ORIGINAL ARTICLE

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Defoliation management affects performance of alfalfa–bermudagrass mixtures in the southeastern United States

John Harling

orcid.org/0000-0002-5312-7174

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Writing - original draft, Writing - review & editing

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Esteban Rios,

Corresponding Author

Esteban Rios

[email protected]

orcid.org/0000-0003-3389-7195

Agronomy Department, University of Florida, Gainesville, FL, USA

Correspondence

Esteban Rios, Agronomy Department, University of Florida, Gainesville, FL 32608, USA.

Email: [email protected]

Contribution: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Supervision, Writing - review & editing

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Cleber Henrique de Souza,

Cleber Henrique de Souza

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Data curation, Investigation, Methodology, Writing - review & editing

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Lynn E. Sollenberger,

Lynn E. Sollenberger

orcid.org/0000-0001-7944-2749

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Formal analysis, Investigation, Methodology, Resources, Supervision, Writing - original draft, Writing - review & editing

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José C. Dubeux,

José C. Dubeux

orcid.org/0000-0001-8269-9959

Agronomy Department, North Florida Research and Education Center, University of Florida, Marianna, FL, USA

Contribution: Conceptualization, Funding acquisition, Methodology, Supervision, Writing - review & editing

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Marcelo O. Wallau,

Marcelo O. Wallau

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Investigation, Resources, Supervision, Writing - review & editing

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John Harling,

John Harling

orcid.org/0000-0002-5312-7174

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Writing - original draft, Writing - review & editing

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Esteban Rios,

Corresponding Author

Esteban Rios

[email protected]

orcid.org/0000-0003-3389-7195

Agronomy Department, University of Florida, Gainesville, FL, USA

Correspondence

Esteban Rios, Agronomy Department, University of Florida, Gainesville, FL 32608, USA.

Email: [email protected]

Contribution: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Supervision, Writing - review & editing

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Cleber Henrique de Souza,

Cleber Henrique de Souza

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Data curation, Investigation, Methodology, Writing - review & editing

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Lynn E. Sollenberger,

Lynn E. Sollenberger

orcid.org/0000-0001-7944-2749

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Formal analysis, Investigation, Methodology, Resources, Supervision, Writing - original draft, Writing - review & editing

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José C. Dubeux,

José C. Dubeux

orcid.org/0000-0001-8269-9959

Agronomy Department, North Florida Research and Education Center, University of Florida, Marianna, FL, USA

Contribution: Conceptualization, Funding acquisition, Methodology, Supervision, Writing - review & editing

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Marcelo O. Wallau,

Marcelo O. Wallau

Agronomy Department, University of Florida, Gainesville, FL, USA

Contribution: Investigation, Resources, Supervision, Writing - review & editing

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First published: 02 February 2023

https://doi.org/10.1002/agj2.21296

Citations: 1

Assigned to Associate Editor Reagan Noland

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Abstract

Overseeding alfalfa (Medicago sativa L.) into grass swards increases forage nutritive value and reduces N fertilizer requirement. In warm climates, establishing and maintaining alfalfa in mixtures with vigorous C4 grasses is challenging, which makes adjustment of defoliation management critical. The objective of this 2-year study was to determine optimal harvest stubble height (SH; 5, 10, and 15 cm) and cutting interval (CI; 2, 4, and 6 weeks) for two alfalfa genotypes (“Bulldog 805,” and breeding line UF Alf Pers 2015 [UF 2015-AP]) in mixture with “Tifton 85” bermudagrass (Cynodon spp.). When statistical differences occurred, shorter SH resulted in greater mixture forage accumulation (FA), with this effect generally more pronounced for UF 2015-AP than Bulldog 805, and for CI of 6 weeks than 2 or 4 weeks. Genotype did not affect alfalfa FA, but alfalfa FA was least for the 2-week CI for all SH levels, and greatest for the 5-cm SH when CI was 4 weeks, and for the 5- and 10-cm SH when CI was 6 weeks. Alfalfa percent cover was generally greater for longer CI, regardless of SH. Mixture crude protein and digestibility were minimally affected by treatments because the expected negative effects of long CI on nutritive value were offset by greater alfalfa proportion in the mixture for these treatments. In this environment, CI was generally the most important factor affecting alfalfa persistence in alfalfa–bermudagrass mixtures. A short SH in combination with longer CI (4–6 weeks) resulted in the greatest mixture and alfalfa FA.

