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FORAGE MANAGEMENT

Persistence of Perennial Cool-Season Grass and Legume Cultivars under Continuous Grazing by Beef Cattle

Agron. Dep., Iowa State Univ., Ames, IA 50011. Journal Paper No. J-18377 of the Iowa Agric. Home Econ. Exp. Stn., Ames, IA, USA

brummer{at}iastate.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Persistence of highly productive forage species in pastures is essential to maximize economic returns from grazing livestock. However, most forage cultivars are neither developed nor evaluated under grazing. The objective of this study was to evaluate several cool-season forage species and cultivars to determine their tolerance to continuous grazing. Three grazing experiments were established in 1996 in central Iowa: (i) 20 cultivars and populations of alfalfa (Medicago sativa L.), (ii) 15 cultivars representing five cool-season legume species, and (iii) 25 cultivars and germplasms of six cool-season grass species. Beef cattle (Bos taurus) continuously grazed the experiments for about four months in 1997 and 1998. Alfalfa yield was measured in adjacent plots. Stand survival ratings were taken each year. Among the alfalfa entries, grazing-tolerant `Alfagraze' showed high persistence but moderate yield. Several new alfalfa populations combined excellent grazing tolerance with yield equal to the best hay-type cultivar. Kura clover (Trifolium ambiguum Bieb.) and white clover (T. repens L.) persisted better than alfalfa, birdsfoot trefoil (Lotus corniculatus L.), and red clover (T. pratense L.), with no loss of stand after two grazing years. Orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.) persisted well, though considerable variation was present among orchardgrass cultivars after the second grazing year. Reed canarygrass (Phalaris arundinacea L.) and smooth bromegrass (Bromus inermis Leyss.) stands were reduced to <10% after one grazing year. Although the severe, continuous grazing used in these experiments is not recommended, it clearly and quickly differentiates among species and cultivars for grazing tolerance.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
IN THE USA, approximately 23 million ha of cropland and 53 million ha of nonarable land, exclusive of range, are used for pasture (Vallentine, 1990). Because most permanent pastures in the upper Midwest consist primarily of Kentucky bluegrass (Poa pratensis L.) and white clover, interseeding more productive species, such as alfalfa and orchardgrass, results in higher forage yields and animal productivity (Rohweder and Albrecht, 1995). However, most temperate forage cultivars were neither selected nor evaluated under grazing, so their utility to pasture systems is unknown (Casler et al., 1998).

A substantial body of literature has unequivocally shown that the genetic constitution of pasture species shifts under grazing (Brummer and Bouton, 1991 and 1992; Smith and Bouton, 1993; Vaylay and van Santen, 1999). Similarly, natural grassland populations with a history of grazing appear more tolerant of grazing pressure than nongrazed populations (Smith, 1998). These results suggest that a superior cultivar for the pasture environment should be selected based on persistence under grazing, at least as a part of its development process. In alfalfa, continuous severe grazing has proven to be an effective means to increase grazing tolerance (Smith et al., 1989; Smith and Bouton, 1993). Alfagraze (Bouton et al., 1991), an alfalfa cultivar selected in this manner, has also shown superior persistence in a continuously grazed system with low cattle pressure (i.e., an undergrazed system) (Bates et al., 1996), suggesting that selecting under a worst-case system (continuous overgrazing) can also improve persistence under a more acceptable management system.

Even though continuous stocking of a pasture may compromise its potential productivity, a considerable hectarage of pasturelands is managed in this manner, with consequent difficulties in maintaining highly productive plants in the sward. Similarly, environmental conditions, particularly drought, may lead to overgrazing and the potential for stand decline. Thus, evaluation of cultivars capable of withstanding the stress of continuous grazing (and, more importantly, overgrazing) would aid selection of appropriate cultivars for pasture systems. Only one report has simultaneously evaluated the performance of a number of forage species and cultivars in a grazing system (Casler et al., 1998). This evaluation was conducted on-farm using rotational grazing management; cultivar performance was not evaluated in a continuously grazed setting. While this evaluation is appropriate for use by producers who practice management-intensive grazing, it may not reflect cultivar performance in pastures managed suboptimally.

