Nitrogen Dynamics in Sunn Hemp Intercropped Tall Fescue Pastures

Incorporating temperate legumes is a strategy for increasing nitrogen (N) in tall fescue (Schedonorus arundinaceus (Schreb.) Dumort, nom. Cons) systems. However, when temperatures are elevated, biological N-fixation (BNF) by temperate legumes is limited. Sunn hemp (Crotalaria juncea L.), a warm-season annual legume, may provide greater N input during the warm season. This 2-year study aimed to (1) determine BNF in sunn hemp-tall fescue mixed pastures and (2) determine N transfer from sunn hemp to tall fescue. The experiment included four replicates of two treatments: tall fescue (TF) and tall fescue intercropped with sunn hemp (TF+SH), arranged in a randomized complete block design. Response variables included δ15N, N derived from the atmosphere (%NDFA), BNF, N concentration, N transferred (%Ntran), N stock, and herbage accumulation (HA). Herbage accumulation was 16% greater in TF+SH compared to TF (p < 0.05). Root mass was 43% greater for TF compared to both species combined in TF+SH (p < 0.05). Herbage N was 40% greater in sunn hemp shoots than tall fescue shoots in TF or TF+SH (p < 0.05). Sunn hemp root N was 34% greater than tall fescue (p < 0.05). NDFA by sunn hemp was 88% and 100% in 2017 and 2018, respectively. BNF by sunn hemp was greater (p < 0.05) in 2018 than in 2017 (53.8 and 44.3 kg ha−1, respectively). The %Ntran from sunn hemp to tall fescue was 13 and 20% in 2017 and 2018, respectively. Interseeding sunn hemp into tall fescue pastures can provide an alternate N source to tall fescue-based forage-livestock systems, increasing herbage accumulation during the summer grazing season.