Abbreviations

CI: cutting interval
CP: crude protein
DM: dry matter
FA: forage accumulation
IVDOM: in vitro digestible organic matter
NDF: neutral detergent fiber
SH: stubble height

1 INTRODUCTION

Incorporating legumes into perennial warm-season grass pastures and hayfields has received increased attention as a potential tool to reduce fertilizer use and increase forage nutritive value (Sollenberger et al., 2019). When in mixture, legumes can provide N for the grass and increase forage crude protein (CP) concentration (Brown & Byrd, 1990). Although use of grass–legume mixtures is common in temperate environments, in warm-climate grasslands vigorous C4 grasses often provide greater competition to legumes than do the C3 grasses common to temperate regions. Thus, the use of temperate legumes in mixture with warm-season grasses presents management challenges and requires particular attention to defoliation management in order to achieve success and encourage wider adoption by producers (Muir et al., 2011). However, under specific management practices, legumes have outcompeted bermudagrass [Cynodon dactylon L. (Pers.)] and contributed to stand decline. For example, White and Lemus (2015) reported suppression of bermudagrass when alfalfa (Medicago sativa L.) was overseeded in Mississippi.

Despite these challenges, the association of alfalfa and bermudagrass has significant potential (Bouton, 2015). Alfalfa is one of the most important forages globally due to its high productivity and excellent nutritive value (Bouton, 2012). While alfalfa is a standard forage of the dairy industry in temperate regions, the search for adapted and persistent alfalfa cultivars for the southeastern United States is ongoing and has proven to be challenging, leading to breeding efforts with the objective of expanding alfalfa growth in subtropical areas (Acharya et al., 2020). In contrast, bermudagrass (seeded and hybrid types) is a persistent and dependable warm-season forage crop in the southeastern United States, with more than 11 million ha currently utilized (Bouton, 2015). However, bermudagrass requires N fertilizer to optimize its forage accumulation (FA) and CP concentration. One way to improve bermudagrass without replacing the crop is to have alfalfa interseeded into the existing grass stand.

Once mixtures are established, defoliation management can be a major determinant of its success. For various grass–legume mixtures in Maryland, cutting at 5 cm provided more forage compared with cutting at 10 cm (Hunt & Wagner, 1963). Jones and Tracy (2018) also reported that shorter harvest height resulted in greater FA for an orchardgrass (Dactylis glomerata L.)–alfalfa mixture. However, stubble height (SH) did not affect “Amarillo” pintoi peanut (Arachis pintoi Krapov. & W.C. Greg.) when it was overseeded into Jiggs bermudagrass, but the shorter SH treatment did have less weed ground cover (Sanchez et al., 2018). Harvest interval also affects forage performance. In Georgia, Hoveland et al. (1996) indicated that 4–6-week harvest intervals resulted in similar alfalfa FA when grown in monoculture, but a 2-week harvest interval had lesser FA than the longer intervals. Other studies supported that, in general, less frequent harvests result in greater FA of grass–legume mixtures, but cutting height and genotype differences can interact with harvest interval to affect optimal harvest management (Wolf & Smith, 1964). For example, Bulldog 805 alfalfa (Bouton et al., 1997) had greatest FA when harvested every 6 week at 15 cm, but the same cutting height with a 2-week frequency resulted in the least FA (Groce et al., 2019). These authors also found a 6-week, 10-cm treatment had the greatest proportion of alfalfa throughout the growing season, while alfalfa proportion decreased across the growing season in the 2-week, 5-cm cutting height treatment. Furthermore, alfalfa–bermudagrass mixtures in Georgia that were harvested every 28–35 days outperformed bermudagrass monoculture (fertilized with 84 kg N ha⁻¹ per growing season) in FA, CP, and digestible nutrients (Hendricks et al., 2020).

Thus, the literature indicates that defoliation management plays a significant role in survival and productivity of grass–legume mixtures in general, and alfalfa and alfalfa–grass mixtures specifically. The current study seeks to build on these findings to assess the extent to which harvest height, harvest interval, and alfalfa genotype affect performance of alfalfa–bermudagrass mixtures in the warm climate, sandy soil environment of northern Florida. The specific objective of the experiment was to define the optimum combinations of harvest SH and cutting interval (CI) for two alfalfa genotypes in mixture with bermudagrass in terms of alfalfa persistence, total and alfalfa FA, and the mixture nutritive value.

