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SEED PRODUCTION

Spring Grazing Reduces Seed Yield of Cool-Season Perennial Grasses Grown in the Southern Great Plains

^a Forage Biotechnology Group, Samuel Roberts Noble Foundation, Ardmore, OK 73401
^b Agriculture Division, Samuel Roberts Noble Foundation, Ardmore, OK 73401
^c Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
^d Dep. of Animal Science, Oklahoma State Univ., Stillwater, OK 74078
^e Dep. of Statistics, Oklahoma State Univ., Stillwater, OK 74078
^f Texas A&M Agric. Research and Extension Center, Overton, TX 75684

^* Corresponding author (aahopkins{at}noble.org)

Received for publication February 15, 2002.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Information is lacking regarding management of cool-season perennial grasses for seed production in the southern Great Plains. Our objective was to determine the effect of spring grazing on seed yield of cool-season perennial grasses grown in the southern Great Plains. Treatments consisting of no spring grazing by beef cattle (Bos spp.), limited spring grazing, and extended spring grazing were applied to ‘Paiute’ orchardgrass (Dactylis glomerata L.), ‘Lincoln’ smooth bromegrass (Bromus inermis Leyss), and ‘Manska’ pubescent wheatgrass (Thinopyrum intermedium subsp. barbulatum (Shur) Barkw. & D.R. Dewey) in 1998 to 2000, and to pubescent wheatgrass alone in 2001. Date of first hollow stem, when elevation of apical meristems became evident, was also determined. Limited vs. no spring grazing led to decreased seed yield in 7 of 10 Species x Year combinations, whereas extended vs. limited spring grazing led to declines in seed yield in six of nine Species x Year combinations. Date of first hollow stem occurred from late March to early April for orchardgrass and smooth bromegrass, and early to mid-April for pubescent wheatgrass. Seed yield of orchardgrass and pubescent wheatgrass decreased as grazing time after first hollow stem increased. Commercial seed production of smooth bromegrass, which averaged 336 kg ha^-1 with no spring grazing, may be feasible in the Southern Plains. However, spring grazing does not appear to be a viable component of seed production systems for cool-season perennial grasses in the Southern Plains based on the likelihood of decreased seed yield associated with spring grazing.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
AN ADEQUATE SUPPLY OF SEED, at a cost acceptable to farmers and ranchers, is necessary for success of forage grass cultivars. Currently, production of cool-season perennial grass seed takes place to a large extent in the Pacific Northwest USA and in western Canada. In those regions, seed production of forage cultivars must compete economically against turfgrass cultivars. Also, species such as pubescent wheatgrass that may be useful as forage in the Southern Plains (Redmon, 1999), may have limited seed production potential under climatic conditions found in areas of the Pacific Northwest, such as the Willamette Valley of Oregon.

Several factors, including relatively low land costs (National Agricultural Statistics Service, 1999) and proximity to markets, may allow growers in Oklahoma, Texas, and surrounding areas of the southern Great Plains to become competitive seed producers of various cool-season perennial grasses. Relative to the Pacific Northwest, major challenges facing producers of cool-season grass seed in the southern Great Plains include greater climatic risks, limited infrastructure for seed processing, and insufficient information concerning management of cool-season perennial grasses for seed production in the region.

In the southern Great Plains, a substantial land area of winter wheat (Triticum aestivum L.) is managed for grazing plus grain production. Livestock weight gains can account for much, and in some cases most, of the net returns to wheat managed as a dual purpose crop (Redmon et al., 1995). The economic potential for production of cool-season perennial grass seed in the Southern Plains might be improved by implementation of a similar dual purpose (grazing plus seed production) system.