Our objective was to determine the relative tolerances to continuous grazing in the upper Midwest among cultivars and species of a number of cool-season grasses and legumes to guide varietal recommendation. The grazing trials also serve as a selection environment for the development of grazing-tolerant populations of a number of species.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Three grazing experiments were established on 17 Apr. 1996 at the Iowa State Univ. Rhodes Research Farm in central Iowa (41°52' N, 93°10' W) on a Downs silt loam soil (fine-silty, mixed, mesic Mollic Hapludalf). The studies included (i) alfalfa cultivars and experimental entries, (ii) cultivars of cool-season legume species, and (iii) cultivars of cool-season grass species. Additionally, a test to measure the hay yield of alfalfa grazing entries was seeded immediately adjacent to the grazing area. Soil tests showed both P and K to be very high at planting, and they remained high throughout the experiment. After establishment, the hay yield test was fertilized annually according to soil test recommendations, and 78 kg ha^-1 N was added annually to the grass tests. No P or K was required for the grazing tests during the experiment. After the second year of grazing, 56 mg kg^-1 P and 172 mg kg^-1 K were present in the legume tests and 43 mg kg^-1 P and 245 mg kg^-1 K in the grass test, levels which were all in the very high category (Voss et al., 1996).

Alfalfa
Twenty alfalfa cultivars and experimental populations were evaluated for grazing tolerance (Table 1) . Cultivar seed was obtained from commercial sources; experimental entries were obtained directly from the breeder. Entries were broadcast seeded into 1.5- by 4.5-m plots at 17 kg ha^-1 using a Brillion seeder (Brillion Iron Works, Brillion, WI). The plot area was bordered on all sides by at least 6 m of tall fescue. The experimental design was a 4 x 5 sextuple rectangular lattice (Cochran and Cox, 1957). For the hay yield evaluation, alfalfa was drilled at 17 kg ha^-1 into 0.9- by 3.7-m plots in a randomized complete block design with four replications. All seed was treated with the correct Rhizobium species. The entire hay plot area was bordered by alfalfa. The selective herbicide EPTC (s-ethyl dipropylthiocarbamate) was applied at 4.67 L ha^-1 for preemergent weed control to both hay and grazing tests.

Grazing
Forage was removed from all plots in mid-July and late August 1996 without collecting yield data. Plots were continuously grazed with beef cattle from 15 May to 12 Sept. 1997 (120 d) and from 16 May to 15 Sept. 1998 (122 d). Stocking was adjusted so that the majority of forage at the commencement of grazing was removed within 3 d. After the initial grazing period, cattle numbers were adjusted to ensure that all regrowth remained below 5 cm in order to maximally stress the plants. Cattle were supplemented with hay as needed to maintain their body condition.

Stand Evaluation
For legumes, stands were rated using two methods: (i) Plant numbers per unit area were determined by counting the number of plants within two 0.1-m² quadrats placed at fixed locations within each plot; and (ii) a visual estimate of percent ground cover was taken. Visual estimates were calibrated in the field by using a series of five photographs depicting stands of <20, 21 to 40, 41 to 60, 61 to 80, and >80% (Thompson et al., 1996). The photographs showed alfalfa plants in early regrowth less than 10 cm in height. Stand percentages were based on ground cover; because the plants had some regrowth, the percentages are higher than basal area estimates. In practice, ground cover estimates can be affected by regrowth height, but by scoring plants within 2 wk after removing cattle in the fall, regrowth was generally short (<10 cm) and similar for all entries. For grasses, each plot consisted of five rows each assigned 20% of the total plot area. Total stand per plot was estimated by summing visual estimates of the percentage (0–20%) of each row occupied by plants.

Initial stands of alfalfa were estimated by plant counts in September 1996. All tests were evaluated for stand immediately prior to the commencement of grazing in 1997 and 1 to 2 wk after the conclusion of grazing each year. From the stand observations, we calculated the percent stand after each year of grazing relative to the initial stand. For each experiment, data were analyzed over and within species using the MIXED and GLM procedures of the SAS statistical software package (SAS Inst., 1990; Littell et al., 1996). Mean separations were based on Fisher's protected LSD (Steel and Torrie, 1980).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Visual Scores Versus Plant Counts
In the two legume studies, both stand counts and visual stand scores were taken after grazing ended in 1997. The two metrics were highly correlated in both the alfalfa trial (r = 0.89) and the legume experiment (r = 0.90), indicating that visual estimates, which are considerably faster to make, produced results similar to the plant counts. Slight inconsistencies between the methods were observed in the alfalfa trials (Table 1). Each of the metrics has flaws. We placed the quadrats at a specific point in each plot to avoid unintentional error, but because grazing creates patchy stands in most plots, plant counts may be biased simply due to chance. Visual observations can also be distorted due to nonuniformity of surviving stands, but basing our scores on a series of photographs detailing stands of varying densities assisted in evaluation (Thompson et al., 1996). Because visual estimates were 5 to 10 times faster than plant counts, only visual scores were taken in 1998.