Core Ideas

Two alfalfa genotypes responded similarly to intercropping with Tifton 85 bermudagrass.
Longer cutting intervals in combination with shorter stubble heights produced greater forage accumulation.
The expected nutritive value decline with longer cutting intervals was limited by greater alfalfa presence.
Optimizing alfalfa–bermudagrass defoliation management increases likelihood of mixture success.

2 MATERIALS AND METHODS

2.1 Experimental site

This research was a 2-year (2019 and 2020) study that was carried out at the Plant Science Research and Education Unit in Citra, FL (29°24′30.99″ N; 82°10′16.08″ W). The plots were fully established ‘Tifton 85' bermudagrass stands and they were interseeded to alfalfa on December 5, 2018. Individual plot area was 1.5 × 4.6 m, and there were 1.8-m alleys of bermudagrass monoculture between blocks and 0.3 m between plots. The soil at the site is a Chipley sand (thermic, coated Aquic Quartzipsamments) with an initial soil pH of 6.8, and Mehlich-3 P, K, and Mg concentrations were 235 (high), 41 (medium), and 35 (medium) mg kg⁻¹. Rainfall and temperature data for 2019 and 2020 were collected from the weather station located at the research site and can be seen in Table 1.

TABLE 1. Rainfall and average monthly high and low temperatures for the Plant Science Research and Education Unit, Citra, FL

Month	Average	2019	2020	Average low	Average high	Average low	Average high
	Rainfall (mm)			Temperature (°C)
				2019		2020
January	80	135	25	7	19	9	22
February	38	31	45	13	25	10	23
March	24	35	12	11	24	14	28
April	107	84	130	14	28	16	28
May	30	37	22	20	32	17	30
June	178	168	188	23	32	22	32
July	118	131	104	23	33	23	33
August	240	296	183	23	32	23	33
September	151	35	266	22	32	23	31
October	64	95	32	20	30	20	29
November	57	39	75	11	23	16	26
December	74	115	32	11	23	6	20
Total	1161	1201	1114	NA	NA	NA	NA

Abbreviation: NA, not applicable.

2.2 Treatments and experimental design

The experimental design was a randomized complete block with three replicates. Treatments were all possible combinations of two alfalfa genotypes (“Bulldog 805” and breeding line UF 2015-AP), three harvest SHs (5, 10, and 15 cm), and three harvest CIs (2, 4, and 6 weeks) in a factorial treatment arrangement, resulting in 18 treatments and 54 experimental units. Bulldog 805 and breeding line UF 2015-AP alfalfas were interseeded into previously established Tifton 85 bermudagrass using a Sukup no-till drill (Sukup Manufacturing Co.).

2.3 Management of plots

Prior to planting alfalfa, the bermudagrass was mowed to 5 cm (November 16, 2018) and sprayed with 420 g of active ingredient ha⁻¹ of glyphosate [N-(phosphonomethyl)glycine] (November 29, 2018) to stunt bermudagrass growth and reduce competition to alfalfa seedlings. Planting of alfalfa occurred on December 5, 2018. The alfalfa seeding rate was 25 kg pure live seed ha⁻¹, and the row spacing was 33 cm. A staging harvest occurred on June 26, 2019 at 20%–30% blooming to implement the SH treatment and standardize the beginning of the regrowth interval treatments. This staging harvest occurred later than anticipated because alfalfa plant vigor was weak, and the delay allowed the alfalfa to fully establish. Based on slow growth and yellowing of seedlings, we hypothesize this was due to slow nodule formation and onset of nitrogen fixation. Thereafter, all plots were harvested according to their designated defoliation interval through December 11, 2019. In 2020, plots were clipped to a 15-cm SH on February 28 to remove winter weeds, but subsequent initiation of the experiment was delayed in 2020 due to the Covid-19 pandemic and associated suspension of research activities. The staging harvest in 2020 occurred on July 22, and regular harvests continued through October 14, 2020. There were a total of 12, 6, and 4 harvests after staging in 2019 for the 2, 4, and 6-week treatments, respectively, while in 2020 there were 6, 3, and 2 harvests, respectively.

For the dates when all CI treatments (2, 4, and 6-week plots) were cut (excluding the last harvest of each growing season), all plots were fertilized with 74 kg of K ha⁻¹ (for a total of 148 kg of K ha⁻¹ in each year). The herbicide pendimethalin [N-(1-ethylpropyl)−3,4-dimethyl-2,6-dinitrobenzenamine] was applied following these same harvests at a rate of 1065 g of active ingredient ha⁻¹ for control of residual weeds. The dates when K fertilizer and herbicide were applied include June 26, 2019, September 18, 2019, February 28, 2020, and July 22, 2020. The plots were flail chopped on February 28, 2020 to 15 cm to remove cool-season weeds and stage them for the 2020 growing season (no data were collected on February 28, 2020).

