nearby weather stations at Lancaster and Sauk City. GDD were calculated summing the difference between average daily temperature and alfalfa’s base temperature (5 C; Sharratt et al. Reference Sharratt, Sheaffer and Baker1989), starting on the first day after planting. Values were averaged across both locations, as data were pooled for analysis.
Experiment Establishment and Management
Field trials in both locations were planted without tillage (no-till). Plots received glyphosate at 1 kg ae ha−1 (Roundup PowerMax®, 540 g ai L−1, Bayer (Monheim am Rhein, Germany)) in the last week of April before planting to control any weeds that had emerged. Based on soil analysis results, fertilizer was broadcast preplant at rates of 202 kg N, 52 kg P, and 344 kg K ha−1 on April 27 at LAR and on April 29 at PDS. Corn (ARL:P0529Q; Pioneer Seed Company (Johnston, IA); LARS:BO4RR11Q-N804Q; Brevant (Indianapolis, IN)) was planted on May 4 at LARS and May 9 at PDS. Alfalfa (431RRLH; Farm Science Company) was interseeded in four rows between each corn row the following day (May 5 at LARS and May 10 at PDS) as described by Grabber et al. (Reference Grabber, Smith, Osterholz and Renz2021b). The seed rate of corn and alfalfa was 74,100 seeds ha−1 and 18 kg PLS ha−1, respectively, with a row spacing of 0.76 m for corn and 0.15 m for alfalfa.
Treatments and Experimental Design
Initially, target treatments were seven S. pumila density ranges (0, 5 to 15, 30 to 50, 70 to 90, 91 to 100, 101 to 120, and >120 plants m−2) established in 3 m by 7.6 m plots arranged in a randomized complete block design with four replicates. To achieve these treatments, S. pumila seeds (75% germination rate) were broadcast on May 5 at LARS and on May 10 at PDS immediately before alfalfa planting. Weed control in weed-free treatments was achieved by applying glyphosate at 1 kg ae ha−1 (Roundup PowerMax®, 540 g ai L−1, Bayer) when alfalfa had four trifoliate leaves. At 25 d after planting alfalfa (DAPa), three 0.76 m2 sampling areas were marked within the center interrow of each plot for subsequent measures. In each sampling area, S. pumila plants were counted and thinned by hand weeding to achieve target densities. This procedure started at 30 DAPa and was repeated biweekly thereafter until the end of June. Annual grasses that were not S. pumila and broadleaf weeds were also removed during this time frame. Two irrigation events (64 and 38 mm) were applied at 31 DAPa (June 11) and 77 DAPa (July 27) at PDS due to the unusually dry spring, but no irrigation was applied in LARS due to infrastructure constraints.
To control alfalfa foliar diseases, plots were treated with fluxapyroxad at 48.6 g ai ha−1 and pyraclostrobin, and 97.4g ai ha−1 (Priaxor®, BASF (Research Triangle Park, NC)) at 75 DAPa in both locations when the corn canopy was beginning to close (V10-V12 growth stages). Lambda-cyhalothrin at 18.2 g ai ha−1 (Warrior II®, Syngenta (Greensboro, NC)) was mixed with fungicide and applied at PDS to control potato leaf hopper (Empoasca fabae), but no insecticides were needed at LARS, as potato leaf hopper was not present.
Measurements
Corn Silage
Silage biomass was harvested on September 12 and 5 at LARS and PDS, respectively, and yields were estimated at the field level at each location. Four representative areas (23 m2) were harvested within each location to determine silage dry matter (DM) yield. This was done by measuring fresh weights, oven-drying samples until constant weight at 60 C, and reweighing to calculate percentage moisture. Moisture was averaged over the four samples within each location (64% and 63% in LARS and PDS, respectively) and used to correct field-level silage yield to a DM basis.
