Mean monthly air temperatures for the winter rye growing period (September through April) and corn growing season (May through September) were generally above normal (compared to 30-year averages reported by NOAA for Arlington, WI, 1980 through 2010) for the double-crop seasons 2011–2012 and 2015–2016, whereas temperatures trended below normal for seasons 2012–2013 and 2013–2014. Monthly precipitation totals were 43% below normal during the 2012 drought with only 29 cm of precipitation falling May through September. June 2014 had 24 cm precipitation (compared to 12 cm normal), which helped to compensate for otherwise droughty conditions that year. Precipitation from September 2015 through September 2016 was 27 cm above normal. This variability in weather conditions over a 5-year study is ideal, because we are better able to discern the potential behavior of this cropping system over time under changing climatic conditions.
Winter rye growth
Winter rye aboveground biomass production in the RyeCC treatment averaged 3.44, 0.34, 0.36, 0.28, and 3.08 Mg dry matter ha−1 in 2012, 2013, 2014, 2015, and 2016, respectively (standard error (SE) = 0.11, 0.04, 0.03, 0.03, 0.08). Mean N content of winter rye aboveground biomass was 129, 12, 12, 10, and 83 kg ha−1 (SE = 6.1, 1.7, 1.1, 1.3, 9.4). Winter rye forage yield in the RyeHarv treatment averaged 5.5, 2.3, 3.0, 3.3, and 6.0 Mg dry matter ha−1 (SE = 0.19, 0.06, 0.23, 0.14, 0.22) with mean N removal of 135, 67, 73, 93, and 105 kg ha−1 in 2012 through 2016, respectively (SE = 8.2, 6.3, 5.1, 8.7, 4.6). The range of these values and the increase in biomass from the time of cover crop termination to the time of forage harvest is in line with previous findings in North Central United States (Krueger et al. 2012; Basche et al. 2016). The tenfold increase in RyeCC aboveground biomass and N uptake in 2012 and 2016 as compared to other years and the twofold increase in rye forage yield in the RyeHarv treatment in the same study years may be attributed to temperatures exceeding normal in the associated rye growth periods (mean monthly temperature 2.25 and 3.25 °C above normal September through April for 2011–2012 and 2015–2016, respectively), particularly in contrast to below normal temperatures during rye growth periods in the other study years (also observed in Martinez-Feria et al. 2016). With climatic shifts in Wisconsin toward higher mean annual temperatures punctuated by earlier onset of spring and later onset of winter (Wisconsin Initiative on Climate Change Impacts 2011), winter cover crops are poised for higher production potential in future seasons, and thus increased ecosystem services such as prevention of soil erosion by both wind and water (Blanco-Canqui et al. 2015) and reduction in soil nitrate concentrations (Martinez-Feria et al. 2016). The high rye biomass values in RyeCC in 2012 and 2016 and in RyeHarv in all years would be sufficient for a 50% reduction in water runoff and 80% reduction in soil erosion according to a meta-analysis done by Ranaivoson et al. (2017). Although belowground biomass was not quantified, previous work suggests that belowground production may match aboveground production of winter rye biomass in this context (Austin et al. 2017; Cates and Jackson 2018).
Averaged over the 5 study years, mean preplant soil NO3-N concentrations for NoCC, RyeCC, and RyeHarv, respectively, were 14.4, 8.3, and 5.3 mg N kg−1 at 0–30 cm and 14.3, 11.5, and 4.6 mg N kg−1 at 30–60 cm; repeated measure analysis demonstrated a main effect of cover crop treatment (Table 2). Annual variation reflected weather trends to some extent in that increased rainfall from September 2012–April 2013 and September 2015–April 2016 may have contributed to low soil NO3-N measured at preplant in 2013 and 2016 at both depths (Fig. 2), whereas low rainfall in September 2014–April 2015 agrees with higher preplant soil NO3-N in 2015 at both depths. Repeated measure analysis also demonstrated a year × cover crop treatment interaction effect on preplant soil NO3-N (Table 2). Accordingly, soil NO3-N data were also analyzed separately for each study year. There was a main effect of cover crop on preplant soil NO3-N at 0–30 cm in 4 out of 5 years (2012, 2013, 2014, and 2016; p < 0.05), indicating RyeCC and RyeHarv treatments significantly reduced soil NO3-N at 0–30 cm as compared to NoCC and the RyeHarv treatment further reduced soil NO3-N compared to RyeCC in 2013 and 2014 (Fig. 2). The reduction in soil NO3-N carried through at 30–60 cm in 2012 (p = 0.0011; RyeCC and RyeHarv < NoCC) and 2014 (p = 0.0149; RyeHarv < NoCC and RyeCC; Fig. 2). These results support our prediction that the rye treatments would significantly reduce soil NO3-N at preplant, thus indicating lower concentrations of environmentally reactive and mobile N in the soil profile at this critical time when significant spring leaching events occur prior to corn planting or crop demand for N (Blanco-Canqui et al. 2015).