2.4 Response variables

2.4.1 Alfalfa cover, FA, and botanical composition

Prior to each harvest, a whole-plot visual estimate of alfalfa proportion was recorded for each plot. At each harvest, the alleys between each block were cut, and each plot length was recorded. Next, a 1.52-m wide strip was cut through each plot using a Wintersteiger plot harvester (Wintersteiger Inc.) at the corresponding treatment SH. The total fresh weight of forage was quantified and a subsample of ∼500 g was taken for dry matter (DM) determination by drying until constant weight at 60°C. A second subsample was taken and separated by hand into alfalfa, bermudagrass, and weed fractions to determine botanical composition. Each fraction was dried as described for the DM sample and weighed to calculate fraction proportion on a DM basis. The remainder of the plot was clipped to the treatment SH and the forage removed from the plot.

2.4.2 Nutritive value

To ensure equal number of nutritive value observations for each level of CI, laboratory analyses were conducted for samples collected when all levels of CI were harvested on the same date or in close proximity to the same day. These dates were September 18, 2019 (a date when all treatments were harvested; referred to as September 2019), August 20, 2020 (2- and 4-week CI treatments were harvested), and September 2, 2020 (6-week CI treatment was harvested) (referred to collectively as August 2020), and October 14, 2020 (a date when all defoliation treatments were harvested; referred to as October 2020). At these dates, the samples taken to determine DM concentration were subsequently ground to pass a 1-mm screen in a Wiley mill (Thomas Scientific) and analyzed to determine nitrogen, neutral detergent fiber (NDF), acid detergent fiber (ADF), and in vitro digestible organic matter concentration (IVDOM). Forage ADF and NDF concentrations were determined using the filter bag technique from the ANKOM procedure (Ankom Technology, 2017a; Ankom Technology, 2017b). The IVDOM analysis was performed using a modification of the two-stage technique (Moore & Mott, 1974). For N analysis, samples were digested using a modification of the aluminum block digestion procedure of Gallaher et al. (1975). Nitrogen in the digestate was determined by semiautomated colorimetry (Hambleton, 1977). Forage CP concentration was calculated by multiplying N by 6.25.

2.5 Statistical analyses

Data were analyzed in R version 3.6.3 (R Core Team, 2021) using the Agricolae (v1.3-5; de Mendiburu, 2021) and Readxl (v1.3.1; Wickham & Bryan, 2019) packages. Because there were differences among levels of CI in actual harvest dates and number of harvests per year, data were organized and analyzed in two ways. The first was across all harvests during the 2-year study to describe total-experiment performance, and the second was by cycles within a year to explore changes over time. A cycle was defined as a 12-week period in which all 2-, 4-, and 6-week CI treatments were harvested at the beginning and end of the 12 week, so during the 12 weeks these treatments were harvested, 6, 3, and 2 times, respectively. There were three cycles throughout the experiment, two in 2019 (began on June 26 and September 18) and one in 2020 (began on July 22). For both types of data organization, genotype, CI, SH, and their interactions were considered as fixed effects, while block was considered as a random effect. When cycle was included in the model, it was considered fixed and treated as a repeated measure. Analysis of variance was conducted for each response variable and effects with significant p values (p ≤ 0.05) were identified in R. For mean separation, Tukey tests were conducted for the main effect or interaction means, as appropriate.

