To date breeding and selection efforts with rye (Secale cereale L.) have not focused on its use as a cover crop, but rather the breeding in rye has focused on making it a more useful grain crop. Finding appropriate rye germplasm containing useful cover crop traits is needed to be able to maximize the effectiveness of the rye cover. Data regarding the diversity of rye germplasm is crucial in helping to modify the way this cover crop is used. There are several useful traits that could be improved to make rye a more effective cover crop, including early seedling vigor, fast biomass accumulation and canopy closure, and earliness to anthesis. These traits can be adapted to specific growing regions because of the wide range of germplasm and the large area in which a rye cover crop can be grown. Of these traits earliness to anthesis is the most important as it constitutes the most limiting factor when dealing with the management of a rye cover. Different varieties and land races can take between 32 to 61 days to go from anthesis to mature grain. There are little current data on the amount of time, heat units, and solar radiation it takes for different rye varieties to reach anthesis from planting. However, there is variability in heading date ranging from 35 to 50 days after initiation of re-growth in the spring. Due to the lack of variability for both maturity and early-season biomass production in the currently available adapted winter rye cultivars in Minnesota, germplasm acquisition and screening was conducted and crosses to move earliness into the MN rye germplasm were made.
Flowering time has been manipulated by plant breeders for a century and by farmers for several millennia. Flowering time is an important agronomic trait for several reasons including: decreasing the time between generations, increasing yield, avoidance of certain stresses, and the ability to manage the crop. This trait has extended the area in which domesticated crop plants can be grown by shortening the flowering time as well as extending the growing season to increase yield (Cockram et al., 2007). Flowering time is also important to the general survival of the plant, mechanisms of flowering evolved to protect the plant from temperature and environmental fluctuations during the year, and provide the plant with the most favorable conditions to grow during the year (Worland, 1996). Flowering time is also a highly heritable trait making it a trait easily manipulated by plant breeders (Rattunde et al., 1991). There has been variability reported in rye germplasm regarding flowering time, with as much as a 15 day difference in time of anthesis (Gregory and Purvis, 1938, Purvis, 1934). Flowering time is controlled differently in different plants; many mechanisms including photoperiod, gibberellins, temperature, and light quality can have a profound impact (Baurle and Dean, 2006). Therefore, when breeding for flowering time it is important to look at the genetic, physiologic, and environmental differences that lead to different maturities.
Envronmental influences can also have a large impact on anthesis date. Some vernalization genes have been linked to stress tolerance genes (Snape et al, 2001, Taeb et al., 1992) therefore stressed situations can cause rye to initiate flowering earlier than expected (Taeb et al., 1992). In field studies the response to vernalization and photoperiod has been shown to be very similar between wheat, barley, rye and oats (Aiken, 1966). Under conditions that are unstressed provides evidence that differences are genetic rather than environmental. When the plant is physiologically developed enough photoperiod will not limit floral initiation (Major, 1983). Temperature and maturity level can cause the importance of the photoperiod on flower initiation to be lessened (Aiken 1966).
Throughout Minnesota the spring planting date for corn (Zea mays L.) and small grains is earlier than the planting date for soybean (Glycine max L.). With the later planting date for soybean, a fall planted rye cover crop would have a longer spring growing period before the rye crop would need to be managed. This would allow for maximal cover crop characteristics increasing their benefit. A delay in the rye management is critical if the cover crop is to have significant biomass production and weed management benefits. The addition of a well-adapted fall planted cover crop to the corn soybean rotation would has the potential of reducing erosion as well as reducing nutrient loss. Many studies have shown that early spring rye management is what protects yields in the following crop later in the year (De Bruin et al., 2005, Westgate et al., 2005, Reddy, Krishna, 2003); in conventional farming systems, where herbicides can be used, timing spring rye management is relatively easy.
