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B D CampbellAgResearch, Palmerston North

Climate Change AndPastures

Summary

n Climate is a key component in thesuccess of New Zealand dairyproduction systems.

n The most up-to-date scientificevidence indicates that climate ischanging and may change more in thefuture. Increases have been observedin ultraviolet radiation and carbondioxide in a range that may affectpastures, and a discernible humaninfluence on climate has been notedthat is expected to increase in thefuture.

n An average increase in pastureproduction of 10-15% is predicted by2050-2100, but the effect may benegative in some regions and higherin others.

n A change in species composition isexpected, with a significant increasein the range and abundance ofsubtropical grasses.

n The nutritive value of forage isexpected to be reduced by thepresence of the subtropical grasses.

n The impact of the changes on farmingwill be highly dependent on the extentto which there is active adaptation of

Introduction

Climate is a key driver of pasture production inNew Zealand. Our predominantly grass feedingsystems are geared to matching animalrequirements to climate-driven feed supply, byadjusting calving dates, feed conservation andother farm management tools.

The past year has been an important one inunderstanding the possibility that climate maychange in the future as a result of releasinggreenhouse gases (predominantly carbondioxide (CO2), methane (CH4) and nitrous oxide(N2O)) to the atmosphere. In 1995, the report ofthe Intergovernmental Panel on Climate Change(IPCC), a compilation of the scientific evidenceon climate change from hundreds of scientistsaround the world, concluded that “the balanceof evidence suggests that there has been adiscernible human influence on global climate”(IPCC, 1995). The report also concluded thatclimate is expected to continue to change in thefuture. The increase of the whole globe surfacetemperature is highly dependent on the rates ofrelease of greenhouse gases, but is expected

farm management to minimise threatsand capitalise on opportunities. Theseactive management opportunitiesinclude adapting use of waterresources, pasture renewal, forageplant genetic resources, grazingmanagement, and types of livestocksystems.

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to be in the range of 1-3.5°C by 2100, relative to1990 (IPCC, 1995). This prediction is still subjectto uncertainty, and there is less confidence inpredictions for regions the size of New Zealandthan there is for the whole planet.

There is also already clear evidence thatchanges are taking place in the earth’satmosphere which are increasing ultraviolet (UV)radiation levels in New Zealand (due to globalproduction of chlorofluorocarbons (CFCs) andother ozone depleting chemicals). Increases arealso being observed in the CO2 concentrationsin the atmosphere. These two changes havedifferent causes, but both are expected to havedirect effects on pastures, in addition to effectsof changes in climate.

As part of a risk-and-opportunity assessmentfor New Zealand and the dairy industry, it isappropriate to begin to consider whatimplications these changes may have on dairypastures in the future.

rates of emission from human activities, and mayrise from the current 358 ppm to a range of 480-680 ppm by 2100, using the lower bounds ofthe DECO and IS92a scenarios (IPCC, 1995).The annual UV doses in New Zealand are alsoexpected to rise at a rate of about 10% perdecade up to about 2050, then subsequentlyrecover if international agreements on CFCreduction strategies are adhered to. NewZealand is also already a high UV environment,relative to similar latitudes in the northernhemisphere.

Pasture Production

At present, prediction of the impacts of theseclimatic changes on production systems isdifficult because of uncertainties about the likelychanges in regional climate and the sensitivityof the systems to climate change. Most of whatcan be said about the changes at the nationalor regional scale is qualitative, rather thanquantitative (Campbell et al., 1996a), and mostanalyses of climate change impacts thereforecontain the words “could” or “might” far moreoften than is desirable. Research is currentlybeing started to try to produce more quantitativeestimates of future changes. The mostquantitative data on the sensitivity of futurepasture production to climate, CO2 and UVcurrently comes from specific experimentalstudies conducted on particular pasture typesor locations, often in artificial controlledconditions.

