Interseeding Warm-season Grasses followed by High Intensity Grazing Enhances Pasture Productivity

Interseeding Warm-season Grasses Followed by High Intensity Grazing Enhances Pasture Productivity

Alexander J. Smart, Department of Animal and Range Sciences, and Vance N. Owens, Plant Science Department, South Dakota State University, Brookings 57007

Abstract

Pasture renovation by interseeding can be an efficient way to introduce native warm-season grasses into low-producing cool-season pastures, provided that the existing sod can be managed to favor establishment and stand persistence. The objectives were to test interseeding warm-season grasses into cool-season pasture using grazing or herbicide sod suppression treatments and the effects of prescribed grazing events on stand development. The grazing treatment was applied using sheep at a high stocking rate to achieve a residual grazing height of < 2.5 cm in autumn and in the spring prior to seedling emergence. The herbicide treatment was an application of glyphosate in early October. Native warm-season grasses were seeded in mid-March (2004 and 2005). Warm-season grass biomass harvested the year after establishment was greater (P < 0.01) for the herbicide sod suppression treatment (5700 kg/ha) than the grazing treatment (20 kg/ha). Three years of prescribed grazing significantly (P < 0.01) enhanced the warm-season grass composition from 56 to 70% and from 1 to 12% in the herbicide and grazed sod suppression plots, respectively. Herbicides are integral to successfully interseeding warm-season grasses into cool-season pastures. Short-duration, high-density grazing can be appropriately timed to increase the competitiveness of warm-season grasses and decrease the cool-season grasses of interseeded stands.

Introduction

Low-producing, eastern South Dakota pastures that have been overgrazed are dominated by introduced cool-season grasses, such as Kentucky bluegrass (Poa pratensis L.), weedy forbs, and lesser amounts of native tallgrass species. Enhancing productivity and wildlife habitat of these pastures is of major concern in light of increasing land values and pasture rental rates. Traditional methods to improve productivity include fertilization and legume interseeding (3). Fertilizing cool-season pastures increases forage production, but is not always cost effective especially with increasing N costs. Limited weed control options deter many producers from legume interseeding. Finally, hunting enthusiasts want taller vegetation structure for nesting cover which introduced cool-season grasses don’t offer compared to native warm-season tallgrasses (2,9).

An alternative method for increasing pasture production and wildlife habitat could involve introducing productive species into existing pastures without complete seedbed preparation. Autumn chemical application has been used to suppress cool-season species and aid in no-till establishment of warm-season grasses the following spring (12,15,16,21). Costs, performance, and effects on the environment are reasons why alternative methods to herbicides are desirable to suppress existing sod. Tillage to suppress sod followed by deferred grazing has been successfully used to renovate pastures (11,16). These methods may not always be as successful as herbicides (13). Prescribed grazing at the appropriate time with high stocking rates may weaken existing vegetation and reduce competition to interseeded plants. This technique has been successfully used with interseeded legumes (18,19). However, very little is known about interseeding warm-season grasses into sod suppressed by prescribed grazing. The objectives of this study were to: (i) test the success of interseeding warm-season grasses with a no-till grass drill into existing cool-season pasture using grazing or herbicide sod suppression; and (ii) monitor the effects of continued prescribed grazing on stand development.

Testing Sod Suppression Techniques

Two experiments were conducted from 2003 through 2007 at South Dakota State University’s Sheep Unit located near Brookings, SD (44°35’N, 96°79’W). Pasture vegetation has been dominated by smooth bromegrass [Bromus inermis (Leyss.) subsp. inermis] and Kentucky bluegrass with minor amounts of quackgrass [Elytrigia repens (L.) Desv. ex Nevski] for > 20 years. The pasture is on a Kranszburg silty clay loam soil (Fine-silty, mixed Udic Haploborolls) with 0 to 2% slope. Climate is continental with a monthly mean maximum temperature of 28°C in July and a monthly mean minimum temperature of -12°C in January from a 30-year (1975-2004) average (20). Average annual precipitation is 58.9 cm with June being the wettest month and 79% of the annual precipitation occurring from April through September (20).

