A cereal rye (Secale cereale L.) cover crop provides a multitude of benefits, including soil conservation and weed suppression. Cereal rye biomass is positively correlated with weed suppression; however, high biomass is not always feasible. The weed-suppression potential of cereal rye grown under conditions that do not support high biomass is unknown. In such cases, other mechanisms, such as reduced soil temperature and nutrient depletion, may contribute to weed suppression. The objective of this study was to determine the impact of cereal rye biomass levels on soil water, temperature, nutrients, and in turn, weed emergence patterns. Cereal rye was planted at four seeding rates (0, 20, 40, and 80 kg ha−1) and terminated at three timings (6, 4, and 2 wk before planting cotton [Gossypium hirsutum L.]). Soil water and temperature were continuously monitored using automatic sensors. Soil nutrient content was analyzed from samples taken before cereal rye planting and at each termination timing. Weed seedling emergence was assessed throughout the summer. Cover crop termination timing had a greater influence on biomass production than seeding rate; delaying termination by 4 wk resulted in approximately 80 to 210% more biomass. High cereal rye biomass levels reduced maximum soil surface (0 to 10 cm) temperature by up to 7 C and thermal amplitude by 10 C before crop planting. Cereal rye significantly reduced soil nitrogen content but had minimal effect on phosphorus and potassium. A minimum biomass production of 2 Mg ha−1 is necessary for 30% to 50% weed suppression, whereas moderate (2 to 4 Mg ha−1) and high biomass levels (∼6 Mg ha−1) provided 60% and 70% and >90% weed suppression, respectively. The time for 50% weed seedling emergence was delayed by 18 d under high cereal rye biomass compared with fallow. Overall, our findings indicate that cereal rye suppresses weeds through multiple mechanisms, explaining suppression even at low biomass levels.

Growing winter cereal rye (Secale cereale) (WCR) has been identified as an effective in-field practice to reduce nitrate-N and phosphorus (P) losses to Upper Mississippi River Basin, USA. In the Midwestern USA, growers are reluctant to plant WCR especially prior to corn (Zea mays L.) due to N immobilization and establishment issues. Precision planting of WCR or ‘skipping the corn row’ (STCR) can minimize some issues associated with WCR ahead of corn while reducing cover crop seed costs. The objective of this study was to compare the effectiveness of ‘STCR’ vs normal planting of WCR at full seeding rate (NP) on WCR biomass, nutrient uptake and composition in three site-yrs (ARC2019, ARC2020, BRC2020). Our results indicated no differences in cover crop dry matter biomass production between the STCR (2.40 Mg ha−1) and NP (2.41 Mg ha−1) supported by similar normalized difference vegetative index and plant height for both treatments. Phosphorus, potassium (K), calcium (Ca) and magnesium (Mg) accumulation in aboveground biomass was only influenced by site-yr and both STCR and NP removed similar amount of P, K, Ca and Mg indicating STCR could be as effective as NP in accumulating nutrients. Aboveground carbon (C) content (1086.26 kg h−1 average over the two treatments) was similar between the two treatments and only influenced by site-yr differences. Lignin, lignin:N and C:N ratios were higher in STCR than NP in one out of three site-yrs (ARC2019) indicating greater chance of N immobilization when WCR was planted later than usual. Implementing STCR saved $8.4 ha−1 for growers and could incentivize growers to adopt this practice. Future research should evaluate corn response to STCR compared with NP and assess if soil quality declines by STCR practice over time.

Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.