Extensive information is available regarding the effect of grazing on seed yield of cool-season annual grasses. Grain yield in wheat has been shown to decrease only if grazing is terminated after first hollow stem, defined as "the growth stage at which hollow stem can first be identified between the crown and growing point in ungrazed wheat" (Redmon et al., 1996). The authors concluded that developmental stage must be monitored in ungrazed wheat because grazing can delay onset of first hollow stem by 14 d or more. Young et al. (1996) concluded that late winter to early spring grazing of annual ryegrass (Lolium multiflorum Lam.) by sheep (Ovis aries) did not decrease seed yield in western Oregon. Evers and Nelson (2000) reported that seed yields in eastern Texas did not decrease where spring grazing was terminated before mid-April for most, though not all, of seven annual ryegrass cultivars examined.

Additional information is needed regarding the effect of spring grazing, and in particular, timing of grazing termination, on seed yield of cool-season perennial grasses. Brown (1980) reported decreased seed yield associated with grazing in two of three Treatment x Year combinations in perennial ryegrass (Lolium perenne) grown in New Zealand. In Australia, Williams and Boyce (1977) reported that spring grazing reduced seed yield of tall fescue (Festuca arundinacea Schreb.) only when grazing was terminated at later dates. Likewise, Watson and Watson (1982) found that tall fescue in Mississippi could be defoliated by clipping as late as 30 March without a decline in seed yield. In contrast, in Alabama Ward et al. (1984) observed that among tall fescue plots defoliated by clipping in late fall, spring defoliation as early as 1 March resulted in decreased seed yield.

Date of first hollow stem, and the effect of grazing termination relative to first hollow stem on seed yield, has not been investigated for various cool-season perennial grasses. First hollow stem is an easily identifiable growth stage, and if closely associated with seed yield as has been found in wheat, may prove useful in determining appropriate grazing termination dates for cool-season perennial grasses used for grazing plus seed production.

Poor persistence has often been a limitation among cool-season perennial grasses grown in the southern Great Plains (Klages, 1929; Malinowski et al., 2003; Redmon, 1999). It is not clear if this restraint might be overcome by permitting seed production to take place in pastures periodically, and thus allowing resulting seedling recruitment, from either the seed rain or seed bank, to maintain acceptable stands. Under similar management scenarios where pastures were rested and allowed to produce seed, seedling recruitment has been reported to be poor for orchardgrass in Japan (Yang et al., 1988) and phalaris (Phalaris aquatica L.) in Australia (Virgona et al., 2000). Tracy and Sanderson (2000) suggested that recruitment from the seed bank might be an important mechanism for the prevalence of Kentucky bluegrass (Poa pratensis L.) in pastures in the Northeast USA, but concluded that reseeding would be necessary when establishing pastures that are to contain an array of useful species. Other researchers have found that recruitment of cool-season perennial grasses from the soil seed bank was minimal, particularly in relation to seed rain (Edwards and Crawley, 1999). As such, substantial levels of seed production would appear to be a prerequisite should seedling recruitment be useful as a mechanism for maintaining stands of cool-season perennial grasses. The economics of this approach would presumably be more favorable if spring grazing (vs. no spring grazing) was possible before seed production.

Knowledge regarding the effect of spring grazing and timing of grazing termination on seed yield would be helpful to define seed production systems for cool-season perennial grasses in the southern Great Plains. Thus, the objective of this research was to determine the effect of spring grazing on seed yield of cool-season perennial grasses grown in the southern Great Plains.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The research was conducted at the Oklahoma State University Wheat Pasture Research unit near Marshall, Oklahoma (36°07' N, 97°36' W; 314 m elev.). Soil type was a Kirkland silt loam (fine, mixed, thermic Udertic Paleustolls). Seed yield data were gathered in 1998 and 1999 to 2001 from paddocks corresponding to those described as Location 1 and Location 2, respectively, by Reuter and Horn (2002). Paddocks of ‘Paiute’ orchardgrass, ‘Lincoln’ smooth bromegrass, and ‘Manska’ pubescent wheatgrass were planted in early September of 1996 (Location 1) or 1997 (Location 2) with 13.4 kg bulk seed ha^-1 using a grain drill. Paddocks were harvested for hay the spring following planting.

