Spring establishment of tall fescue utilizing sunn hemp as a companion crop under rotational grazing management
Authors:   , Brandl Chrisee Sue 
Published: April 2024

[EMBARGOED UNTIL 12/01/2024] Tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) is the predominant perennial cool season grass used for forage in Missouri. It demonstrates a bi-modal seasonal yield distribution with forage production periods observed in the spring and fall and a lack of forage production in the summer and winter months. Establishing tall fescue as part of a total renovation or a partial renovation of existing pastures is recommended to be performed in the fall rather than the spring due to more desirable environmental conditions and the ability to establish roots prior to subsequent spring growth. However, spring establishment of tall fescue may be desirable as management opportunities arise. Introducing warm season annual legumes, such as sunn hemp (SH, Crotalaria juncea L.), may improve summer forage productivity, mitigating the summer slump as well as serving as a companion crop during spring tall fescue establishment. We hypothesized that the inclusion of SH in combination with spring planted tall fescue would benefit tall fescue establishment the subsequent spring while providing grazeable forage throughout the growing season. The objectives of this study were to 1. evaluate the effects of SH seeding rate and grazing on subsequent spring tall fescue plant density and 2. evaluate the effects of SH seeding rate, grazing, and seasonal productivity on herbage accumulation and forage nutritive value. The experiment was a split block design in the years 2020 and 2021; Whole-plots were assigned SH seeding rates 0, 11, 22, and 34 kg ha-1) and sub plots were assigned as GRAZED and NON-GRAZED treatments. There were 4 GRAZED replications and 4 NON-GRAZED replications. Differences were considered significant at P [less than or equal to] 0.05 and data regressions were considered significant at P [less than or equal to] 0.1. Sunn hemp was separated from all other pasture plant species that will be referred to as other grasses and forbs henceforth. Tall fescue spring establishment was greatest at 0 and 22 kg ha-1 SH seeding rates and grazing resulted in greater TF plant density than non-grazed treatments. Soil test results revealed that ammonium increased in the subsequent spring. In contrast to ammonium, the cation exchange capacity and magnesium decreased in the subsequent spring. Sunn hemp herbage accumulation was greatest at 22 and 34 kg ha-1 SH seeding rates and 120 days after planting (DAP) in grazed treatments. In contrast to SH herbage accumulation, other grasses and forbs herbage accumulation declined at greater SH seeding rates with grazing imposed. Crude protein (CP) concentrations increased in other grasses and forbs as SH seeding rate increased with and without grazing. Sunn hemp neutral detergent fiber digestibility (NDFD), in-vitro true digestibility (IVTD), and CP concentrations were greater in grazed treatments compared to non-grazed treatments. Other grasses and forbs digestible neutral detergent fiber (dNDF) concentrations were greater in the non-grazed treatments compared to the grazed treatments. With grazing imposed, other grasses and forbs neutral detergent fiber (NDF), NDFD, dNDF, IVTD, and CP concentrations decreased with DAP. Without grazing, other grasses and forbs NDF and acid detergent fiber (ADF) concentrations increased with DAP whereas NDFD and IVTD concentrations decreased with DAP. With grazing imposed, SH NDF concentrations increased with DAP whereas IVTD and CP concentrations decreased with DAP. Without grazing, SH NDF and ADF concentrations without grazing increased with DAP whereas NDFD, IVTD, and CP concentrations decreased with DAP. Sunn hemp plant density decreased with DAP and increased as SH seeding rate increased with the greatest plant density at the 34 kg ha-1 seeding rate with grazing imposed and in NON-GRAZED treatments. Sunn hemp plant density was greater in the NON-GRAZED treatments compared to the GRAZED. A positive correlation was observed for soil temperature and SH herbage accumulation and SH plant density such that soil temperature increased as SH herbage accumulation and SH plant density increased. A negative correlation was observed for photosynthetically active radiation (PAR) and SH herbage accumulation such that PAR decreased as SH herbage accumulation increased. Sunn hemp plant density and SH herbage accumulation in NON- GRAZED treatments were positively correlated. Sunn hemp herbage accumulation increased as SH plant density increased. A negative correlation was observed for TF plant density and seasonal herbage accumulation of all pasture plant species such that TF plant density decreased as seasonal herbage accumulation increased. Our data suggest that the combination of SH and grazing can benefit spring planted TF establishment. Sunn hemp herbage accumulation increased significantly with rate and DAP providing greater amounts of grazeable forage during the summer months. Other grasses and forbs herbage accumulation decreased significantly with greater SH seeding rates, suggesting that SH can reduce competition from other plant species. The increase in ammonium the subsequent spring suggests that animal excreta from grazing and plant residues were beneficial to the persistence of TF planted in our study. Based on correlations of environmental factors and SH herbage accumulation, grazing can potentially alter the canopies microclimate to lower soil temperature and increase PAR through the removal of vegetation. In summary, the inclusion of SH at any seeding rate and the application of grazing is a promising practice to benefit spring planted TF establishment.