Alfalfa
After planting, the seed of one entry was found to have been misidentified by the breeder; its results are not reported. In the grazing trial, all entries established well, having virtually identical stand densities at the end of 1996 (Table 1). No winter injury was evident in spring 1997 prior to grazing. After being continuously grazed for four months in 1997, stands of the grazing-tolerant check cultivar Alfagraze had over 80% ground cover; grazing-intolerant cultivars, including the check `Apollo', were severely reduced by grazing. A second year of grazing weakened all stands, but the grazing-tolerant cultivars and experimental entries were clearly differentiated from grazing-sensitive entries. Our experiment met the guidelines in both years outlined by the standard test to characterize alfalfa grazing tolerance, both for separating the tolerant and intolerant check cultivars and for recommended stands of the checks (Bouton and Smith, 1998). Entries with persistence similar to Alfagraze are considered to be grazing-tolerant. After the second year, these included `Xgrazer', ZG9533, 3T136, ZG9430, ZG9438, `620', `Amerigraze 401 + Z', and `Cut `n' Graze.'

Alfalfa stands in the hay yield trial were excellent for all cultivars throughout the experiment (data not shown). The cultivar 5454, included as a high-yielding check due to its consistently superior performance throughout Iowa (Brummer and Crim, 1997), yielded an average of 15.6 Mg ha^-1 for the two postestablishment years (Table 1). Several grazing-tolerant entries yielded as well as 5454.

Alfagraze was developed as a dual-purpose cultivar, having tolerance to continuous grazing pressure but also producing high forage yields when managed for hay (Smith et al., 1989; Bouton et al., 1991). In this study, Alfagraze showed clear persistence advantages over grazing-intolerant cultivars. Several experimental populations and one new cultivar (XGrazer) persisted similarly to Alfagraze, indicating that excellent grazing tolerance can be successfully selected in different populations, as previously reported by Smith and Bouton (1993). Although the hay yield of Alfagraze was substantially lower than that of 5454, newer grazing-tolerant populations (e.g., Xgrazer and ZG9533) had yields similar to the commercial check. Pest resistance profiles of the newer grazing entries are substantially improved from Alfagraze, particularly for phytophthora root rot (Table 1). This experiment conclusively demonstrates that grazing tolerance can be successfully combined with both high yield and disease resistances.

Cool-Season Legumes
Alfalfa and red clover established better than the other species evaluated (Table 2). After the second year of grazing, the white and kura clover entries were clearly superior to the others; their stands postgrazing were unchanged from those pregrazing. White and kura clover plots were visually distinct throughout the grazing season, with plants forming a very prostrate canopy over the plot area. Most of the loss in stand for alfalfa, red clover, and birdsfoot trefoil occurred during the first grazing year, with little subsequent loss observed. Surviving plants may represent the grazing-tolerant fraction of each cultivar's population, as shown previously in alfalfa (Smith and Bouton, 1993).

Several differences were observed among entries of a species (Table 2). Alfagraze maintained one-third of initial stands after two grazing seasons. Conversely, not a single plant of the grazing-intolerant cultivar Apollo remained in any plot after the second year of grazing. Among the trefoils, the rhizomatous cultivar Steadfast had superior stands to all other cultivars after the first season of grazing, despite having the poorest initial stands. After a second season, its stands were only superior to `Leo'. The loss of birdsfoot trefoil stands in our study concurs with previous results obtained under rotational grazing with sheep (Sheaffer et al., 1992).

Two other differences after the first grazing year were noteworthy: (i) `Mammoth' red clover had superior grazing tolerance to all other red clover entries and (ii) `Endura' kura clover was superior to `Rhizo'. In neither case was the difference maintained after the second year of grazing. Mammoth red clover usually persists no more than 2 yr, even under clipping management; therefore, its loss in 1998, the third year after establishment, was expected. Between the two kura clover cultivars, we have found that Endura produces more biomass in the establishment and first full production year than Rhizo, but that Rhizo equals its performance in subsequent years (Brummer, unpublished data, 1998). Similarly, Rhizo stands in this study were initially poorer than Endura but equal after two grazing years.

Continuous grazing has the potential to severely impact the stands of alfalfa, red clover, and birdsfoot trefoil. Grazing-tolerant alfalfa shows marked superiority over cultivars not developed for this use. Birdsfoot trefoil is adapted to permanent, low-maintenance pastures (Beuselinck and Grant, 1995), but no trefoil cultivars performed better than Alfagraze for persistence. Our results suggest that alfalfa developed for grazing, particularly newer cultivars with high resistance to root rot fungi, could be used in these systems as well.

