Persistence of Heavily-Grazed Cool-Season Grasses in the Central Great Plains

Keith R. Harmoney, Kansas State University Agricultural Research Center, Hays 67601

Abstract

High temperatures and drought conditions lead to reduced forage production and eventual over-utilization if stocking rates are not adjusted accordingly. Grass survival under this environmental stress is important for future production and resource conservation. Eight introduced and two native perennial cool-season grasses, selected for adaptation to the Great Plains and released for production from 1942 to 1992, were heavily grazed for two seasons at two locations to monitor recovery and persistence. Barton and Flintlock western wheatgrass and Jose tall wheatgrass increased tiller density by over 100 tillers/ft² after two seasons of heavy grazing. Barton and Flintlock western wheatgrass had the greatest plant frequencies each sampling period. One season of heavy grazing reduced Lincoln smooth bromegrass plant frequency by 22% on an upland soil, and frequency remained lower than other cultivars for the duration of the test. During recovery from two seasons of grazing, Alkar tall wheatgrass and Barton and Flintlock western wheatgrass had three of the greatest forage yields at both locations. The native western wheatgrasses and introduced tall wheatgrasses used in this trial showed greater persistence potential compared to other introduced cultivars, and may be useful for cool-season grass areas of the central Great Plains that receive heavier grazing pressure.

Introduction

Grazing animal-sward interactions are especially important in environments where high temperatures and drought conditions lead to forage over-utilization if stocking rates are not adjusted accordingly. Grass survival under this environmental stress is important for future production and resource conservation. Response to defoliation stress from grazing has recently been a useful selection criteria for persistence of new cultivars of some perennial cool-season grasses (1,2,7) and legumes (1), but has been used less extensively to test older cultivars.

Growth of perennial cool-season grasses usually occurs from April to June and from late August through October in the central Great Plains (11), but environmental stress due to extremely hot, dry conditions during the late spring, summer, and early autumn coupled with defoliation from grazing could limit growth and persistence of some perennial cool-season grasses. Plant and tiller density over time are useful indicators of grass persistence (3,8).

This project evaluated production and stand persistence of native and introduced cool-season grass cultivars while undergoing severe defoliation stress from heavy grazing in the variable climate of west-central Kansas. Grasses used in this study were selected for adaptation to the Great Plains and were released for production from 1942 to 1992.

Testing Cool-Season Grasses for Persistence under Heavy Grazing

Ten perennial cool-season grasses were seeded at two locations (upland and lowland sites), each in a randomized complete block design with four replications near Hays, KS. Individual cultivar plots were 30 × 12 ft. The upland location was a Harney silt loam soil (fine, smectitic, mesic Typic Argiustoll), and the lowland location was a Roxbury silt loam soil (fine-silty, mixed, mesic Cumulic Haplustoll). Locations were three miles apart. Daily precipitation was collected from a central location between sites throughout each year.

Grasses included ‘Lincoln’ smooth bromegrass (Bromus inermis Leyss.), ‘Bozoisky’ Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski], ‘Luna’ and ‘Manska’ pubescent wheatgrass [Thinopyrum intermedium ssp. barbulatum (Shur) Barkw. & D.R. Dewey], ‘Alkar’ and ‘Jose’ tall wheatgrass [T. ponticum (Podp.) Barkw. & D.R. Dewey], ‘Slate’ and ‘Oahe’ intermediate wheatgrass [T. intermedium (Host) Barkw. & D.R. Dewey], and the only native grasses in the study, ‘Barton’ and ‘Flintlock’ western wheatgrass [Pascopyrum smithii (Rydb.) Love]. Plots were seeded at 30 pure live seeds/ft² in a firm and clean-tilled seed bed. Seeding occurred on 10 April 2000 at the upland site and 28 April 2000 at the lowland site. Plots received N at 40 lb/acre in the form of ammonium nitrate prior to seeding, and an additional 40 lb/acre in late March in 2001 through 2005. Grasses had excellent establishment in 2000 and were very productive in 2001 when annual precipitation was 28% above normal. Grasses were stressed in 2002 when annual precipitation was only 76% of normal. Plots also were sprayed with 1 qt active ingredient 2,4-D/acre (2,4-Dichlorophenoxyacetic acid) following the first harvest in 2000 and 2001 to reduce broadleaf weed competition.