Herbicide [(2,4-D) 2,4 dichlorophenoxyacetic acid] was applied at a rate of approximately 0.8 kg a.i. ha^-1 on 18 Oct. 1996 at Location 1, and 3 Mar. 1998 at Locations 1 and 2. Fertilizer was applied in accordance with forage production needs (Table 1); N fertilizer was not applied in spring 2000, primarily because of untimely wet field conditions. Fall grazing occurred at Location 1 (29 Aug.–3 Oct. 1997) and Location 2 (23 Sept.–3 Nov. 1999; 2–17 Nov. 2000) when forage availability was adequate. Fall stocking rates ranged from 420 to 1200 kg body weight ha^-1. Height of residual forage following fall grazing was approximately 10 cm in 1997, and 15 cm in 1999 and 2000. Before spring grazing, 4.86 by 4.86 m exclosures were constructed to impose the no spring grazing treatments. After grazing began, adjoining exclosures were sequentially constructed to impose limited spring grazing and extended spring grazing treatments (Table 2). Either six (Location 1) or four (Location 2) replications per species were placed randomly within paddocks (Fig. 1). Exclosures were located in a new area of a given paddock each year. Growing beef cattle were used for all grazing. Spring stocking rates ranged from approximately 1100 to 1500 kg body weight ha^-1, and were adjusted so that initial forage allowance (kg forage/100 kg body wt.) was approximately equal across species within a given grazing season. Redmon (1999) and Reuter and Horn (2002) give additional information regarding stocking rates and animal growth performance from these paddocks for 1997 to 1999.

View larger version (39K): [in this window] [in a new window]   Fig. 1. Arrangement of no spring grazing (NG), limited spring grazing (SG), and extended spring grazing (SG+) treatments in paddocks of smooth bromegrass, pubescent wheatgrass, and orchardgrass at Marshall, OK. Location 1 and Location 2 correspond to those paddocks described by Reuter and Horn (2002). Not to scale. R = replication.

A small plot combine was used to harvest seed from a 3.04 by 4.86 m area in the center of each exclosure. Seed was subsequently dried overnight at approximately 38°C, sieved, cleaned, and weighed. Seed harvest dates, depending on year, occurred in early to mid-June for smooth bromegrass and orchardgrass, and in late June to late July for pubescent wheatgrass.

Because Genotype x Treatment x Year interactions were large, seed yield data were subsequently analyzed by year, as a randomized complete block design using a mixed model. Replications were nested within species and considered random, whereas species and treatments were considered fixed. First hollow stem data were analyzed as a randomized complete block design with years as replications; years were considered random and species fixed. Data from the no spring grazing treatment for 1999 to 2001 were analyzed to examine the effect of age of stand on seed yield of each individual species. For such analyses, a randomized complete block design was used with age of stand considered fixed, and replications considered random. The PROC Mixed procedure of SAS (SAS Inst., 1999), with the LSMEANS PDIFF option was used to compare treatment, first hollow stem, and age of stand differences. The PROC REG procedure was used to perform linear and quadratic regression analyses. Seed yield from all treatments was regressed as the dependent variable against the number of spring grazing days. Seed yield from the limited spring grazing and extended spring grazing treatments was also regressed as the dependent variable against (i) number of days after first hollow stem that grazing occurred; and (ii) number of days between grazing termination and seed harvest.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Precipitation totals for February through May were greater than 254 mm in each year of the research, which is not uncommon for the region. From 27 May through June 30 1998 precipitation totaled 9 mm, with temperatures at or above 37.8°C (i.e., 100°F) on 9 d. By contrast, during this same period in 1999, 2000, and 2001, rainfall amounts of 205, 164, and 143 mm, respectively, were recorded and temperatures never exceeded 37.8°C during May or June. Due to later maturity, seed yield of pubescent wheatgrass was probably affected most by the late spring drought of 1998 (Table 3).