3 RESULTS AND DISCUSSION

3.1 Mixture DM accumulation

There was a three-way interaction between SH, CI, and genotype for mixture FA across the experimental period (p = 0.014). Because genotype was the only factor in the three-way interaction for which the main effect was not significant, the SH × CI interaction was explored separately for each genotype (Figures 1 and 2). For Bulldog 805, the 6-week CI in combination with a 5-cm SH had greater mixture FA than any other treatment (Figure 1), while for UF 2015-AP the same treatment combination had greater FA than any of the 15-cm SH treatments (Figure 2). For both genotypes, there was no effect of SH when CI was 2 or 4 week, although for UF 2015-AP the general pattern of FA response favored shorter SH. These findings are similar to Hunt & Wagner (1963), who evaluated the effect of SH on clover-grass mixtures (Ladino clover [Trifolium repens L.] with “Potomac” orchardgrass, “Lincoln” smooth bromegrass [Bromus inermis Leyss.], and “Alta” tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.]), and Jones & Tracy (2018) who compared orchardgrass monocultures with alfalfa–orchardgrass mixtures under various SH treatments. Both studies showed that shorter SH allowed for greater mixture FA. Shepard et al. (2018) also found that a shorter SH allowed for greater FA when studying rhizoma peanut (Arachis glabrata Benth.) pastures. However, in contrast to the results of this study, they also found that FA decreased with longer CIs. The difference in results between the current study and Shepard et al. (2018) could be due to species differences, with the more decumbent rhizoma peanut being better adapted to frequent defoliation. Quinby et al. (2021) reported harvest intervals of 35 days or longer enable greater FA, a result which coincides with the 6-week CI in this study allowing for the greatest amount of FA. Despite alfalfa genotype having limited impact on mixture FA in the current study, more data assessing genotype effects are needed because performance in mixtures has received limited attention in most legume cultivar evaluation protocols (Muir et al., 2014). Differences in competitive relationships in mixtures due to plant growth habit (Sollenberger et al., 2012) and to amount of N fixed by the legume and made available for use by companion species (Ta & Faris, 1987) should be investigated.

Details are in the caption following the image — **FIGURE 1**
Open in figure viewer

Interaction of harvest stubble height (SH) × cutting interval (CI) for dry matter (DM) forage accumulation (FA) of Bulldog 805 alfalfa–Tifton 85 bermudagrass mixtures summed across three 12-week harvest cycles during 2019 and 2020. Interaction means having the same letter did not differ (p > 0.05). Standard error (SE) = 562 kg DM ha⁻¹.

To explore changes in responses over time, cycle was included in the model. There was a four-way interaction between SH, CI, genotype, and cycle for mixture FA (data not shown, p = 0.026). There were a few exceptions within treatments in the ranking of the cycles, leading to the interaction, but for both Bulldog 805 and UF 2015-AP genotypes, most defoliation management treatments (i.e., combinations of CI and SH) had greater mixture FA during the first (began on June 26, 2019) than the second cycle (began September 18, 2019) of 2019. The lesser FA observed in Cycle 2 was due to these harvests occurring later in the growing season when shorter days and cooler temperatures reduced total FA, primarily through reduction in bermudagrass FA. Bermudagrass FA in short-daylength months is restricted by photoperiod (Sinclair et al., 2003), even if temperature and soil moisture conditions favor rapid growth.

When differences due to SH occurred, shorter SH resulted in greater mixture FA, and this effect was generally more pronounced for UF 2015-AP than Bulldog 805. Murphy et al. (1977) observed the effect of different CI and SH treatments on eight subtropical forage mixtures. They also found that a shorter SH of 5 cm cut every 6 weeks provided the greatest mixture FA, while a taller SH of 13 cm cut every 3 weeks had the least. The shorter SH allowed for a larger amount of the FA to be harvested, while the longer CI gave the forage more time for regrowth, an advantage bermudagrass capitalizes upon due to its ability to initiate regrowth quickly following mowing (Zhang et al., 2020).

3.2 Alfalfa forage DM accumulation, proportion in the DM, and cover

Alfalfa genotype did not significantly affect total alfalfa FA in this study where UF 2015-AP averaged 1160 kg DM ha⁻¹ and Bulldog 805 averaged 1120 kg DM ha⁻¹ (p = 0.747). There was a two-way interaction between SH and CI for alfalfa FA across the three cycles (p = 0.006). Alfalfa FA increased as SH decreased for the 4- and 6-week CI treatments, but there was no effect of SH when CI was 2 weeks (Figure 3). Greatest alfalfa FA occurred when clipped to 5-cm SH every 4 or 6 week, but these means were not different from the 10-cm SH when clipped every 6 week. When alfalfa monocultures were harvested to 5 or 10 cm every 32–40 days in Florida (López et al., 2017), no difference due to SH was found. In the current study, we also found no difference between these two SH treatments for the 6-week CI. The relatively low alfalfa FA across the 2 years (Figure 3) can be attributed to the late initiation of harvesting in 2019, due to slow alfalfa establishment, and in 2020, due to Covid-19 restrictions.