However, in organic systems rye should be managed at anthesis to ensure that the rye cover will not regrow. If managed before anthesis there will be significant re-growth that will impact soybean yield through competition for nutrients and water (De Bruin et al., 2005). Typical rye varieties grown in Minnesota reach anthesis in early June. A rye variety that reaches anthesis in mid- to late-May would complement this system. Therefore an important cover crop trait is earliness to anthesis. Another important trait is early biomass accumulation, which could add ground cover and quicken canopy closure, thereby reducing weed seed germination and vigor.
Short-term project objectives:
-Greater understanding of planting date on effectiveness of a rye cover crop at different latitudes in Minnesota;
-Disseminate results to Minnesota farmers regarding rye as cover crop.
Intermediate-term project goals:
-Comparison of rye to related fall-planted species;
-Develop a greater understanding of variability in rye germplasm through the growing out of rye accessions to evaluate for quick fall biomass, larger amounts of early-season spring biomass, and an earliness to anthesis while still maintaining appropriate winter hardiness for northern tier states in the US.
The Germplasm Information Network (GRIN) has 1960 accession of rye in their database (Germplasm Information Network, 2006). Accessions were chosen from GRIN based on: the latitude where they were collected, tillering capacity, and grain filling period.
Since rye is largely self-sterile with sterility estimates ranging from 96 to 99% (Stoskopf, 1985), a breeding strategy of recurrent mass selection with the desired result of producing an open pollinated variety of rye that has better cover crop characteristics than current varieties was pursued. An open pollinated cultivar is the best way to approach this breeding effort because the most important characteristic for a rye cover crop is flowering date, which ishighly heritable, h2=0.82 (Rattunde et al., 1991). High heritability makes selection possible on a single plant basis (Fehr, 1991).
The overall strategy was adapted from Geiger and Miedaner (2007), where they outlined several breeding strategies from the University of Hohenhiem rye breeding program in Stuttgart, Germany. These methods were adopted with the goal of developing an open pollinated variety of rye that has more earliness to anthesis, early season biomass, and more N uptake.
The strategy used was as follows: In 2006-2007 all heads (including tillers) from each mother plant within the polycross were placed in bags marked with the ID of the mother. Each mother plant from each replication is unique and formed the basis for its own half-sib family, which was used for later selections. In 2007-2008, progenies of the selected mother heads (half-sib famlies) were evaluated in un-replicated drilled observation head rows (1.22m x 0.2m) at two locations. Remnant seed of all half-sib families were kept. In 2008-2009, remnant seed of the selected half-sib families was multiplied by open pollination in plots with foliar isolation (walls). In 2009-2010, multi-environment trials of the multiplied seed of the advanced half-sib seed will occur with one to two replications per environment and will be evaluated for cover characteristics with the most desirable lines advanced to become a potential variety.
Forty-four accessions were planted at the Minnesota Experiment Station in St. Paul (45”N lat) planted on September 5, 2006 and October 17, 2006 for observation. Of the 44 accessions, 6 were from Dr. Thomas Miedaner of the University of Hohenhiem in Stuttgart Germany, 33 were from the USDA GRIN rye germplasm collection in Boulder Colorado, and 5 were named varieties grown in the northern tier of the U.S. and known to be of suitable winter hardiness. The germplasm was chosen based in part on latitude data showing which regions had the potential for earliness as well as the appropriate winter hardiness, and included both forage and grain types.
On April 16, 2007 four plants were selected from each of the 44 accession rows and transplanted in four randomized polycross blocks. A polycross maximizes randomness allowing for maximum amount of variation to be present (Fehr, 1991). Seed from each plant becomes a half-sib family. Within the polycross blocks plants were spaced 0.5 meters apart. The transplants were observed in both the polycross blocks as well as the accession rows, looking for characteristics of earliness to anthesis and vegetative growth. Plants that had not reached anthesis by May 25th were killed and removed from the polycross blocks by uprooting. This left 21 accessions remaining within each of the four polycross blocks. Plant introductions that were earlier in maturity than the check cultivar (Rymin) with good biomass production were allowed to pollinate. Unselected plants were killed to prevent cross-pollination. The 21 accessions from each of the four polycross blocks were hand-harvested on July 11, 2007 to obtain the half-sib familes.