Before this trial began, from 2000-2002, when grasses reached approximately 10- to 12-inch heights, plots were intermittently grazed to a 2-inch residual height within three days of stocking by 550- to 850-lb steers (Bos taurus). The upland site was grazed twice in 2000, and three times in both 2001 and 2002. The lowland site was grazed twice in 2000, four times in 2001, and three times in 2002. Adequate rest, at least 35 days, was allowed between stocking dates. Plot and border area totaled 0.8 acres at each location, and a connected 0.3 to 1.0 acre waste area was added, depending on the location, for livestock watering, supplemental hay feeding, and cattle lounging.

Beginning in 2003 and continuing through 2004, plots were heavily grazed to place severe defoliation stress on the stands. On 1 May 2003, 14 steers averaging 755 lb were stocked when grasses reached approximately an 8-inch height. Animals grazed two days, until residual height was approximately two inches. At this time, all but three of the animals were removed. The three remaining animals grazed on plots for 83 days and were fed supplemental hay in an adjacent location. Two animals were restocked from 15 September until 10 October 2003, a critical time period for cool-season grasses to accumulate energy reserves for the next season. At each location, plots supported 436 steer grazing days, which is equivalent to a stocking rate of 10.9 animal unit months (AUM) per acre. Grasses remained below a 2-inch height from 3 May 2003 until dormancy in late autumn.

On 6 May 2004, 17 steers averaging 610 lb were stocked when forage reached approximately an 8-inch height, and all steers were removed after two days when residual height was approximately two inches. Warm, dry spring conditions in late April and May did not allow for any regrowth, and plots were not restocked until 7 June 2004. At this time, two steers were stocked from 7 June through 24 September 2004. Plots supported a total of 337 steer grazing days, equivalent to a stocking rate of 6.9 AUM per acre, in 2004. Following the initial spring grazing, grasses again remained below a 2-inch height through the rest of the season. In 2005, grasses were not grazed and were allowed to accumulate biomass until harvested for dry matter yield in late June. All cultivars had produced seedheads by the time of harvest.

Grass frequency was measured in the spring and autumn of 2003 and 2004, prior to and following seasonal grazing. Frequency was also measured in the spring of 2005 to monitor recovery following the winter. The method used to determine frequency of grasses was slightly altered from that of Vogel and Masters (11). A metal frame, 40 × 40 inches, was divided into 10 segments each 40 × 4 inches. The frame was placed over three adjacent seeded rows so that one 40- × 4-inch segment was on top of each row. Each 40- × 4-inch segment was further divided into 10 units, each 4 × 4 inches. At three random locations in each plot, the number of 4- × 4-inch units with a live plant base within the frame was recorded and expressed as a percentage of the total number of units possible (maximum of 90). Tillers within two randomly located frames, each 1 ft² in opposite halves of a plot, were counted to estimate tiller density during the autumn of 2002 to 2004, and the spring of 2005. Vegetation from two 1-ft² frames, randomly located in opposite halves of the plot, was also hand clipped to a 1-inch height and dried in a forced-air oven to measure herbage dry matter yield response in late June of 2005. Subsample measurements were averaged to provide one value for each trait within the experimental unit for statistical analysis.

Analysis of frequency and tiller density data was performed using the Proc Mixed procedure of SAS (SAS Institute Inc., Cary, NC), with locations, cultivars, and years as fixed effects and replication as the random effect. Frequency percentage data were arcsin√ transformed for analysis and comparison of means, but actual percentage values rounded to the nearest whole number are presented in tables. Yield data were analyzed using Proc Mixed with locations and cultivars as fixed effects and replication as the random effect. If interactions existed with years, data were analyzed by year. Effects were significant if the probability level was less than P = 0.05.

Weather. Total annual precipitation was similar and slightly above the long-term average each year of the trial (Table 1), but the pattern of precipitation differed greatly. In 2003, a wet spring was followed by an extremely dry and hot July, with only 0.01 inches of rain, that provided short-term precipitation and heat stress in addition to the applied severe grazing stress. Ample autumn precipitation offered potential for growth and recovery following summer moisture stress. The winter of 2003 and spring of 2004 provided little moisture for 2004 spring growth. Above normal precipitation fell in June and July of 2004, but rainfall was well below normal in May of 2005, a period when growth is normally expected from cool-season grasses. Extreme temperature stress also occurred during the study. In 2002 to 2005, low temperatures were below 0°F a total of 4, 4, 15, and 7 days, respectively. High temperatures rose above 100°F for 16, 24, 6, and 17 days in 2002 to 2005, respectively.