Experimental design was a randomized complete block with three replications in both experiments. Sod suppression treatments (herbicide or grazing) were each applied to 588 m² plots and randomly assigned to each block. Herbicide was applied in autumn and grazing in autumn and spring at a high stocking rate [3.8 to 5.9 animal unit months (AUM)/ha; where one AUM equals the forage demand for one month based on 454 kg of body weight] to achieve a residual vegetation height of < 2.5 cm after each grazing period. In Experiment I, the grazing treatment started on 5 October 2003 with 25 yearling ewes each weighing approximately 60 kg grazing each replication for 2 days (3.8 AUM/ha). On 22 March 2004, the entire pasture was seeded with 4.6, 4.3, and 3.6 kg/ha pure live seed of ‘Bison’ big bluestem (Andropogon gerardii Vitman), ‘Sunburst’ switchgrass (Panicum virgatum L.), and ‘Tomahawk’ indiangrass [Sorghastrum nutans (L.) Nash], respectively with an 8-row, 1.5-m wide Truax FlexII No-till drill (Truax Company Inc., New Hope, MN). These seeding rates were higher than recommended because of the potential high competition from the existing cool-season grasses. In spring the grazing treatment was applied on 7 May 2004 with 45 mature ewes each weighing approximately 90-kg grazing each replication for 1 day (5 AUM/ha). Regrowth in each replication was grazed for 1 day on 19 May 2004 with the same 45 ewes. Herbicide treatment was glyphosate (isopropyl amine of N-[phosphono-methyl] glycine) applied at 0.83 kg/ha a.i. on 2 October 2003 using a small plot sprayer. On 29 June 2004 picloram (4-amino-3,5,6-trichloropicolinic acid) was applied at a rate of 0.22 kg/ha a.i. to control broadleaf weeds in the herbicide treatment. No herbicide was necessary in the grazing treatment. In Experiment II, the grazing treatment started on 9 September 2004 with 35 yearling ewes each weighing approximately 60 kg grazing each replication for 2 days (5.3 AUM/ha). On 22 March 2005, the entire pasture was seeded with 5.2, 5.1, and 4.9 kg/ha pure live seed of Bison big bluestem, ‘Dakota’ switchgrass, and non-certified indiangrass harvested in South Dakota, respectively with the same no-till drill. In spring the grazing treatment continued on 1 May 2005 with 53 mature ewes each weighing approximately 90 kg grazing each replication for 1 day (5.9 AUM/ha). Regrowth in each replication was grazed for 1 day on 8 May 2005 with the same 53 ewes. Glyphosate (0.83 kg/ha a.i.) was applied 10 October 2004 using a small plot sprayer. No herbicides were necessary to treat weeds in either the herbicide or grazing treatments during summer 2005.

Application of Follow-up Prescribed Grazing after Establishment

Experimental plots were subjected to heavy grazing in autumn 2005, spring and autumn 2006, and spring 2007. Autumn (September and October) and spring (April and May) grazing occurred with 60 ewes weighing approximately 90 kg each for 2 to 4 days (2.3 to 4.5 AUM/ha) to achieve a residual plant height of < 5 cm. When regrowth occurred plots were regrazed. The cut-off date for spring grazing was mid-May each year to avoid grazing warm-season grasses. In 2005 and 2006, summer grazing by ewes occurred for 2 to 4 days in late July to a residual height of 15 cm (1.7 to 3.4 AUM/ha). In 2007, both experiments received summer grazing (3.4 AUM/ha) by ewes on 20 June to a residual height of 10 cm.

Measuring Stand Development

Biomass estimates were made one year after interseeding on 20 July 2005 and 17 July 2006 for Experiments I and II, respectively. Additional biomass estimates (2 and 3 years after interseeding) occurred on 17 July 2006 and 25 July 2007 for Experiment I and were used to calculate dry matter composition of cool- and warm-season grasses. The second year of biomass estimates for Experiment II occurred on 25 July 2007. Four 0.25 m² sub-samples were randomly collected from each treatment and replicate by clipping vegetation near the soil surface with hand shears. Samples were brought to the laboratory and sorted into warm-season perennial grass, cool-season perennial grass, and weeds. Samples were dried in a forced air oven at 60°C for 1 week and weighed.