While all 84 mother plants were harvested, not all produced enough seed to be planted for evaluation in fall 2007. Of the 84 half-sib families 70 had enough seed to be planted at two locations with two replications while 6 had enough seed to be planted at two locations with one replication. On September 5, 2007 the 76 half-sib families were planted (40 seeds per 1.22 m of row) at Roseau (49”N lat) and to St. Paul (45”N lat). Remnant seed was kept of all half-sib families. In early November plants were scored for emergence, vigor, and plant height and were scored for heading and anthesis starting in the middle of May, 2008. Accessions were ranked for earliness in both years and then ranks were compared across years. In November 2007 plants were given a visual biomass ranking based on plant height, plant spread and number of plants in the head row. Differences in years were observed and explained though an analysis of Growing Degree Days (GDD), solar radiation and precipitation. These data were obtained from the University of Minnesota Climatology Working Group (http://climate.umn.edu/climatology.htm, 2008). GDD were calculated in Fahrenheit for comparison to historic totals recorded by Nuttonson in 1957. Forty degrees Fahrenheit was used for final evaluations because little vegetative growth takes place below this temperature (Nuttonson, 1957).
During 2007 there were clear differences within the material. The rye was much earlier to anthesis than the usually reported early June anthesis date, likely due to the unusual nature of 2007 which had GDD and solar radiation outside a 95% confidence interval indicating the year was likely to happen only once in twenty years. The earliest accession Wintergrazer 70 reached anthesis on May 18, six days earlier than Rymin, the standard variety in Minnesota. This accession was initially released in the southern United States, but had no problem surviving the winter in Minnesota in 2007. A total of 17 accessions reached anthesis earlier than Rymin and 4 accessions reached anthesis at the same time as Rymin. This was promising evidence for genetic variability for anthesis date in rye as there was separation between the accessions. Not all the accessions within the polycross had similar seed yields. There was also some ergot (Claviceps purpurea) in the polycross, which reduced the yield. The difference in yield limited what could be done for testing in the subsequent year. The range was from less than one gram of half-sib family seed to over 50 grams. Half-sib families behaved in a similar manner across locations with respect to germination rate and visual assessment of biomass (Data not shown). Plant count was unrelated to plant height. There were clear differences in biomass with only 44 of the 276 head rows (15%) receiving the top score for biomass (4) and even fewer (10.5%) had the highest score in more than one rep or location.
None of the half-sib family rows had winter injury or winter-kill problems. All the material was substantially later to anthesis than the mother plants in 2007, but there was still separation between the half-sib families and Rymin. Rymin reached anthesis at St. Paul on June 10, 2008. This was two weeks later than it reached anthesis in 2007. There were still differences between the half-sib families and Rymin with respect to anthesis, in this case with the earliest half-sib families being 6 days earlier. Out of a total of 10 half-sib families that had an earlier anthesis date than Rymin only seven of the half-sib famliles also had an earlier heading date. Since the main objective of the screening effort was to find material that was earlier to anthesis these seven families were kept for future screening.
There was also a clear effect of year. There was not however a clear rank change between families between the years, the progeny of the earliest accessions were the earliest half-sib rows in both locations with the exception of one family (plant ID 225), which was descended from a mother plant with a later anthesis date. This provides more evidence that there is genetic variability in the rye, by showing that despite environmental differences between years there was still separation in anthesis date. Based on the high heritability reported for the flowering trait this was expected. It was also expected that this earliness can be maintained in future crosses.
Impact of Year
The weather patterns between 2006-2007 and 2007-2008 were quite different. These differences can explain the two-week difference in anthesis date for all rye germplasm screened in 2007 and 2008. The first year, 2006-2007, was much earlier than a more typical year like 2007-2008. If the difference between the rye germplasm between years is ignored the subsequent difference is likely explained. Three major environmental characteristics that greatly impact growth and maturity of rye were growing degree-days (GDD), solar radiation, and precipitation. In 2006-2007 GDD and solar radiation greatly deviated from long-term averages.