Despite the excessive grazing pressure we applied to these plots, we could not damage stands of kura or white clover. The high P and K levels in these plots probably contributed to the clover persistence. Kura and white clover are superior pasture species, particularly for pastures at risk of overgrazing. In many heavily grazed pastures, white clover becomes the primary legume, but invading ecotypes typically have low yield (Gibson and Cope, 1985). Previously, kura clover has been shown to have moderately high yield and excellent persistence under rotational grazing with sheep (Sheaffer et al., 1992; Peterson et al., 1994). Our research extends these findings to continuous grazing with beef cattle. Therefore, we suggest that kura clover be strongly considered for pasture systems given its productivity and grazing tolerance.

Cool-Season Grasses
Although `Citadel' perennial ryegrass was initially included in the test, its stand was reduced to 9% after the 1996–97 winter (i.e., before grazing), so it was deleted from further analyses.

Orchardgrass had the best stands prior to grazing (Table 3) . The sole prairie bromegrass entry, BW7, had only 52% ground cover; it sustained some winter damage despite having been selected for winterhardiness. After the first grazing season, stands of smooth bromegrass, reed canarygrass, and prairie bromegrass cultivars were below 10%, but orchardgrass and tall fescue stands were over 80% (Table 3). The persistence of orchardgrass fell after a second grazing season, but tall fescue was less affected.

Orchardgrass stands after 2 yr ranged from 31% to 73% of the initial values (Table 3). The two Canadian cultivars, AC Nordic and AC Splendor, had the poorest survival, probably because Iowa is outside their area of adaptation. However, none of the cultivars or populations developed in Iowa performed particularly well. `Dawn' and `Duke' have superior winterhardiness and are among the highest-yielding cultivars in Iowa (Brummer, 1998; unpublished data, 1997), but were among the poorest-surviving cultivars. As with alfalfa, high yield can exist with grazing tolerance; `Benchmark' is one of the highest-yielding cultivars in Iowa (Carlson et al., 1991) and had good grazing tolerance. The experimental population 850 also had excellent grazing tolerance.

Among the tall fescue entries, `Stargrazer' had poorer stands than either `Kentucky 31'or `Johnstone' after two grazing years. Johnstone is reported to be noninfected with the fungal endophyte Neotyphodium coenophialum, but Kentucky 31 is infected; however, we did not verify the presence or absence of the endophyte in these samples. Though Johnstone may be trending toward lower persistence than Kentucky 31, removal of the endophyte has less effect under our conditions than those in the southern USA, where marked differences in performance between infected and noninfected tall fescue are observed (Bouton et al., 1993). More research should be conducted to determine the effect of fungal endophytes on tall fescue performance in the upper Midwest. If endophyte-free cultivars persist well, cultivation of tall fescue could be expanded in Iowa and neighboring states, particularly for use as stockpiled forage for fall and winter grazing.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The validity of using severe, continuous grazing to identify useful cultivars may be questioned because well-managed pastures typically do not receive the treatment these experimental plots have received. Yet a cursory survey of pasturelands virtually anywhere in the USA clearly shows that many pastures are overgrazed, either because of poor management or unfavorable environmental conditions. This study identified species and cultivars that performed well under worst-case conditions. Those cultivars that survive under severe, continuous grazing are unlikely to be eliminated due to grazing pressure when grown in a well-managed pasture.

Continuous heavy grazing accentuates differences among cultivars and may also reflect persistence characteristics that may not become obvious in a rotationally grazed study in its first 2 to 3 yr, the length of time most studies are conducted. For example, under rotational grazing with sheep, Sheaffer et al. (1992) showed that birdsfoot trefoil stands did not decline until the fourth grazing year; our results showed trefoil stand declines within the first year of grazing. Thus, after 1 yr, clear trends among cultivars were evident in the legumes. Smith and Bouton (1993) suggested that two grazing years provide the most consistent results for ranking alfalfa cultivars. For grasses, we found that 2 yr of grazing were needed to differentiate among cultivars of tall fescue and orchardgrass. Severe continuous grazing may allow the identification of grazing-tolerant plants in a shorter time than it takes to thin stands under rotational grazing or than would be observed in well-managed rotationally grazed pastures.

This study clearly demonstrates that discrimination among cultivars and species is possible under continuous grazing and points to particular entries that may be ideally suited to lower-maintenance pastures. Finally, species or cultivars that did not perform well in this study may be acceptable in a rotationally grazed pasture management system, at least for the short term.SAS Institute 1990

	ACKNOWLEDGMENTS

 
We thank Ron Sealock, Jim Kubick, and the rest of the Rhodes Farm Crew for assisting with this research. Thanks also to Lloyd Crim for technical assistance and to an anonymous reviewer for constructive comments. This research was partially funded by the Iowa Crop Improvement Association.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Project No. 2569, supported by Hatch Act and State of Iowa Funds.

Received for publication April 23, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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