Mean in-season soil NO3-N collected just prior to sidedress, 0–30 cm, was 14.0, 11.2, and 5.7 mg kg−1 (SE = 1.2, 0.5, 0.4) across study years in NoCC, RyeCC, and RyeHarv, respectively, and repeated measure analysis showed main effect of cover crop and year × cover crop treatment interaction effect (Table 2). Analysis within each year demonstrated a main effect of cover crop on in-season soil NO3-N at 0–30 cm at sidedress (p < 0.05, all years); RyeHarv had significantly lower soil NO3–N than that of NoCC and RyeCC (all years; in-season data not graphed).
The reduction in NO3-N in RyeHarv measured at preplant and extending into the corn growing season compared to RyeCC is expected given that the rye in the RyeHarv treatment grew for a longer period of time, producing more measured aboveground biomass and presumably more root biomass (Austin et al. 2017), with an increased likelihood of net N immobilization upon decomposition (Martinez-Feria et al. 2016). After the decrease in soil NO3-N concentrations observed at preplant in RyeCC relative to NoCC, the in-season “rebound” of soil NO3-N in RyeCC to concentrations similar to those observed in NoCC implies timely release of N from the rye cover biomass. This is desirable and indicates that nutrient release timing may meet crop need, and immobilization is less likely to decrease corn yield.
The lack of a cumulative trend in soil NO3-N as the study years progressed and weak responses of rye forage yield and corn silage yield attributable to N rate (section 3.4, Table 2) indicate that excess N was either lost from the system, as estimated by the N balance approach (Section 3.6, below), or maintained to some extent in decomposing biomass, potentially contributing to soil carbon accumulation (Poeplau and Don 2015).
Repeated measure analysis demonstrated main effects of year, cover crop, and N rate on corn silage yield and on total forage yield (Table 2). As predicted, there was no statistically significant difference between corn silage yield in NoCC or RyeCC, and total forage yield in the RyeHarv treatment was significantly greater than NoCC or RyeCC, despite corn silage yielding 13% and 14% lower in RyeHarv compared to NoCC and RyeCC, respectively. On average, mean total forage production over 5 years was 6–7% higher in the RyeHarv treatment than corn silage alone in NoCC (Table 2), which is an encouraging result for a moderate adjustment to an already highly productive cropping system. This suggests one potential avenue toward intensification focused on increased production without exacerbating environmental issues such as soil and water quality, as advocated by Cassman (1999). Annual differences seen in mean yields reflect the main weather patterns observed: Drought in 2012 combined with a slight increase in mean monthly temperatures resulted in the lowest yields of any year. Decreased corn silage yields also were associated with excess rainfall and increased temperatures in 2016, though only a moderate decrease in total forage yield (Table 2). The highest yielding years were 2014 and 2015, which were the most typical study years regarding temperature and precipitation. Repeated measure analysis also demonstrated an advantage in corn silage and total forage yields at the overapplication rate of 179 kg N ha−1 rate compared to the underapplication rate of 67 kg N ha−1, though no clear differences in yields between the recommended rate of 112 kg N ha−1 and either the over or underapplication rates (Table 2).