There were large differences between 2006-2007 and 2007-2008. The year of 2006-2007 was consistently above the upper limit of the 95% confidence limit of the long term average for GDD while 2007-2008 was consistently within the same confidence limit. According to Nuttonson (1957) there was not a meaningful difference when calculating number of growing degree days needed to satisfy the requirement for growth of winter rye if calculations start on March 1st or at emergence in the fall or if base temperature was 32°F or 40°F when calculating GDD to evaluate differences in growth from emergence to physiologic maturity or heading to physiologic maturity (Nuttonson, 1957). Our data agrees with this as GDD trends were the same regardless of base temperature (32°, 40° or 50° F) but disagrees with regard to their being no difference if calculated from planting or from March 1st. Calculating GDD from March 1st resulted in 2007-2008 being below the lower limit of the 95% CI and from planting the year was within the CI. Since the behavior of the 2007-2008 was fairly typical, it seems important to calculate from planting rather than March 1st. This decrease in heat units greatly impacted the maturity of the half-sib families as once the plant has been vernalized, increasing temperature will increase the speed of all developmental phases (Davidson and Christian, 1984).
In the former Soviet Union winter rye was planted on average two weeks before rye in North America, it headed approximately two-weeks earlier on average as well, but ripened at the same time (Nuttonson, 1957). An inverse relationship appears to exist between the fall planting date temperature and earliness (Purvis, 1948). The tillers of rye plants also generally flower earlier than the main stem (Gregory and Purvis, 1938). Late sown rye develops more slowly in the spring and depending on how late it is sown in the fall, it can be up to two-weeks behind rye seeded at the standard time (Nuttonson, 1957). This implies that it is very important especially for organic farmers to sow rye early in the fall (September) in order to achieve the earliest anthesis date of the rye and therefore be able to mechanically manage in the most efficient manner.
Solar radiation provided a similar story to GDD. 2006-2007 had the highest solar radiation through May in the last 10 years, while 2007-2008 was one of the five lowest totals in the last 10 years and a 3-8% overall decrease from 2006-2007 going from mid-May to mid-June. The solar radiation by month there was a 20% decrease in solar radiation in April for 2006-2007 compared with 2007-2008, which contributed to the delay in maturity of the rye. 2006-2007 was above the 95% for solar radiation during all the intervals examined, while 2007-2008 was always within the intervals. Biomass production is linearly related to solar radiation before the rye goes into the reproductive cycle of development (Green, 1987, Monteith, 1977). Major contributors to photosynthesis and therefore crop growth are the amount of light intercepted and the rate at which leaves can assimilate CO2. The intersection of high temperature and high light interception will provide the greatest amount of growth, if water is not limiting (Clough, 1984). While total plant growth is impacted more heavily than growth stage, low light intensity can influence a plant in much the same way day length does (Kemp, 1984).
There was an increase in precipitation in April, May and June in 2007-2008 from 2006-2007. The only month that was outside the 95% confidence interval of the 30-year average precipitation was April 2008, where there was more than normal precipitation. Therefore during April and May there was enough soil moisture for the rye to grow and this did not limit its physiological development.
There was a deficiency in heat units and solar radiation in April, the heat unit deficiency continued into May but solar radiation normalized. The precipitation indicates that there was enough available moisture for the rye to grow. There was clearly an energy deficiency in 2006-2007 that led to a delayed maturity. The combination of low temperatures and low solar radiation intensity in April delayed the maturity of the rye in 2007-2008 relative to 2006-2007. In 2006-2007 there was enough heat and solar radiaiton to allow the rye to complete its life cycle more quickly, while in 2007-2008 the lifecycle was delayed by suboptimal but more normal conditions. This indicates that while there is genetic variability within the rye germplasm there may not be enough to overcome environmental characteristics of the upper Midwest in every year. Rye is generally a long day plant, which means that it responds by flowering after long light periods interrupted by short dark periods. Rye usually requires greater than a 14-hour day to flower (Burger et al., 2007). However, early flowering types tend to flower at low leaf numbers, and seem to be rather insensitive to photoperiod and temperature (Aitken, 1966, Gott et al., 1955). In this respect rye behaves as a facultative long day plant, flowering can be initiated if conditions are appropriate; good heat, solar radiation and precipitation time to anthesis can be increased.