Seed yield decreased with spring grazing in 7 of 10 Species x Year combinations (Table 3). In some cases, as few as 5 d of spring grazing led to decreased seed yield. Seed yield decline associated with limited spring grazing ranged from 0% (orchardgrass in 1998 and pubescent wheatgrass in 1998 and 2001) to as much as 59% (orchardgrass in 1999) relative to no spring grazing. Further decline accompanied extended spring grazing, with seed yield of limited spring grazing exceeding that of extended spring grazing in six of nine Species x Year combinations (Table 3).

Delayed maturity was often observed in grazed plots (data not shown). All orchardgrass plots were harvested on the same day in 1998, with the greatest amount of shattering evident for the more mature no spring grazing plots. We speculate these shattering losses in 1998 probably masked a seed yield decline due to spring grazing of orchardgrass. In subsequent years, any effect of grazing on maturity was taken into account when timing seed harvests. Brown (1980) also noted delayed maturity for grazed perennial ryegrass managed for seed production. Grazing may have application as a management technique where delayed maturity of grass seed crops is desired.

First hollow stem occurred consistently within the last week of March to the first week in April for orchardgrass and smooth bromegrass (Table 4). These results somewhat parallel those of Canode et al. (1972), who reported internode elongation, a growth stage described similarly to first hollow stem, occurring on 19 March for ‘Manchar’ smooth bromegrass and 26 March for ‘Latar’ orchardgrass grown at Pullman, WA. Pubescent wheatgrass was later maturing (P < 0.05) than both orchardgrass and smooth bromegrass, with first hollow stem occurring as late as 21 April in 2001. Target dates for grazing termination were no more than 10 d, or 15 to 25 d, of grazing after first hollow stem for the limited and extended spring grazing treatments, respectively, although these targets were not fully achieved in 1998 and 2001 (Table 4).

Results of regression analyses provided further evidence of the detrimental effect of spring grazing on seed yield. As the total number of spring grazing days increased, seed yield decreased (P < 0.01) for all species (Fig. 2). Likewise, an increased length of recovery period between grazing and seed harvest led to increased seed yield for all species (Fig. 3). Seed yield of orchardgrass and pubescent wheatgrass declined with increased length of grazing time following first hollow stem (Fig. 4), although for reasons that are not clear, no such relationship was evident for smooth bromegrass. The effect of grazing before first hollow stem on seed yield varied. Limited spring grazing of smooth bromegrass in 2000, which terminated on the date of first hollow stem, led to a 49% decline in seed yield compared with no spring grazing. In comparison, seed yield of the no spring grazing and limited spring grazing treatments of pubescent wheatgrass in 2001 did not differ (P = 0.22), perhaps because grazing was terminated 2 d before first hollow stem for the latter treatment.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Relationship between seed yield and number of spring grazing days for orchardgrass, pubescent wheatgrass, and smooth bromegrass.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Relationship between seed yield and number of days between grazing termination and seed harvest for orchardgrass, pubescent wheatgrass, and smooth bromegrass.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Relationship between seed yield and number of days after first hollow stem that grazing occurred for orchardgrass and pubescent wheatgrass.

In the absence of spring grazing, seed yields of Manska pubescent wheatgrass were consistently the least (P < 0.01) of the species examined, with a maximum yield of 157 kg ha^-1 in 1999. This contrasts with reports of average seed yields in excess of 400 kg ha^-1 for the cultivars Manska, Reliant (Berdahl et al.,1993), and Greenleaf (Henry Najda, personal communication, 2002) in the northern Great Plains of the USA and Canada. The near failure of pubescent wheatgrass seed production in 1998 was not likely due to a lack of vernalization, as winter temperatures were similar that year to 1999, 2000, and 2001 (data not shown), but rather can probably be attributed to late spring drought.

Seed yields of orchardgrass, in the absence of spring grazing, were generally less compared with that reported for other areas. Yields from commercial orchardgrass seed fields in Oregon from 1997 to 2000 averaged 969 kg ha^-1 (Oregon Agricultural Statistics Service, 2001), whereas yields averaged from 83 to 853 kg ha^-1 for a number of orchardgrass cultivars evaluated from 1997 to 2001 in western Canada (Alberta Agriculture, Food, and Rural Development, 2002).