The repeated measure analysis also demonstrated a year × cover crop treatment interaction effect (Table 2); therefore, the following analysis of yield data by study year was completed to tease out annual responses. Notably, corn silage yields in RyeCC and NoCC treatments were statistically similar in each study year. The significant effect of cover crop treatment on corn silage yield in 2012 and 2014 (Table 3) was driven by a 15% decrease in corn silage yield in RyeHarv treatment compared to RyeCC and NoCC treatments in 2014 and 34% decrease in 2012 under drought conditions. Corn planting was delayed in the RyeHarv treatment in 2014, 2015, and 2016, which may have contributed to decreased corn yield in that treatment in 2014, though unapparent in 2015 or 2016. Reductions in corn silage yield in the RyeHarv treatment are offset by yield of rye biomass harvested as forage, as is demonstrated by the statistically similar total forage yields in study years 2012 through 2015 (i.e., lack of significant effect of cover crop treatment), and significant increase in total forage yield in RyeHarv in 2016 by 23% and 26% as compared to corn silage production in NoCC and RyeCC treatments, respectively.
Though repeated measure analysis reflected increased yield with 179 kg N ha−1 compared to 67 kg N ha−1, analysis within each year demonstrated that N rate only had a significant effect on yields in 2013, for both corn silage and total forage yields (Table 3). This may be explained by the generally lower soil NO3-N concentrations at preplant in 2013 (Fig. 2), possibly due to N immobilization resulting from the combination of high rye biomass production and drought conditions in 2012 that may have impeded rye decomposition. Alternatively, soil NO3-N may have decreased due to excess rainfall from September 2012–April 2013. The applied N apparently made up for any potential deficit in soil NO3-N with regard to yield, similar to other studies demonstrating potential for increased N rate to offset potential N immobilization and resultant crop yield loss (Martinez-Feria et al. 2016; Ranaivoson et al. 2017).
The neutral effect on corn silage yield in RyeCC is an expected (Practical Farmers of Iowa and Iowa Learning Farms 2019) yet still encouraging response as others have periodically found rye cover to result in corn silage yield declines, particularly under high levels of rye biomass growth (Krueger et al. 2012; Marcillo and Miguez 2017). The greater total forage yield in the RyeHarv system (Table 2), without the requirement more N in most years (Table 3), demonstrates that sustainable intensification is possible when connected with dairy production (manure input and forage output) in North Central United States. There was no apparent cumulative treatment effect on yield as the study progressed over five seasons, given that the low yields in the first study year (2012) were attributed to severe drought.
Forage quality analysis
Analysis of corn silage quality parameters across all years indicated a statistically significant effect of N rate on the crude protein content of corn silage (p = 0.0147). Mean crude protein at 67 kg N ha−1 was 7.97%, which was a significant reduction from 8.43% at 179 kg N ha−1. The other quality parameters evaluated indicated no significant effect of cover crop or N rate treatments or treatment factor interaction. The following are mean values for corn silage quality parameters over the course of the study (and SE): 8.22% crude protein (0.12), 24.1% acid detergent fiber (0.36), 43.8% neutral detergent fiber (0.52), 65.1% total digestible nutrients (0.22), 1.47 Mcals kg−1 net energy for lactation (0.01), and 1543 kg Mg−1 milk production (9.04).
Analysis of rye forage quality was completed to ensure that the rye forage was a high-quality feed option. The following are mean values for the quality parameters over the course of the study (and SE): 17.6% crude protein (0.94), 29.3% acid detergent fiber (1.0), 49.0% neutral detergent fiber (1.8), 70.6% total digestible nutrients (0.91), 1.59 Mcals kg−1 net energy for lactation (0.026), and 1759 kg Mg−1 milk production (43.8). Compared to corn silage, the rye forage had higher crude protein content and higher associated milk production estimates. However, the acid detergent fiber, which is a largely undigestible fraction (Hoffman and Shaver 2004), was also typically greater in rye forage.
Though somewhat distinct feeds with unique nutrient characteristics, the rye harvested as forage generally measures up to corn silage as a high-quality forage option, and as such the yields were not adjusted for nutrient content. Thus, total forage yield is a simple sum of dry matter yields of corn silage and rye harvested as forage. Though beyond the scope of this paper, a full nutrient and economic analysis of this production system could assess nuanced trade-offs in forage quality and overall dietary structure. Further, there may be some cultural limitations to overcome as rye forage is mainly considered a feed option for pre-lactational heifers. Our results, however, provide evidence that intensifying a corn silage production system in this way over multiple years has the potential to increase total biomass with higher crude protein content and associated milk production estimates and thus may help to encourage adoption as this system can increasingly be seen as a net benefit to dairy operations.