Statistical Analysis

Statistical analysis of biomass estimates one year after establishment were computed using PROC MIXED (17) with a mixed model. Treatment, experiment, and treatment × experiment were considered fixed effects. Block, block × experiment, and treatment × block × experiment were considered random effects. Warm-season and cool-season grass dry matter composition estimates were computed using PROC MIXED (17) with a mixed model and year as a repeated measure. Experiments were analyzed separately because years following interseeding were not the same for each experiment for the repeated measures analysis. Treatment, year, and treatment × year were considered fixed effects. Block, block × year, and treatment × block × year were considered random effects.

Weather

Conditions were excellent for establishment of warm-season grasses during the two experiments. Establishment year mean maximum monthly temperatures for Experiment I during the growing season were slightly below the 30-year average whereas they were near the average during Experiment II (Table 1). Annual precipitation during the establishment year was 8 and 43% above the 30-year average for Experiments I and II, respectively (Table 1). More importantly, precipitation during the critical months (May-July) of the establishment year was approximately 38% above the 30-year average for both experiments (Table 1).

Table 1. Average maximum monthly temperature and monthly precipitation
for May through September during the seeding year (2004 and 2005) for Experiment I and II, respectively, near Brookings, SD.

	Month	Year		30-year mean
	Month	2004	2005	30-year mean
Mean Maximum Temperature (°C)	May	18.3	18.4	20.0
	Jun	22.8	26.2	25.0
	Jul	26.1	28.2	27.8
	Aug	21.1	26.3	26.1
	Sep	23.9	24.7	21.7
Precipitation (cm)	May	15.7	9.6	7.4
	Jun	6.9	15.2	10.9
	Jul	11.2	8.8	8.1
	Aug	2.3	8.9	7.6
	Sep	15.7	19.4	6.9
	Annual	63.5	84.1	58.9

Warm-season Grass Biomass

Averaged over both experiments, the use of grazing as a sod suppression method was not as effective in reducing competitiveness of the existing sod compared with using herbicide (P < 0.01), as indicated by warm-season grass production one year after interseeding (Table 2). During the establishment year, seedlings emerged in early June and grazing was stopped by mid-May to minimize seedling damage. Grazing probably would have been feasible until the end of May, especially during Experiment I because of low mean maximum temperatures during May-June (Table 1). Contrary to the grazing treatment, minimal cool-season grass was observed in the herbicide treatment during the summer of the establishment year in both experiments. Low observed cool-season grass cover also resulted in greater weed pressure in the herbicide treatment, and in Experiment I it was necessary to spray these plots in late-June with a broadleaf herbicide whereas it was not necessary in the grazed plots. Warm-season grass establishment in our study was similar to those that used herbicides and no-till drills to interseed warm-season grasses into existing sod (11,12,15,16,21).

Table 2. Warm-season perennial grass (WSG), cool-season perennial grass (CSG), and weeds sampled in mid-July one year after interseeding warm-season grasses into existing cool-season pasture with a no-till drill and using grazing or herbicide to suppress existing sod. Results are averaged over 2 experiments, one seeded in 2004 and the other seeded in 2005 near Brookings, SD.

Sod suppression method	WSG	CSG	Weeds^x	Total
Sod suppression method	(kg/ha)
Grazing	20 b^y	4,470 a	190 a	4,680 b
Herbicide	5,770 a	4,600 a	320 a	10,690 a
Standard error of difference	711	485	174	649

^x Speices were foxtails (Setaria spp.) and Canada thistle (Cirsium arvensis L.).

^y Means within a column followed by similar letters are not statistically different (P < 0.05).

Increases in herbage production largely depended on site potential and productivity of the pre-treated vegetation. In previous Midwest studies, existing vegetation was dominated by introduced cool-season grasses such as Kentucky bluegrass and smooth bromegrass while native warm-season grasses were either non-existent or minimal contributors to total production (11,12,16). In south central Nebraska, researchers showed an increase in total herbage of approximately 1500 kg/ha by interseeding warm-season grasses with a no-till drill and using herbicide sod suppression versus untreated vegetation (11). In northeast Iowa, biomass in interseeded areas treated with herbicide was twice that compared with unseeded areas because of the success of warm-season grass establishment (12).