Of the cultivars examined, Lincoln smooth bromegrass may have the most potential for commercial seed production in the southern Great Plains, based on an average yield of 336 kg ha^-1 where spring grazing did not occur. Although direct comparisons are not possible because of differences in cultural practices, row spacing, etc., the seed yields we observed were greater or comparable to yields of Lincoln reported from western Canada (Knowles et al., 1970; Knowles and Christie, 1972), and Iowa (Jessen and Carlson, 1985). Seed yields for Lincoln exceeding 600 kg ha^-1 have been reported from Iowa in 1-m rows (Trupp and Carlson, 1971) and under irrigation in eastern Washington (Van Keuren and Canode, 1963). Although spring application of 2,4-D, at rates of 1.7 to 2.2 kg ha^-1, has been observed to reduce seed yield in smooth bromegrass (Canode, 1974), the rate we used was less and probably did not have a negative impact.

Additional information is needed regarding persistence and seed yield potential of a wide range of cool-season perennial grass populations in the southern Great Plains. The cultivars used in this research were chosen based on preliminary results of small plot trails conducted at several locations in Oklahoma by L.A. Redmon (unpublished data, 1996), as well as favorable information regarding performance of animals grazing Manska (Moore et al., 1995). The three cultivars also represented a range in both maturity (with Manska being the latest) and plant type (with Paiute being a bunchgrass whereas Lincoln and Manska form sods via rhizomes). It is possible that various cultivars or species would have produced different seed yields. For example, Van Keuren and Canode (1963) reported cultivar differences for seed yield among the three species examined in the present research, and Berdahl et al. (1993) reported greater seed yield for Reliant pubescent wheatgrass compared with Manska.

A number of factors may have been associated with the variability we observed in seed yields of the three cultivars across years. Presumably differences in the amount and timing of precipitation had a major impact on seed yields. Other factors that may have affected seed yield variability across years include variation in stand age, fall grazing, and N rates. Van Keuren and Canode (1963) reported generally declining seed yields as stands of Lincoln smooth bromegrass and ‘Topar’ pubescent wheatgrass aged. Similarly, in the present research where no spring grazing was imposed, seed yield decreased (P < 0.05) over time for Lincoln (1999–2000) and Manska (1999–2001), but increased (P < 0.05) for orchardgrass (1999–2000). The effect of fall grazing or defoliation on seed yield of cool-season perennial grasses has been reported to range from negative to positive (Lambert, 1956; Green and Evans, 1957; Evans, 1975). Seed yields in 2000 might have been greater if additional N had been applied in spring, as indicated by previous research in orchardgrass and other species (Nordestgaard, 1983; Young et al., 1999). Clearly, additional information regarding management practices, such as row spacing, fall grazing, fertilizer application rates and timing, irrigation, and residue management, will be needed to optimize seed yields, as well as seed quality, of cool-season perennial grasses grown in the Southern Plains.

As noted, persistence of cool-season perennial grasses can be inadequate in the southern Great Plains. Seed yields of extended spring grazing treatments indicate that in many instances 20 to 40 or more d of spring grazing can occur and yet allow seed production in excess of 25 kg ha^-1, which is approximately twice the recommended seeding rate for these cultivars in the Southern Plains (Redmon, 1999). However, Mortimer (1976) reported that virtually all orchardgrass seeds from a seed rain died or were lost to predation, and others have concluded that the seed bank is not a major source of seedling recruitment for cool-season perennial grasses (Edwards and Crawley, 1999). This seems to indicate that frequent production of ample amounts of seed would be needed for a system relying on seedling recruitment to maintain pastures of cool-season perennial grasses. Further research will be needed to determine the feasibility of such a strategy. With this possible exception, management of cool-season perennial grasses for spring grazing plus seed production will probably not be practical in the southern Great Plains.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
This research is based on work supported in part by the Cooperative State Research, Education, and Extension Service, USDA, under Agreement no. 93-34198-8410, 97-34198-3970, and 99-34198-7481.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

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