Table 1. Monthly, annual, and long-term average precipitation for Hays, KS,
from 2003-2005.

Month	Precipitation (inches)
Month	2003	2004	2005	Long-term average
January	0.01	0.32	1.11	0.52
February	0.42	0.68	1.62	0.64
March	2.19	2.03	2.99	1.98
April	3.74	1.49	2.32	2.18
May	2.31	1.55	1.58	3.14
June	4.50	4.27	3.00	2.64
July	0.01	7.45	2.33	3.76
August	2.99	1.76	3.04	2.93
September	6.46	1.96	1.93	1.62
October	0.39	1.77	2.67	1.40
November	0.15	0.78	0.76	1.22
December	0.43	0.00	0.29	0.65
Annual	23.60	24.06	23.64	22.68

Tiller Density Trends

No location × cultivar × year interaction resulted for tiller density. However, a cultivar × year interaction resulted for tiller density, so data presented are grouped across locations. In 2002, Barton had greater tiller density than five of the other nine grass cultivars, while the remaining nine cultivars were not different from each other (Fig. 1). From 2002 to 2003, no cultivar had a significant change in tiller density after the first season of grazing. In 2003, Barton western wheatgrass still had greater tiller density than all cultivars except Flintlock western wheatgrass and Jose tall wheatgrass. From 2003 to 2004, Barton and Flintlock western wheatgrass and Jose tall wheatgrass increased tiller density by over 100 tillers/ft², even after two seasons of heavy grazing, and these three cultivars had greater tiller densities than all other cultivars in 2004 (Fig. 1). Barton had another significant increase in tiller density from 2004 to 2005, and had greater tiller density than all other cultivars except Flintlock in 2005. Oahe intermediate wheatgrass, Luna pubescent wheatgrass, Bozoisky Russian wildrye, and Lincoln smooth bromegrass did not change tiller density from 2002 to 2005, while all other cultivars increased tiller density from 2002 to 2005 despite heavy grazing in 2003 and 2004. The lowland location also had significantly greater tiller density than the upland location, 209 and 131 tillers/ft², respectively (data not shown) across years and cultivars.

Fig. 1. Tiller density of ten perennial cool-season grass cultivars from 2002 to 2005 averaged over upland and lowland sites near Hays, KS. Autumn tiller density in 2002 was prior to heavy grazing, while autumn tiller density in 2003 and 2004 was after seasons of heavy grazing. Tiller density was measured again in the spring of 2005 to quantify winter survival following defoliation. TW = tall wheatgrass, WW = western wheatgrass, RW = Russian wildrye, SB = smooth bromegrass, PW = pubescent wheatgrass, IW = intermediate wheatgrass.

Plant Frequency Trends

Upland. Western wheatgrass frequency was consistently high because of its spreading rhizomes and subsequent tiller emergence between rows. Both western wheatgrasses, both intermediate wheatgrasses, and Lincoln smooth bromegrass had the greatest frequencies in the spring of 2003 prior to heavy grazing (Table 2). Autumn frequency in 2003 was significantly lower, less than 81%, for Lincoln smooth bromegrass and Bozoisky Russian wildrye following one season of heavy grazing. In the spring and autumn of 2004, Lincoln smooth bromegrass and Bozoisky Russian wildrye again had the lowest plant frequencies of all cultivars (Table 2). Lincoln smooth bromegrass began with one of the greatest plant frequencies in the spring of 2003, but finished with one of the lowest frequencies at each sampling period after the onset of heavy grazing (Fig. 2). However, Barton and Flintlock western wheatgrasses were found in over 99% of the frames in each of the sampling periods, even after heavy grazing (Table 2). All other grasses had over 93% frequency at each of the sampling periods.

Table 2. Plant frequency from 2003 to 2005 of ten cool-season grass cultivars when heavily grazed during the 2003 and 2004 growing seasons on an upland soil near Hays, KS.

Grass^x	Frequency (%)
	2003		2004		2005
	Spring^y	Autumn	Spring	Autumn	Spring
Alkar TW	94 e	93 b	98 bcd	97 bc	98 ab
Barton WW	100 a	100 a	100 a	100 a	100 a
Bozoisky RW	92 e	81 c	94 de	89 cd	92 cd
Flintlock WW	100 ab	100 a	100 a	100 a	100 a
Jose TW	96 de	95 b	98 abc	99 ab	97 bc
Lincoln SB	98 bc	76 c	87 e	78 d	86 d
Luna PW	96 de	94 b	94 cd	93 bc	97 bc
Manska PW	97 cd	96 b	98 abc	99 ab	99 ab
Oahe IW	98 cd	95 b	97 bcd	99 ab	98 ab
Slate IW	99 abc	98 ab	99 ab	99 ab	98 ab

^x TW = tall wheatgrass; WW = western wheatgrass, RW = Russian wildrye, SB = smooth bromegrass, PW = pubescent wheatgrass, IW = intermediate wheatgrass.