Alfalfa and Setaria pumila
All measurements for alfalfa and S. pumila were conducted within the 0.76 × 0.76 m2 sampling area determined at 25 DAPa. At 35 DAPa, S. pumila density (plants m−2) was estimated. At 40, 70, 105, and 160 DAPa, percentage alfalfa and S. pumila cover were visually estimated. To document initial alfalfa establishment, alfalfa plants (plants m−2) were counted in early summer (40 DAPa) within one sampling area within each plot at each site (n = 28). To ensure individual plants were identified, all alfalfa plants within sampling areas were destructively harvested by uprooting plants to a depth of 20 cm. Alfalfa plant density was counted in all remaining sampling areas within each plot at each site after corn silage harvest (160 DAPa; n = 56). At this time, alfalfa crowns were collected and then separated into shoot and root biomass. These samples were then dried at 105 C until constant weight and then weighed to determine alfalfa shoot and root biomass accumulation at the end of the growing season.
Modeling and Statistical Analysis
Setaria pumila populations varied substantially within plots and did not conform to target treatment densities. Therefore, S. pumila populations were considered continuous rather than categorical treatments (Kohmann et al. Reference Kohmann, Sollenberger, Dubeux, Silveira, Moreno, da Silva and Aryal2018). Regression analysis was used with the nls() function to determine the relationships between alfalfa response variables and S. pumila density using the R Studio platform (R Development Core Team 2021). Data across sites were visually inspected and considered similar in their responses among sites, and therefore were pooled for analysis. Setaria pumila density was chosen as the determinant variable, as this is a common metric used to assess weed competition and results are easily adopted by stakeholders. For all responses, three models were compared: linear, two-parameter concave, and linear plateau, all commonly used with establishment and yield experiments (Larson et al. Reference Larson, Renz and Stoltenberg2016; McCartor and Rouquette Reference McCartor and Rouquette1977; Ratkowsky Reference Ratkowsky1990). The best model was selected based on a combination of visual assessment of residuals, normality, and root-mean-square error (RMSE).
Percentage alfalfa cover at 40, 70, 105, and 160 DAPa was best described by a two-parameter concave model shown in Equation 1 (Ratkowsky Reference Ratkowsky1990):
$$f\left( x \right){\rm{ }} = {\rm{ }}1/\left( {a + bx} \right)$$
where 1/a is the percent alfalfa cover when S. pumila population is zero, b is the rate of decline of alfalfa cover with increasing S. pumila density, and x is S. pumila density in June. Fall alfalfa plant density and shoot and root biomass at 160 DAPa were fit to a linear plateau model (McCartor and Rouquette Reference McCartor and Rouquette1977) described in Equation 2:
$$f\left( x \right) = a-b\left( {x-c} \right){\rm{if}}\,x \le c;{\rm{otherwise}},f\left( x \right) = c$$
where a is the point where alfalfa plant density, shoot, or root biomass reached a plateau, b is the rate of change in those responses as S. pumila density increased (before reaching the plateau), c is the S. pumila density at the join point of the linear and plateau response of alfalfa, and x is S. pumila density in June.
Results and Discussion
Temperature and precipitation during the growing season (April to November) were atypical at LARS and PDS (Table 1). Average monthly temperature was 5% to 16% and 3% to 10% greater than the 30-yr average at LARS and PDS, respectively, except July, which had temperatures similar to the 30-yr average. Monthly precipitation was below average at both locations, and the total for the growing season amounted to only about 57% of the normal expected at LARS and PDS. With irrigation applied at PDS in June and July, the precipitation deficit was partially offset, and the total for the growing season reached 70% of the expected normal. Corn silage yields at LARS and PDS were 7.5 and 16.7 Mg ha−1, respectively. In prior studies, yields of corn silage grown with interseeded alfalfa typically approached or exceeded 20 Mg ha−1 at these sites under near-normal precipitation (Grabber, Reference Grabber2016, Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Dias and Renz2023, Reference Grabber, Bjorneberg and Rogers2024; Osterholz et al. Reference Osterholz, Renz, Lauer and Grabber2018).
Table 1. Monthly 2023 weather and 30-yr historical averages at Lancaster (LARS) and Prairie du Sac (PDS) Wisconsin during the growing season. a
a Sources: National Weather Service, https://www.weather.gov/wrh/Climate?wfo=arx (accessed: March 5, 2024); Wisconet, Wisconsin’s Environmental Mesonet, https://wisconet.wisc.edu (accessed: July 21, 2024).
b Irrigation was available at PDS only; total volume of water, consisting of precipitation plus irrigation, is presented for that location.
