Nitrogen fertilizer rate effects on yield and botanical components of summer annual forage mixtures

Abstract

Summer annual grass-legume mixtures may provide supplemental grazing for livestock when cool-season pastures are less productive, but nitrogen fertilizer recommendations for these mixtures are not well established. Inputs to these systems are often high, so optimizing N fertilizer rates may increase appeal to producers. This study evaluated the effects of increasing botanical diversity and N fertilizer application on the yield and botanical composition of summer annual mixtures in four environments in Kentucky. Nitrogen fertilizer rates of 0, 56, 112, 168, and 224 kg N ha -1 were applied to a sudangrass [Sorghum bicolor (L.) Moench × Sorghum sudanese (P.) Stapf] monoculture, a three-species mixture, and an 11-species mixture. In three out of four environments, sward biomass increased as N application increased (average of 15 kg dry matter (DM) ha -1 increase per kg N ha -1 ; p < .05). Mixture complexity had no effect on forage DM accumulation in three out of four environments (4,000, 5,830, and 7,280 kg DM ha -1 averaged over mixture for three environments; p > .05). Mixtures were overwhelmingly dominated by grasses, resulting in low functional diversity. Legumes did not respond to N (p > .05), but their contribution to sward DM was <4%. Species compatibility should be a priority when utilizing multi-species mixtures. If a stronger legume component is desired, care must be taken to provide management that favors these species, such as reduced grass-seeding rates to limit competition, especially during initial establishment.

Nitrogen fertilizer rate and timing effects on smooth bromegrass interseeded with sorghum‐sudangrass

“…However, N fertilizer rate and timing did not affect sorghum‐sudangrass mass, suggesting that it has limited effects on the success of sorghum‐sudangrass when interseeded into perennial cool‐season grass pastures. This contrasts with seeding in conventionally prepared seedbeds, where linear N responses have been observed (Mercier et al., 2021). The lack of effects on cool‐season grass mass in summer also appears to show that the N fertilizer rate and timing treatments do not increase competition by smooth bromegrass in interseeded pastures, the assessment of which was a principal objective in the design of this research.…”

“…The lack of treatment effects illustrates the somewhat limited ability of practitioners to get more production from pastures in summer if interseeding annual warm‐season grasses fails (Table 1). Alternative strategies might be to use perennial cool‐season grass pastures that have been mixed with legumes (Hillhouse et al., 2021), to seed separate pastures of summer annuals (Mercier et al., 2021), or to use perennial warm‐season grass pastures (Keyser et al., 2020). In Kentucky, summer annual forage mixtures that contained sudangrass produced 1.79 to 3.25 ton acre –1 , depending on the environment, when seeded into conventionally prepared seedbeds that had been rotovated, cultivated, and disked (Mercier et al., 2021).…”

“…Alternative strategies might be to use perennial cool‐season grass pastures that have been mixed with legumes (Hillhouse et al., 2021), to seed separate pastures of summer annuals (Mercier et al., 2021), or to use perennial warm‐season grass pastures (Keyser et al., 2020). In Kentucky, summer annual forage mixtures that contained sudangrass produced 1.79 to 3.25 ton acre –1 , depending on the environment, when seeded into conventionally prepared seedbeds that had been rotovated, cultivated, and disked (Mercier et al., 2021). The forage mass of mixtures also tended to increase linearly as the N fertilizer rate increased (Mercier et al., 2021).…”

“…izer rates and timing affect the establishment and productivity of annual warm-season grasses (Mercier et al, 2021). In the research completed by Guretzky et al (2020Guretzky et al ( , 2021, N application occurred after annual warm-season grass emergence in early summer to minimize competition from existing perennial cool-season grasses.…”

“…sudanense ) have been shown to have 100 to 214% greater forage accumulation than unseeded pastures (Guretzky et al., 2020) and greater mid‐ and late summer yields of crude protein and digestible organic matter (Guretzky et al., 2021). Critical to the success of interseeding pastures is an understanding of how N fertilizer rates and timing affect the establishment and productivity of annual warm‐season grasses (Mercier et al., 2021). In the research completed by Guretzky et al.…”