^y Means within columns followed by the same letter are not different
(P > 0.05).

Fig. 2. Lincoln smooth bromegrass (A) and Barton western wheatgrass (B) autumn frequency in late October 2004 at the upland location four weeks after grazing concluded for the season. Lincoln smooth bromegrass plant frequency and tiller density were significantly lower than for either of the western wheatgrass cultivars after two seasons of heavy grazing. Notice the short stature of both grasses. For reference, the frame is made of 3/4 inch diameter electrical conduit.

Lowland. At the lowland location, no sampling period × cultivar interaction resulted, so data were averaged across all sampling periods. Barton and Flintlock western wheatgrasses had 100% frequency at each sampling period (Table 3). The tall wheatgrass cultivars, Jose and Alkar, had the lowest frequencies at just under 95%. All other grasses averaged over 95% frequency across the five sampling periods. Even though statistical differences were present, plant frequencies at the lowland location remained high for all grasses even after the onset of heavy grazing. Averaged across years of sampling and cultivars, grass frequency was only slightly greater at the lowland site compared to the upland site, 98% and 96%, respectively (data not shown).

Table 3. Plant frequency of ten cool-season grass cultivars
averaged over five sampling periods (from 2003-2005) when
heavily grazed in the 2003 and 2004 growing seasons on a
lowland soil near Hays, KS.

Grass^x	Frequency (%)^y
Alkar TW	94 d
Barton WW	100 a
Bozoisky RW	98 bc
Flintlock WW	100 a
Jose TW	95 d
Lincoln SB	98 c
Luna PW	97 c
Manska PW	99 b
Oahe IW	99 b
Slate IW	99 b

^x TW = tall wheatgrass; WW = western wheatgrass,
RW = Russian wildrye, SB = smooth bromegrass,
PW = pubescent wheatgrass, IW = intermediate wheatgrass.

Yield After Recovery From Grazing

During the summer of 2005, following consecutive seasons of excessive defoliation, stands were allowed to recover and accumulate forage dry matter until June. Yield response depended on the location. Alkar tall wheatgrass had the greatest yield at both locations, but was not different than Flintlock western wheatgrass at the upland location or Barton western wheatgrass at the lowland location (Fig. 3). Lincoln smooth bromegrass, Luna and Manska pubescent wheatgrass, Bozoisky Russian wildrye, and Slate intermediate wheatgrass had among the lowest yields at both locations, producing 3000 lb/acre or less. Although Alkar and Jose tall wheatgrass had the lowest frequencies at the lowland location, they also had two of the greatest yields. The pubescent and intermediate wheatgrasses had equal to the greatest frequencies at one or both locations, but had some of the lowest yields. Because plant frequency and survival was greater than expected for most cultivars, tiller density (R = 0.56) appeared to be a better indicator of productivity and yield for these grasses after heavy grazing than did frequency (R = -0.03).

Fig. 3. Dry matter herbage yield of ten perennial cool-season grass cultivars at upland and lowland sites in the summer of 2005 following two consecutive seasons of heavy grazing in 2003 and 2004 near Hays, KS. TW = tall wheatgrass, WW = western wheatgrass, RW = Russian wildrye, SB = smooth bromegrass, PW = pubescent wheatgrass, IW = intermediate wheatgrass.