In Cycle 1 (late June to mid-September 2019), the total alfalfa FA for the 6-week CI treatment was 660 kg DM ha⁻¹ (Figure 4), and the mixture at this time was comprised of 18% alfalfa (Figure 5), resulting in a total mixture FA of 3670 kg DM ha⁻¹. This is equivalent to 55 kg DM ha⁻¹ week⁻¹ of alfalfa FA and 250 kg DM ha⁻¹ week⁻¹ of bermudagrass FA, for a total of 305 kg DM ha⁻¹ week⁻¹ of mixture FA in late summer. Johnson et al. (2001) reported that a bermudagrass monoculture fertilized with 78 kg N ha⁻¹ per cutting (every 4 weeks) in Ona, Florida produced 5120 kg ha⁻¹ of bermudagrass FA over a 12-week period in late summer, which is equivalent to a bermudagrass FA rate of 427 kg DM ha⁻¹ week⁻¹. In Citra, FL, monocultures of UF2015-AP alfalfa produced 890 kg DM ha⁻¹ of alfalfa over a 7-week period in late summer (127 kg DM ha⁻¹ week⁻¹; unpublished data). In general, there are limited data comparing alfalfa–bermudagrass mixtures with the monocultures of these species. However, based on the values described above, if a producer grew bermudagrass and alfalfa as monocultures in Florida with 18% of their land devoted to alfalfa and 82% devoted to bermudagrass fertilized with 78 kg N ha⁻¹ every 4 weeks, they could expect to produce more total FA than the mixture during late summer, but alfalfa FA would be less and large amounts of N fertilizer would be required to achieve these levels of bermudagrass FA. Thus, alfalfa–bermudagrass mixtures in Florida during late summer are a feasible option for growers prioritizing the alfalfa proportion of their production. Additionally, bermudagrass has a limited stockpiling window in the fall (Scarbrough et al., 2001), which also requires additional N fertilizer inputs to the system. Further research could explore using this mixture as a stockpiling alternative in the southeast.

When analyzed with cycle in the model, there was a cycle × CI (p = 0.005) interaction (Figure 4). In Cycles 1 and 3, alfalfa FA did not differ between the 4- and 6-week CIs and both were greater than 2 weeks. For Cycle 2, alfalfa FA was greater for the 6-week CI than either the 4- or 2-week CI. The shorter CI of 2 weeks consistently underperformed compared with the longer CI treatments. These results are similar to those from a study assessing CI levels of 20, 30, and 40 days for alfalfa monocultures, which found the CI of 40 days (∼6 weeks) had greater FA than the other CIs (İbrahim et al., 2019). In another study evaluating an alfalfa monoculture harvested at different CI and SH treatment levels (Birch et al., 1975), longer CI provided greater alfalfa FA and the tallest SH (12 cm) reduced FA relative to short SH, similar to what was seen in our study. Additionally, alfalfa FA was generally greater for Cycle 1 than Cycles 2 or 3 (Figure 4). Similarly, İbrahim et al. (2019) reported alfalfa monoculture FA decreased from year to year of a 3-year study.

Alfalfa botanical composition in FA across the three cycles was affected by the SH × CI interaction (p = 0.036). For the 4-week CI, alfalfa proportion in mixture FA was greater for the 5- than the 15-cm SH (Figure 6). There were no differences among levels of SH for any other CI treatment. Where differences in alfalfa proportion existed, they favored shorter SH and longer CI. For an alfalfa–bermudagrass mixture in Georgia, USA, a 10-cm SH resulted in a greater proportion of alfalfa than a 5 or 15 cm SH when CI was 6 weeks (Groce et al., 2019), while in this study, there was no difference among levels of SH within the 6-week CI.

When cycle was included in the model, there was a two-way interaction between CI and cycle (p < 0.001). For Cycles 1 and 3, there was no effect of CI on alfalfa proportion, but in Cycle 2 the levels of CI were ranked 6 > 4 > 2 weeks, with the 6-week treatment approaching 40% alfalfa at the end of the first year of defoliation (Figure 5). However, the percentages of alfalfa reported in this study, with the greatest mean below 40%, were less than those in alfalfa–bermudagrass mixtures (>50%) in Georgia (Brown & Byrd, 1990). While both experiments tested the same species combination in the southeastern United States, Brown & Byrd (1990) planted bermudagrass in May followed by alfalfa in September of the same year. They indicated the bermudagrass did not yet cover the entire planted area when alfalfa was seeded. In the current study, the bermudagrass was planted the previous year and had achieved full cover prior to alfalfa planting. Additionally, another study assessing alfalfa–bermudagrass mixtures recorded a maximum proportion of alfalfa of 50% when grown in mixture with bermudagrass (Cinar & Hatipoglu, 2014), a finding similar to that of the current study.