Educational & Outreach Activities
Breeding Rye as a Cover Crop in the Upper Midwest, ASA-CSSA-SSA International Annual Meeting, Indianapolis, IN
Conference Poster Presentations
Understanding the Effect of Fall Planting Date of Four Cover Crops, ASA-CSSA-SSA International Annual Meeting, Houston Texas
Rye in a Cellulosic Corn System, Biofuels, Bioenergy and Bioproducts from Sustainable Agricultural and Forest Crops, Bloomington, MN
Rye as a Cover Crop for Organic Food Production, Midwest Organic Research Symposium, WI
Community Invited Speaker
Strategies for Managing Cover Crops with a Focus on Rye, Rural Advantage 3rd Crop Producer Meetings, Fairmont, Minnesota
Speeches about rye cover crops at various extension meetings throughout Minnesota
Despite the difference in anthesis date between the optimal 2006-2007 and the normal 2007-2008 there appears to be a four to six day difference between the early half-sib families and Rymin the standard variety grown throughout Minnesota. The difference in year can be explained through differences in heat units (GDD), solar radiation, and precipitation, as the difference between the half-sib families and Rymin was maintained in both years.
Since there was still a difference in anthesis date this provides good evidence that the difference is genetic rather than spurious. However, yearly variation can have a much larger impact than genetic difference within the germplasm as the difference in antheisis between the optimal 2006-2007 and the normal 2007-2008 was 17 days. The early lines will continue to be evaluated in the hope of obtaining germplasm that will consistently reach anthesis 7-10 days before Rymin.
Rye is used successfully as a cover crop in both organic and conventional systems in rotation with soybean, corn, small grains, potato, canning crops, and sugarbeet. Growers use rye for many different reasons including erosion control, weed control, disease management, forage, to increase soil organic matter, and to increase diversity (time, space, growth form, and phenology) in their current rotations.
Rye has been grown as a cover crop in monoculture and in a bi-culture with a variety of legume species (Baldwin and Creamer, 2000; Odhiambo and Bomke, 2001). When rye is grown alone it can immobilize nitrogen and make it unavailable to the subsequent crop. This tendency can be alleviated if grown in combination with a legume through the addition of the nitrogen produced by the legume. When a legume is grown in monoculture it produces a lot of nitrogen but it may not be available to the plant at the proper time to be utilized (Baldwin and Creamer, 2000).
Growing rye in a bi-culture creates an intermediate effect between those of the monoculture rye and the monoculture legume (Ruffo and Bollero, 2003). This system however has been hard to implement in organic production systems because the legumes that have been used as part of the bi-culture (such as hairy vetch and crimson clover) can be very persistent and are difficult to kill without the use of herbicides. It has also been difficult to implement this system in the northern Midwest section of the United States because of winter tolerance limitations in many legume species (Ruffo and Bollero, 2003).
Since it is not possible to use synthetic herbicides in organic systems, there are several other methods for managing the rye cover crop: undercutting, mowing, rolling, tillage or grazing. The different methods of mechanically killing the plant all have different advantages and disadvantages (Baldwin and Creamer, 2000).
• Undercutting kills the plant by severing the roots from the aboveground part of the plant
• Mowing and grazing cuts the plant above the ground
• Rolling crimps the plant causing it to die more slowly, which allows weed control to persist longer (Lu et al., 2000)
• Tillage disturbs the root system, killing the plant if the tillage results in root desiccation
When rye is mechanically managed by mowing, the date of management becomes very important in determining the amount of re-growth, the amount of weed suppression, and how well the subsequent crop will become established (De Bruin, et al., 2005). The earlier the management date, the shorter the residue will persist on the soil, but the more re-growth will occur.
Determining the method and management date of the rye cover crop is dependent on the goals of the rye in the cropping system. It is important to recognize the diversity of systems that rye fits into and to change management practices for each cropping system.