Relationship between June Setaria pumila Density and Alfalfa Cover
Setaria pumila density in June ranged from 0 to 460 plants m−2. This resulted in different amounts of S. pumila cover, which ranged from 0% to 95% throughout the growing season. A concave function best described the relationship between S. pumila density and alfalfa cover in July, August, and October (Figure 1), for which all parameters were significant (P < 0.0001; RMSE = 12, 17, and 13, respectively). However, the parameter that describes change in alfalfa cover relative to S. pumila density b in June was not significant (P = 0.109) and approached zero, suggesting S. pumila competition was not limiting alfalfa growth at that time. Chu et al. (Reference Chu, Cassida, Singh and Burns2022) reported similar results, as the critical period for weed control for alfalfa did not begin until late June in a weed competition study using Japanese millet [Echinochloa esculenta (A. Braun) H. Scholz] as a surrogate for annual weeds in alfalfa interseeded systems in Michigan.
Figure 1. Effect of Setaria pumila density in June (x) on alfalfa cover at 70, 105, and 160 d after planting alfalfa (DAPa) (July, August, and October, respectively). Points represent measured responses; the continuous line represents the response estimated by the regression model; and the dashed lines represent the 95% confidence interval. The relationship between alfalfa ground cover and June S. pumila density was established using data pooled across two locations (n = 112). GDD, growing degree days.
Competition between S. pumila and alfalfa was evident between July and October, as the b parameter that described change in alfalfa cover relative to S. pumila density was significant (P < 0.0001; Figure 1). The effect of June S. pumila density on alfalfa cover was greatest in August, when the b parameter was nearly 3-fold greater than in July. The pronounced increase in the b parameter during August likely occurred because the adverse effects of abiotic and biotic stress on stand loss and defoliation of interseeded alfalfa are most pronounced in late July and August when plants are subjected maximal shading from corn and defoliated from foliar disease and insects (Grabber et al. Reference Grabber, Osterholz, Riday, Cassida, Williamson and Renz2021a, Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Dias and Renz2023). These effects declined by October, because corn silage harvest in early September alleviated competition and S. pumila did not resprout after harvest. This allowed surviving alfalfa plants, especially those subjected to additional stress from moderate to high populations of S. pumila, to regrow and increase ground cover. While the relationship between S. pumila density and alfalfa cover was significant, alfalfa plant density is recommended for evaluating successful alfalfa establishment, as the degree of stand loss and defoliation of interseeded alfalfa during late July and August and regrowth following silage harvest are highly dependent on other factors that cannot always be controlled (Grabber et al. Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Dias and Renz2023; Osterholz et al. Reference Osterholz, Ruark, Renz and Grabber2021b). In particular, alfalfa leaf defoliation (often caused by potato leaf hopper and alfalfa foliar diseases) and fall precipitation have been identified as important factors for alfalfa plant survival during this time frame (Grabber et al. Reference Grabber, Smith, Osterholz and Renz2021b). These factors make it difficult to rely solely on cover as an assessment of establishment success.
Relationship between June Setaria pumila Density and Fall Alfalfa Density
June S. pumila density impacted alfalfa density in October. A linear plateau function best described the relationship of all three models evaluated (RMSE = 49), with all parameters significant (P < 0.0001; Figure 2). Alfalfa density when no weeds were present was 242 plants m−2 and decreased linearly with increasing June S. pumila density by 0.8 alfalfa plants m−2 for every 1 S. pumila plant m−2. However, when June S. pumila density was ≥125 plants m−2, the alfalfa population remained constant (145 plants m−2) under the relatively dry growing conditions of this experiment.
Figure 2. Relationship between Setaria pumila plant density in June (x) and alfalfa plant density in October (160 d after planting alfalfa [DAPa]). Points represent measured responses; the continuous line represents the response estimated by the regression model; and the dashed lines represent the 95% confidence interval. Data were pooled across two locations (n = 112). GDD, growing degree days.