When percentage alfalfa cover was assessed with cycle in the model, there was a two-way interaction between SH and CI (p < 0.001). Alfalfa cover was greater for the 6- than the 2-week CI when SH was 10 or 15 cm, and when SH was 5 cm it was greater for both 4- and 6-week CIs than for 2 weeks (Figure 7). Greatest cover ranged from approximately 25% to 35% and was associated with either 4- or 6-week CIs (Figure 7). Alfalfa cover decreased over time from 25% in Cycle 1 to 19% and 20% in Cycles 2 and 3, respectively (p < 0.001). When comparing cover estimates taken after one and two complete growing seasons of defoliation, there was an effect of CI (p < 0.001), with alfalfa cover increasing from 14% to 40% as CI increased from 2 to 6 weeks (data not shown). There also was a date effect as cover decreased from 30% (range of 19% to 43% across levels of CI) after 1 year of defoliation to 22% (range of 8%–37% across levels of CI) after 2 years of defoliation (p = 0.014). Similar results were found by Aponte et al. (2019) while studying binary mixtures of alfalfa with several cool-season grasses. They concluded alfalfa proportion was greatest in the mixtures for the first year at 77%–99% but decreased to an average of 50% by the third year. While these percentages are greater than those in the current study, the pattern of decreasing alfalfa percentage in the grass–legume mixture over time is the same. These results are also similar to those of Hoveland et al. (1996) who reported that alfalfa in mixture with crabgrass (Digitaria sanguinalis L. Scop.) or bermudagrass performed best with CIs of 4–6 weeks rather than 2 weeks.

3.3 Forage nutritive value

For most nutritive value responses, there were greater differences among sampling dates than due to CI or SH, and genotype did not affect the response. Mixture CP concentration was affected by an SH × sampling date interaction (p = 0.021) and a CI main effect (p < 0.001). As CI increased from 2 to 6 weeks, forage CP decreased from 130 to 115 g kg⁻¹. CP concentration was greater for all SH levels for the August 2020 than the September 2019 sampling date, but only the 15-cm SH in August 2020 had greater CP than the 10- and 15-cm SH levels in October 2020 (Table 2).

TABLE 2. Interaction of harvest stubble height (SH) × sampling date for acid detergent fiber (ADF), crude protein (CP), and in vitro digestible organic matter (IVDOM) concentrations of alfalfa–Tifton 85 bermudagrass mixtures across two alfalfa genotypes and two cutting intervals during 2019 and 2020.

Harvest date	5 (cm)	10 (cm)	15 (cm)	5 (cm)	10 (cm)	15 (cm)	5 (cm)	10 (cm)	15 (cm)
	ADF			CP			IVDOM
	g kg⁻¹
September 2019	380ab	388a	384ab	105c	103c	102c	440 cd	416d	435 cd
August 2020	336d	337d	337d	124ab	124ab	137a	576a	578a	578a
October 2020	344d	360c	369bc	128ab	117bc	116bc	487b	456bc	450c
Standard error	4			4			7
p-value	0.005			0.021			0.023

Note: Interaction means followed by the same letter within a response variable did not differ (p > 0.05).

CP and IVDOM were relatively low because of delayed initiation of harvesting in both years (described earlier), resulting in samples being taken for analysis when bermudagrass had become more dominant in the mixtures (Harling et al., 2022). Mixture IVDOM concentration was affected by date × SH (p = 0.023) and date × CI (p < 0.001) interactions. Similar to the CP response, IVDOM concentrations were greater in August 2020 than in September 2019, and in this case also greater than in October 2020 (Tables 2 and 3). Differences among levels of SH within a date occurred only for the October 2020 sampling date when the 5-cm SH had greater IVDOM than the 15-cm SH (Table 2). The 2-week CI had greater IVDOM than the 6-week for both sampling dates in 2020 (Table 3), but there was no difference among levels of CI for the September 2019 sampling date. Typically, nutritive value increases as the CI decreases for forages. For example, Grev et al. (2017) found that a shorter CI increased alfalfa nutritive value. Similar responses were reported for alfalfa CP (Min, 2016) and in vitro digestibility (Albrecht et al., 1997). Other studies with bermudagrass monocultures showed decreased IVDOM and CP, with increased fiber concentration, when the CI increased (Burton et al., 1963; Coleman et al., 2004). In the current study, CI had less of an effect than expected. This is attributed in part to the gradual increase in alfalfa contribution with longer CI. This greater alfalfa proportion and greater nutritive value of alfalfa than bermudagrass likely ameliorated the generally observed positive effect of shorter CI on mixture nutritive value. This conclusion is supported by Quinby et al. (2020) who found greater alfalfa persistence in mixtures with bermudagrass with less frequent harvests, leading to greater nutritive value in treatments with longer versus shorter intervals between defoliation events.