For example, the management date will be different if the rye is grazed or if followed by corn. Interseeding soybean into rye has been shown to be a successful agronomic practice, especially in organic production systems (Porter et al., 2005). Soybean can be grown after rye without negatively impacting yield (Reddy, 2003; Ateh and Doll, 1996; Liebl et al., 1992; Moore et al., 1994; Ruffo et al., 2004). However, timing of both fall planting, spring management and planting of soybean will impact rye biomass accumulation and soybean vigor (Maloney et al., 1999; Westgate et al., 2005).
Currently, much of the research involving rye in cropping systems is attempting to quantify benefits associated with the use of the rye cover crop and finding the optimal ways to manage rye to ensure getting the most benefit from nutrient scavenging, erosion control, carbon sequestration, water use efficiency, and insect management while not adversely affecting the economics of the farming operation. To date many systems have been used in order to maximize both the economical and the environmental aspects of the rye cover crop.
Different systems perform better in different geographical areas. In southern Ontario, Canada, rye in a continuous corn system allowed that system to be maintained without yield loss (Ball Coelho et al., 2005). In Minnesota, much of the acreage that uses rye as a cover crop is devoted to potato production systems grown on sandy soils prone to erosion. Rye is an excellent cover after a potato crop due to its scavenging ability and early vigor.
One of the most promising systems to date seems to be using rye as a cover after corn and before soybeans. It has been shown that no yield depression is seen when rye is grown before the soybeans. The soybeans can be planted directly into the rye and the rye can therefore be killed at a relatively late date (Porter et al., 2005). This allows maximization of the benefits of the cover crop while still maintaining the soybean cash crop yield. This system has great potential as it can be used in both organic and conventional systems. Aerial seeding of rye into standing corn allows the cover crop to become established earlier in the fall and has proved successful. This practice works especially well in silage corn and sweet corn. As the RyeGro model predicts (Feyereisen et al., 2006), early establishment of rye in the fall increases fall biomass and speeds canopy closure.
When examining Minnesota in particular, there are differences between regions. In SE Minnesota possible uses for rye include planting rye as cover crop following a canning crop, corn or soybean, then grazing the rye in both the fall and the spring before killing the rye either chemically or mechanically to ensure good growth of the subsequent crop. Rye grows well in almost all environments and landscape positions. This attribute, in addition to its early establishment and vigorous spring growth make rye an excellent choice to prevent erosion. Rye also helps build soil organic matter, which will benefit subsequent crops.
In central Minnesota rye is used as a cover crop following a potato crop to help soak up excess nutrients in the soil and prevent wind and soil erosion of the sandy soils prevalent in that area. Both NW and SW Minnesota have the potential to use rye as a cover crop in organic situations for weed control as well as to control leaching and help prevent erosion. In organic systems rye not only provides cover, but the allelopathic effect contributes to weed control in the following crop. In a conventional system rye can be used to help control nitrate leaching and erosion, as well as to help prevent phosphorous runoff. Rye can also be used to prevent wind erosion and alleviate sugarbeet root maggot when grown in rotation with sugarbeets (Stordahl et al, 1992).
Areas needing additional study
Additional research on rye is needed to continue to find ways to maximize its benefit as a cover crop in both conventional and organic systems, particularly in northern regions of the United States. Such research can help producers effectively manage the cover crop and the subsequent crop in terms of energy, economics and the environment. This information would provide different options for growers to reach their individual goals with the cover crop. Research on germplasm enhancement, aerial seeding, grazing in a mixed livestock-cropping operation, influence on carbon sequestration in continuous corn with the residue removed, and in organics will continue to fine-tune production practices which will allow for greater adoption of rye as a cover crop.
Rye has been shown to be an effective cover crop. While it has been investigated in many locations, it is most intriguing in northern temperate regions because of its excellent winter hardiness coupled with its broad adaptability. Work to date has shown rye’s tremendous potential to fill a niche in northern climates as an environmentally helpful cover crop following many different cash crops. There is still work to be done in terms of maximizing the efficiency of the cover and breeding varieties that are specifically designed to be cover crops.