EFFECTS OF HORSE GRAZING IN SPRING ON SURVIVAL, RECRUITMENT, AND WINTER INJURY DAMAGE OF SHRUBS

4 Pages·1995·1.8 MB·English

by  Dennis D Austin

GreatBasinNaturalist55(3),© 1995.pp.267-270
EFFECTS OF HORSE GRAZING IN SPRING ON SURVIVAL,
RECRUITMENT, AND WINTER INJURY DAMAGE OF SHRUBS
Dennis D. Austin^ and Philip Urness^
J.
—
Abstract. Theuseofdomesticgrazerstoshiftthegrowthadvantagetowardshrubsisacommonlyappliedtoolon
winter ranges managed primarily for big game. Results from horses grazing in spring indicated grazing also benefits
shrubsurvival,seedlingreciaiitment,andreducedwinterinjuiydamageonsomespeciesofshrubs.
Key words: winter range, range management, mule deer horses, shrubs, browse, Utah, revegetation. n^ountain big
sagebrush, Douglasrabhitbrush, truemountainmahogany.
Onwinterranges managed primarily forbig seasons following planting, all seedlings were
game, the management alternative often removed by hand and rototiller weeding.
selected to maintain the desired mixture of Between 1983 and 1987 the three paddocks
shrubs and understory herbage is grazing by received equal use by mule deer {Odocoileus
livestock in spring. Numerous studies have re- honionus) in winter and no livestock grazing.
ported the benefits ofspring livestock grazing A detailed description of the site is found in
to maintain and improve stands of shrubs on Olsen-Rutz and Urness (1987).
winter ranges (Christensen and Johnson 1964, This study was conducted during the six
Smith and Doell 1968, Jensen et al. 1972, Hull growing seasons between spring 1987 and fall
and Hull 1974, Reinerand Umess 1982, Austin 1992. In spring 1987 all shrub seedhngs that
et al. 1994, and others). However, information had become established from seeds were re-
is limited concerning shrub responses to the moved from each paddock by hand pulling to
effects oflivestock grazing with respect to (1) minimize soil disturbance. The number ofsur-
survival of individual mature plants, (2) seed- viving, previously transplanted shrubs within
lingrecruitment, and (3) winter injuiy damage. each clusterwas counted.
In this studytheresponses ofshrubs to domestic Paddocks were randomly assigned agrazing
horse grazingtreatments in springare reported treatment by horses as heavy, moderate, or
for Artemisia thdentota var. vaseyana [Rydb.] protected. Three to sevenhorses were used, de-
Beetle (mountain big sagebrush), Chrysothcun- pending upon herbage production, to obtain
nits viscidiflonis [Hook.] Nutt. (Douglas rabbit- utilization levels of 35-50% and 65-80% for
brush), and Cercocarpus montamis Raf (true moderate and heavy treatments, respectively.
mountain mahogany). Horseswere selectedas grazersbecauseoftlieir
high foraging selectivity for grasses and avoid-
Methods ance ofshrubs, and the managerial opportunity
to manipulate the herbaceous understory to
The studv site, located on the foothills eaWst improve shrub growingconditions (Reinerand
of Logan, UX 4r46' N latitude, lir47' Umess 1982). Paddocks were grazed yearlybe-
longitude, at 1600 m elevation, contained three tween 1 May and 30 June 1987-1991. In 1992
50 X 50-m adjoining paddocks. Within each all paddocks were rested from grazing. In the
paddock the three browse species were hand- moderately and heavily grazed paddocks,
planted from transplants in spring 1983 in 5 X herbage production, comprised almost entire-
5 clusters of25 plants, with 1 m between plant ly of annual grasses, and percent utilization
centers. Seven clusters were planted in each were determined from four paired 1-m^ bas-
paddock, with each cluster separatedbyamin- keted and unprotected plots, randomly placed
imum of20 m. Before planting, all vegetation in spaces between clusters. Baskets were con-
was removedby root plowing; for two growing structedfiom 1.2-m-highnettingwire supported
iRangelandResources,UtahStateUniversit>',Logan,UT84322-5230.
267
268 Great Basin Naturalist [Volume 55
by steel fence posts. Plots were reestablished moderately and heavily grazed treatments din-
and relocated yearlybefore grazing. ing all years except 1987. Herbage production
In fiill 1992all survivingshrubswerecounted in protected plots was not different from the
by cluster, all seedlings within 10 m of each moderately and hea\'ily grazed treatments dur-
clusterwere counted, and percent winterinjui") ing the first three years. However, the heavily
damage was visually estimated. Winter injury grazed treatment had lowerproduction during
was defined as the amount ofdead stems and the last two years, suggesting that heavy graz-
twigs as a percentage of total dead plus live ingbyhorses reducedproduction ofherbage.
stems and twigs. Damagewas estimated at five-
Shrub Sunival
unit increments from to 95%.
Because we were not able to replicate the Horse grazingincreasedsuwivaloilArtemisia
three paddocks established for the previous (P = .01) and Cercocarpus (P = .10) but had
study, we considered clusters as experimental no effect on Chnjsothamnus (Table 1). All
units. We agree with Hurlbert (1984), who three species declined in numbers between
described this experimental design as "simple 1987 and 1992 (P = .001).
pseudoreplication," but because ofconstraints In 1987 the number of surviving plants
of time, space, and costs, this design was the among treatments for Aiionisia, ChrysotJiam-