TABLE 3. Interaction of harvest cutting interval × sampling date for acid detergent fiber (ADF), neutral detergent fiber (NDF), and in vitro digestible organic matter (IVDOM) concentrations of alfalfa–Tifton 85 bermudagrass mixtures across two alfalfa genotypes and two stubble heights during 2019 and 2020.

Harvest date	2 (weeks)	4 (weeks)	6 (weeks)	2 (weeks)	4 (weeks)	6 (weeks)	2 (weeks)	4 (weeks)	6 (weeks)
	ADF			NDF			IVDOM
	g kg⁻¹
September 2019	380ab	379ab	393a	740a	723ab	715ab	431e	430e	430e
August 2020	322e	329de	357c	647 cd	644d	680bcd	613a	597a	525b
October 2020	341d	360c	373bc	678bcd	692bc	695ab	491c	466 cd	436de
Standard error	4			10			7
p-value	0.006			0.046			<0.001

Note: Interaction means having the same letter within a response variable did not differ (p > 0.05).

For forage NDF concentration, there was a two-way interaction between CI and sampling date (p = 0.046). Within individual sampling dates, there were no differences among the levels of CI, but NDF was less for the 2- and 4-week CI of the August 2020 sampling date than for September 2019 (Table 3). Alfalfa–bermudagrass mixture NDF concentration in the current study was generally greater than in previous work (Hendricks et al., 2020), likely because alfalfa proportion was less in the current study than in the previous studies. Mixture ADF concentration was affected by both CI × sampling date (p = 0.006) and SH × sampling date (p = 0.005) interactions. For both interactions, ADF was less for all levels of the treatment factor for the August 2020 sampling date than the 2019 sampling date, and it was less for two of three levels of the treatment factor for the August than the October 2020 sampling dates (Tables 2 and 3). Unlike the NDF response, there was an effect of CI on ADF within a sampling date for both 2020 sampling dates, and ADF increased with increasing length of CI.

4 CONCLUSIONS

Overall, longer CIs (closer to 6 weeks) in combination with shorter SHs (closer to 5 cm) produce greater mixture FA, greater alfalfa FA, and greater alfalfa proportions in the mixture. The effect of SH was reduced as CI decreased, and genotype had little effect throughout the experiment, resulting in similar performance of both Bulldog 805 alfalfa and UF 2015-AP alfalfa when in mixture with Tifton 85 bermudagrass. While longer CIs resulted in the expected decrease of IVDOM and increase of ADF for August and October 2020 sampling dates, the impact of CI in this study was not as pronounced for the September 2019 sampling date, as well as all sampling dates regarding NDF. Lesser differences in nutritive value between the CIs were likely associated with increased presence of alfalfa in the longer CIs, offsetting some of the reduction in nutritive value that is typically seen with longer regrowth periods. Despite the lack of genotype effect in this experiment, further research into the effects of various combinations of genotypes in alfalfa–bermudagrass mixtures is recommended. In conclusion, CI was generally the most important factor affecting alfalfa persistence in alfalfa–bermudagrass mixtures in this environment, with 4–6-week CI favored, and short SH in combination with longer CI typically resulted in the greatest total mixture and alfalfa FA.

AUTHOR CONTRIBUTION

John Harling: Data curation; formal analysis; investigation; methodology; software; visualization; writing—original draft; writing—review and editing. Esteban Rios: Conceptualization; data curation; investigation; methodology; project administration; resources; supervision; writing—review and editing. Cleber Henrique de Souza: Data curation; investigation; methodology; writing—review and editing. Lynn E. Sollenberger: Formal analysis; investigation; methodology; resources; supervision; writing—original draft; writing—review and editing. José C. Dubeux: Conceptualization; funding acquisition; methodology; supervision; writing—review and editing. Marcelo O. Wallau: Investigation; resources; supervision; writing—review and editing.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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Defoliation management affects performance of alfalfa–bermudagrass mixtures in the southeastern United States