Application

Stocking rates and extent of defoliation in this experiment were significantly greater than would be recommended for any production system. Despite severe defoliation, total stand survival based on frequency data was greater than expected across all cultivars, but the tiller densities that made up the stands were lower for some cultivars. It was evident that the western wheatgrass and tall wheatgrass cultivars had greater tiller densities and yields compared to other cultivars following excessive defoliation. In a prior study, Russian wildrye retained greater biomass and tiller number than intermediate wheatgrass when defoliated, but both grasses still declined in tiller number and biomass as defoliation level increased (5). Hendrickson et al. (6) also showed that intermediate wheatgrass stands could be short-lived in the northern Great Plains, because eight intermediate wheatgrass cultivars had negative tiller replacement ratios after the third year of the stand. In the present study after the third year of the stand, although tiller densities were less than western wheatgrass and tall wheatgrass, Bozoisky Russian wildrye and all four intermediate and pubescent wheatgrass cultivars either retained or slightly increased tiller number despite heavy grazing. Jose tall wheatgrass had satisfactory production during one season of close and frequent defoliation in Texas, but did not persist well into the second season in one of two trials (8). The hot, dry summer period of a Texas growing season is longer than that experienced in Kansas, and may have been the main reason for less persistence in the Texas trial than what was observed in the present study. Annual precipitation was close to normal for all years of the present study, so years well below normal precipitation in conjunction with close defoliation may have a greater impact on persistence than what was observed. Similar to the present findings, Jose tall wheatgrass and Barton western wheatgrass were two of the most persistent perennial cool-season grass cultivars under close grazing in seeded and transplanted Oklahoma trials (7). The western wheatgrass and tall wheatgrass cultivars used in the present study also had the greatest yields and tiller densities when grazed intermittently with ample rest between grazing events in a previous Kansas trial (4).

Most improved grass cultivars released from the US were not developed with heavy grazing to test persistence (9), but the western wheatgrass and tall wheatgrass cultivars developed and released decades ago showed excellent persistence in this study. Since western wheatgrass is native to the region, it was expected that it may persist and produce more forage than most other species under the climatic and grazing conditions experienced from 2003 to 2005. Persistence following heavy grazing stress not only serves as a selection tool for forage breeders developing new cultivars, but it also serves as a screening tool for already developed cultivars for production and persistence potential in environments under less favorable conditions. For stands that may experience extreme stress after establishment, and especially near areas of intense utilization (watering sites, corrals, travel corridors, etc.), native western wheatgrass and introduced tall wheatgrass appear to be more capable of persisting and producing biomass than other cool-season grasses tested in this environment. These grasses should provide persistent early spring or late autumn forage in the central Great Plains, especially under less severe defoliation management to maintain grass health and vigor.

Acknowledgment

Journal Paper 07-43-J of the Kansas Agricultural Experiment Station, Manhattan, KS.

Literature Cited

1. Brummer, E. C., and Moore, K. J. 2000. Persistence of perennial cool-season grass and legume cultivars under continuous grazing by beef cattle. Agron. J. 92:466-471.

2. Casler, M. D., Undersander, D. J., Fredericks, C., Combs, D. K., and Reed, J. D. 2000. An on-farm test of perennial forage grass varieties under management intensive grazing. J. Prod. Agric. 11:92-99.

3. Great Plains Agricultural Council. 1966. A stand establishment survey of grass plantings in the Great Plains. Nebraska Agric. Exp. Stn., Great Plains Council Report 23.

4. Harmoney, K. R. 2005. Growth responses of perennial cool-season grasses grazed intermittently. Online. Forage and Grazinglands doi:10.1094/FG-2005-0105-01-RS.

5. Hendrickson, J. R., and Berdahl, J. D. 2002. Responses of a grazing tolerant and a grazing sensitive grass to water and defoliation stress. J. Range Manage. 55:99-103.

6. Hendrickson, J. R., Berdahl, J. D., Liebig, M. A., and Karn, J. F. 2005. Tiller persistence of eight intermediate wheatgrass entries grazed at three morphological stages. Agron. J. 97:1390-1395.

7. Hopkins, A. A. 2005. Grazing tolerance of cool-season grasses planted as seeded sward plots and spaced plants. Crop Sci. 45:1559-1564.

8. Malinkowski, D. P., Hopkins, A. A., Pinchak, W. E., Sij, J. W., and Ansley, R. J. 2003. Productivity and survival of defoliated wheatgrasses in the Rolling Plains of Texas. Agron. J. 95:614-626.

9. Moser, L. E., and Hoveland, C. S. 1996. Cool-season grass overview. p. 1-14. In L.E. Moser et al. (ed.). Cool-season forage grasses. ASA/CSSA/SSSA, Madison, WI.

10. Vogel, K. P., and Masters, R. A. 2001. Frequency grid - a simple tool for measuring grassland establishment. J. Range Manage. 54:653-655.

11. Waller, S. S., Moser, L. E., and Anderson, B. 1986. A guide for planning and analyzing a year-round forage program. Nebraska Cooperative Extension Bulletin EC 86-113-C. University of Nebraska, Lincoln.