Adequate alfalfa plant density is critical to maximize productivity in alfalfa interseeded systems. In solo-seeded alfalfa fields, densities >107 plants m−2 are required to maximize forage harvested (Sheaffer et al. Reference Sheaffer, Drewitz and Jungers2023). Several studies suggest that alfalfa plant densities >200 plants m−2 are required by the fall of the establishment year to maximize first-cut yield the following year (Grabber et al. Reference Grabber, Smith, Osterholz and Renz2021b, Reference Grabber, Bjorneberg and Rogers2024; Osterholz et al. Reference Osterholz, Dias, Grabber and Renz2021a). In our study, fall plant density of alfalfa exceeded 200 plants m−2 at S. pumila densities <50 plants m−2. This threshold is similar to Zhou et al.’s (Reference Zhou, Hou, Liu and Huang1992) finding that S. pumila densities needed to be <32 plants m−2 to ensure successful establishment of solo-seeded alfalfa. As noted earlier, this study was conducted under relatively dry growing conditions; thus, S. pumila effects on alfalfa establishment may differ if intercropping is carried out under normal to wet growing conditions. Weed densities are frequently used as decision support tools for weed management (Larson et al. Reference Larson, Renz and Stoltenberg2016) due to their easy adoption by crop consultants and farmers. We recommend assessing S. pumila density at 35 to 45 DAPa (June in Wisconsin) to determine whether additional weed management activities are needed to ensure successful establishment of alfalfa in this interseeded system.
Relationship between Setaria pumila Density and Fall Alfalfa Shoot and Root Biomass
Similar to alfalfa plant density at October, the relationship between June S. pumila density and alfalfa shoot and root biomass were best described by a linear plateau function (RMSE = 47 and 26, respectively), in which all parameters were significant (P < 0.0001; Figure 3). Shoot and root biomass both decreased by 0.7 g m−2 for each S. pumila plant m−2. Shoot biomass decreased until S. pumila density reached 66 plants m−2, while root biomass declined and plateaued at S. pumila densities of 47 plants m−2. While removal of alfalfa shoots during corn silage harvest and its regrowth likely impacted these observations, these results suggest shoot biomass is more sensitive than root biomass at higher S. pumila densities in this system. The impact from S. pumila was on alfalfa plant survival, because surviving plants had similar root or shoot biomass per plant regardless of the S. pumila density (data not shown).
Figure 3. The relationship between Setaria pumila plant density in June (x) and alfalfa shoot and root biomass in October (160 d after planting alfalfa [DAPa]). Points represent measured responses; the continuous line represents the response estimated by the regression model; and the dashed lines represent the 95% confidence interval. Data were pooled across two locations (n = 112). GDD, growing degree days.
Results provide strong evidence that annual grasses, estimated using S. pumila, can reduce establishment of alfalfa interseeded into corn silage. As S. pumila populations increased in June, alfalfa responded by sharply reducing ground cover during July and August when competition from corn and foliar damage from disease and insects are also typically most pronounced in this intercropping system. Setaria pumila densities above 50 plants m−2 in June reduced postestablishment stand density of alfalfa in October to less than 200 plants m−2, a level previously established as a benchmark for maximizing first-cut yield of alfalfa the following year. This suggests weed control efforts should be initiated if annual grass populations exceed 50 plants m−2 in June. Several options exist for annual grass weed management in the interseeded system, including preemergence applications of residual herbicides (alachlor, pendimethalin) or postemergence applications of glyphosate (in RR systems only) (Osterholz et al. Reference Osterholz, Dias, Grabber and Renz2021a). Additional herbicide options need to be explored for controlling annual grasses that emerge after alfalfa emergence as no options are currently registered for use for late-season annual grass applications in this system if conventional alfalfa varieties are used. Setaria pumila populations need to be below the 50 plants m−2 threshold before July, as reductions to alfalfa cover were observed in this time frame (Figure 1). Because environmental conditions can affect alfalfa establishment in the interseeded system, additional research is needed to confirm the validity of this threshold. Despite this limitation, our results provide information to crop consultants and farmers as to the level and timing of management of a difficult to control and impactful weed species in this interseeded system.
Acknowledgments
The authors would like to thank Charlton Rodriguez and other the students and staff from the Renz Laboratory, as well as personnel at the Lancaster Agricultural Research Station and Matthew Volenec, Prairie du Sac Research Station, for support during the establishment, maintenance, and harvesting of experiments.
Funding statement
The authors declare no conflicts of interest.
Competing interests
No competing interests have been declared.
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