Cool-season Grass Biomass

Averaged over both experiments, cool-season perennial grass production in late-July was not different (P = 0.80) between grazing or herbicide sod suppression methods one year after interseeding (Table 2). Absence of differences in cool-season grass biomass was likely due to the fact that the herbicide rate (glyphosate 0.83 kg/ha a.i.) was not high enough to cause complete kill the previous year. Recommended rates to completely kill grass sod are usually > 3.0 kg/ha a.i. (7) and often require multiple applications (4). Short-term efficacy of glyphosate applied to tall fescue (Festuca arundinaceae Schreb.) has been well documented (6,8,10,23). It is likely that vegetation reestablished from buds on rhizomes since smooth bromegrass, quackgrass, and Kentucky bluegrass are highly rhizomatous (1,5,14).

Weed Biomass

Averaged over both experiments, weed production in late July was not different (P = 0.48) between grazing or herbicide sod suppression methods one year after interseeding (Table 2). Predominant weeds included foxtails (Setaria spp.) and Canada thistle (Cirsium arvensis L.). Existing cool-season grass sod, coupled with excellent establishment of warm-season grasses in the herbicide treatment, provided excellent competition against weeds.

Follow-up Prescribed Grazing Effects on Stand Development

In Experiment I, three years of short-duration, high-density grazing increased the dry matter proportion of warm-season grass from 56 to 70% (P < 0.01) in the herbicide sod suppression plots and from 1 to 12% (P = 0.05) in the grazed sod suppression plots (Fig. 1A). Warm-season grass percent utilizing herbicide sod suppression averaged 64% compared with 6% with grazed sod suppression (P < 0.01). Cool-season grass percentage decreased from 40 to 28% (P = 0.06) in the herbicide sod suppression plots and from 99 to 84% (P = 0.03) in the grazed sod suppression plots (Fig. 1B). Cool-season grass percent following herbicide sod suppression averaged 33% compared with 92% with grazed sod suppression (P < 0.01). In Experiment II, two years of short-duration, high-density grazing increased warm-season grass percentage from 48 to 74% (P < 0.01) using herbicide sod suppression, but no increase (P = 0.43) in warm-season grasses was noted with grazed sod suppression (Fig. 1C). Warm-season grass percent in the herbicide sod suppression plots averaged 61% compared with 4% for the grazed sod suppression plots (P < 0.01). Cool-season grasses decreased (P = 0.04) from 49 to 23% in the herbicide sod suppression plots, but did not decrease (P = 0.56) in the grazed sod suppression plots (Fig. 1D). Cool-season grass percentage averaged 36 and 90% (P < 0.01) utilizing herbicide sod suppression and grazed sod suppression, respectively.

Fig. 1. Effect of short-duration, high-density grazing on warm-season and cool-season grass dry matter composition, reported as a percentage, when sampled in July following interseeding in 2004 for Experiment I (A, B) and 2005 for Experiment II (C, D) near Brookings, SD. Grazed sod suppression treatment is solid line with open circles and herbicide sod suppression treatment is solid line with open squares. Standard error bars are shown for each sampling point.

Short-duration, high-density grazing was successful in helping shift species composition toward more warm-season grasses. Both experiments showed that two years of short-duration, high-density grazing was not enough to significantly shift the warm-season grass percent in the grazed sod suppression plots (Fig 1. A,C); however, in this case significant shifts to warm-season grasses had occurred by the third year in Experiment I. We suspect that warm-season grass plant recruitment and improved plant vigor under high cool-season grass competition (grazed sod suppression plots) would be slow because of low initial warm-season plant densities and plant size. Apparently, long-term prescribed short-duration, high-density grazing is required to shift species composition toward warm-season grasses in pastures with low initial warm-season grass composition. Plant community models with grazing as a tool to drive change have been proposed and tested (22). These data support our understanding of grazing in models proposed by the Natural Resources Conservation Service.

Application

Low-producing cool-season pastures can be enhanced through renovation techniques such as interseeding warm-season grasses, provided that existing sod is suppressed. Autumn application of glyphosate effectively suppressed existing vegetation to establish warm-season grasses. Our grazing treatment (heavy autumn + heavy spring with a target of residual vegetation height of < 2.5cm after each grazing period prior to seedling emergence) resulted in lower warm-season grass establishment. Several years of follow-up prescribed short-duration, high-density grazing enhanced warm-season grass stands of both sod suppression treatments. We believe that the best management practice to interseed warm-season grasses into existing cool-season pasture is to use herbicides for sod suppression and timely application of grazing to reduce cool-season grass competition.

Journal Series Number

South Dakota State University Agricultural Experiment Station Journal Series Number 3607.

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