The N balance presented here is meant to serve as a means of treatment comparison and relative indicator of potential system excess based on accounting of known inputs and outputs and does not represent N loss measured through edge-of-field monitoring, which is inherently problematic because the mechanisms of N loss are nonpoint source (e.g., nitrate leaching, soil erosion by wind or water, N2O emissions; Kladivko et al. 2014). Analysis of the cumulative N balance over the 5-year study period indicates a significant effect of both cover crop and N rate treatments (p < 0.0001 for both treatment factors; Fig. 3) with no treatment factor interaction (p = 0.1864). Cumulative 5-year N balances for NoCC, RyeCC, and RyeHarv were 646, 667, and 403 kg N ha−1, respectively. Cumulative N balance of RyeHarv was about 40% lower than NoCC or RyeCC (p < 0.0001), indicating that a large quantity of N was removed in the RyeHarv system that otherwise would have been susceptible to environmental loss without double-cropping. NoCC and RyeCC were statistically similar, though this calculation in no way captures the amount of N taken up by the growing rye cover crop. The fate of rye biomass-N is unknown, but the decreased soil NO3-N concentration at preplant and sidedress in RyeCC relative to NoCC (Table 2) indicates that a portion of N was tied up in the terminated rye biomass, though not to such an extent that corn silage yield indicated cover crop-related N immobilization in RyeCC (Table 2). The aboveground N content of rye biomass in the RyeCC treatment exhibited extremes (129, 12, 12, 10, and 83 kg N ha−1 in the respective study years) and represents over 35% percent of the cumulative N balance for RyeCC. This uptake of N, though not removed from the system, potentially slows and buffers leaching loss to the environment (Tonitto et al. 2006; Austin et al. 2017) and might contribute to the accumulation of soil carbon, based on substantial evidence that unharvested cover crops can contribute to the accumulation of soil carbon in a review by Poeplau and Don (2015).
N balances across cover treatments by N rate treatment were 366, 531, and 818 kg N ha−1 for 67, 112, and 179 kg N ha−1, respectively (p < 0.0001 for all treatment differences). Interestingly, the applied N rate × 5 (years of application) is nearly equivalent to the respective N balances, which underscores the lack of yield benefit in all but one study year from the added N beyond 67 kg N ha−1 (Table 3), and also serves to conceptually convert the excess applied N directly to potential environmental loss. The reduced N balance achieved by double-cropping with winter rye as a forage bolsters potential for sustainable intensification (Cassman 1999). Overall, the cumulative N balances are high and demonstrate excess due to the high application rate of manure, though we only expect 30–40% of manure-N to be available to the crop (Laboski et al. 2006). This is a negative characteristic of manure-based systems, though other research demonstrates that manure has a high carbon content and can contribute to soil organic carbon stabilization (Kirchmann et al. 2004). In this study, we inadvertently applied a range of manure-N rates, which could be attributable to variability of manure characteristics and the inherent challenges of calibrating machinery for plot-scale application. Improvements to manure-N management can play a large role in decreasing N balances and thus potentially reducing N loss from these systems.
This research is novel because we demonstrated consistent levels of agronomic production (Tables 2 and 3) associated with implementation of a winter rye cover crop in manured continuous corn silage over five seasons with a range of weather conditions in WI, where corn silage covers almost twice the area of any other North Central state. Soil coverage was substantially increased over the course of a year (Fig. 1), thus mitigating vulnerability to soil degradation and potentially improving water quality as forecasted by reductions in N balance and soil NO3-N (Figs. 2 and 3). Continued research is needed to inform precise management of tightly coupled rotational systems that are accessible and attainable by growers interested in sustainable intensification. This is how growing market demands can be met in the short term while simultaneously mitigating trade-offs in environmental quality over the long term. Areas of future interest may include comparison of other winter cover/forage crops such as triticale, potential for climate change mitigation with respect to carbon storage in high biomass removal systems, and potential for managed N to enhance soil carbon accumulation in agricultural systems such as these.