only option. Consequently, we recognize that mis, and Cercocarpus was not different (Table
differencesbetween treatments couldbecaused 1). However, in 1992 the number ofsui-viving
by inherentdifferences between paddocks, but Artemisia plants among treatments was differ-
argue that potential spatial error is low due to ent (P = .005). The protected treatment had
adjoining paddocks, identical use during the lower survival than both the moderately and
three years preceding our experiment, simple heavily grazed treatments (P = .001), but the
grazing treatments applied, and lack ofdiffer- moderatelyandheavily grazed treatments were
ences in the number of surviving seedlings not different. Similarly, for Cercocarpus the
among paddocks for each species (P > .10) at number of sui^viving plants among treatments
the beginning ofthe experiment. was different (P = .03). The protected treat-
T tests of the means were used to deter- ment had lower sui-vixal than both the heavily
mine differences between grazed and protect- (P = .005) and moderately (P = .10) grazed
ed plots within and among paddocks. Forplant treatments, but the moderately and heavily
survival a split-plot design using repeated mea- grazed treatments were not different. For
sures (1987 and 1992 data) analysis ofvariance Chrysothammis, no differences were found.
determined treatment and year effects. One-
way ANOVAs assessed differences among Seedling Recruitment
treatments for species within years. For seed- ForArtemisia, seedlingrecruitment was sig-
lingreciTjitment, because all seedlings were re- nificantly different among treatments (P = .08).
moved in 1987, one-way ANOVAs were used The heavily grazed treatment had more seed-
for species within years. For winter injury lings than the protected and moderately grazed
damage, differences between treatments were treatments (P = .05). No differences among
analyzed using chi-square tests. A significance treatments were found for Chnjsothamnus,
level ofP < .10was usedforall tests. and no seedlings were counted for Cercocar-
pus (Table 1).
Results and Discussion Although the low numbers of seedlings
counted in this study require inteipretive cau-
Horse Grazing
tion, results are consistentwith other studies in
Horse use reduced herbage at the end of which livestock grazing was reported to in-
the grazing period during all years in both the crease shiiib densitv (Stewart 1941, Christensen
moderately and heavily grazed treatments (P and Johnson 1964, Hull and Hull 1974).
< .10), except in 1987when neither treatment Furthermore, the results from this study, that
was different from protected plots. Mean herb- horse grazing in spring resulted in higher sur-
age utilization during all years was 46% in the vival of mature plants and increased seedling
moderately grazed treatment and 71% in the establishment for several species of shrubs,
heavily grazed treatment. Following grazing, are consistent with reports of increased pro-
remaining herbage was different between the duction of shrubs following livestock grazing
1995] Shrub Responses to Grazing 269
Table 1. Plantsundval(total niiinber/paddock), seedlingrecruitment(total niimber/paddock),andwinterinjin-\'dam-
age(mean%pershmb)o{Artemisia tridentata (ARTR), Chnjsothainniisviscidiflorus (CHVI),andCercocarpusmontanus
(CEMO),asaffectedbyheavy(H),moderate(M),andprotected(P)horsegrazingtreatments'.
Species Treatment 1987 1992
Shrubsuni\al Number/paddock
ARTR 119
CHVI
CEMO
Seedlingrecnjitment^
ARTR
CHVI
Winterinjur}'damage'^
CEMO
270 Great Basin Naturalist [Volume 55
Literature Cited Nelson, D. L., andC. ETiernan. 1983. Winterinjurv-of
sagebrush and otherwildland shiiibs in thewestern
AustinI,mpaD.ctsD.,ofF.muJ.leUrdneeesrs,anadndhorS.seL.grDaizihnigiaomn. t1r9a9n4s.- URnaintgeedE.Sxtpaetresi.menUtSSDtaAtioInntReersmeaorucnhtaPianpeEroIrNeTs-t31a4n.d
planted shrubs for revcgetation. Journal of Range Olsen-Rutz, K. M.,and RJ. Urness. 1987.Comparability
Management47:8-11. offoraging behavior and diet selection oftractable
CHR1STEN.SEN, E. M.,ANDH. B.JoHNSON. 1964. Presettle- and wild mule deer. Utah Division ofWildlife
ment vegetation and vegetational change in three Resources Publication No.88-3.
valleys in central Utah. Brigham Young University Reiner, R. J., and P J. Urness. 1982. Effects ofgrazing
ScienceBulletin, BiologicalSeries IV(4): 1-15. horses managed as manipulators ofbiggame winter
Hull,veAg.etCat,ioJnr.,ofaCnadchMe.VaKl.leHy,ulUlt.ah19a7n4d. IPdraehsoe.ttJloeumrennalt RiGGS,raRn.geA..,JoainudTiaPlJo.fURrannegsse.M1a9n8a9.geEmffeenctts3o5:fg5o6a7t-5b7r1o.ws-
ofRange Management27:27-29. ing on Gambel oak communities in northern Utah.
HURLHERT, S. H. 1984. Pseudoreplication and the design JournalofRangeManagement42:354—360.
ofecological field e.xperiments. Ecological Mono- Smith, A. D., and D. Doell. 1968. Guides to allocating
graphs54: 187-211. foragebetweencattleandbiggameonwinterrange.
Jensen, C. H., A. D. Smith, .\nd G. W. Scotter. 1972. Utah DivisionofFishandGame Publication No. 68-
Guidelines forgrazingsheep on rangelands used by 11.
big game in winter. Journal ofRange Management Stewart, G. 1941. Historicrecordsbearingonagriculture
25:346-352. andgrazingecologyin Utah. Ecolog>'39:362-375.
Jensento,bCi.ttHe.r,bnainsdhrPelJ.atUerdnteosss.pri1n9g79s.hWeienptgerrazcionlgd.dJaoimmaigael Received 7Fehnuinj1994
ofRangeManagement32:214-216. Accepted24January1995