Although estimates of the rate of increase inUV levels is quite precise, and effects on pastureare likely (Campbell and Grime, 1993), it is notpossible to give accurate predictions of theeffects on pasture because the sensitivity ofpasture is unknown. In laboratory studies, adepression in yield is often observed withelevated UV (Campbell and Grime, 1993).Similarly for CO2 effects, most data comes fromshort-term laboratory studies. For example,Newton et al. (1994) exposed ryegrass and whiteclover based pastures to 700 ppm CO2 for 217days under three different temperature regimesand observed 8% higher growth on average thanthose grown at 355 ppm. This increase was onlyevident at the two higher temperature regimes(16°C and 22°C). Research is currently inprogress to provide more realistic estimates ofUV and CO2 effects.

Future Climate Scenarios

The current ability of climate scientists to simulateregional climate is not sufficient to make detailedpredictions of future regional climate change.However, plausible scenarios of future regionalchanges in temperature and precipitation havebeen produced (Mullan, 1994), based on anumber of assumptions.

An example of the latest scenarios for NewZealand for a 1°C increase in global surfacetemperature is presented in Figure 1. Thewarming over New Zealand is less than theglobal average, with slightly greater warming inthe South Island than the North Island andslightly greater warming in winter than in summer.A greater frequency of high temperatureextremes, and a reduced frequency of frosts, isalso predicted. There are marked differencesbetween regions in the magnitude and sign ofthe rainfall changes, with a larger precipitationincrease (or smaller decrease) for summer thanfor winter, and for the North Island than for theSouth Island. It is expected that there will be ageneral increase in rainfall intensities and in thefrequency of occurrence of heavy rainfall eventsunder a warming scenario.

The change in CO2 concentrations over thistime frame will depend very much on the net

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Figure 1: Examples of (a) local annual average temperature (°C) change per degree of globalwarming, and local precipitation change (%) in (b) summer and (c) winter per degreeof global warming. The examples represent the weak (Rank 4) and strong (Rank 2)case responses, from a comparison of five global climate models (GCMs) from ananalysis by Mullan (1994). Shading is used to highlight precipitation decreases(shaded) and increases (hatched).

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There are difficulties in scaling up theseobservations to produce predictions of thesensitivity of regional pasture production toclimate change. Some attempts have beenmade (e.g. Salinger et al., 1990). Otherpredictions have been produced usingmathematical models (Martin et al., 1991), butthese involved the use of un-validated modelsand several of the assumptions (such as a largegrowth response to CO2 based on significantlyenhanced photosynthetic rates) now appearunrealistic. Effort is currently being concentratedon producing updated model estimates.

An example of the expected rate of changein pastures given the above climate scenario (butnot taking into account changes in UV levels) isprovided by Salinger et al. (1990). Here, ascenario of 500 ppm CO2 concentration andaverage temperature increase of 1.5°C ispredicted to result in:

• an increase in average production ofryegrass-white clover pasture of 10 to 15percent and higher winter production,

• restricted summer production in east coastareas, due to decreased rainfall associatedwith increased temperature and net radiation,

• an extension of subtropical grass pastures asfar southwards as Taranaki and Manawatu,with an associated marked summerproduction peak of subtropical grasses, butlittle increase in winter production,

• an increase in the incidence of pests anddiseases of pastures and livestock,

• a reduction in soil nitrogen release in springwith higher winter temperatures, as well asfaster nutrient cycling,

• increased year-to-year variability in animalproduction in eastern and southern regions,but decreased variability for dairy productionfor high rainfall regions such as the Waikato.

drought, heavy rainfall events), longer termchanges in climate means, and increases inatmospheric CO2. This has implications for bothproductivity and nutritive value.

A trend to a decrease in the proportion ofryegrass is expected in warmer, northernregions, with an associated increase insubtropical “summer” grasses. Ryegrass is alsoexpected to become less prominent in drier eastcoast areas and in areas with increased damagefrom pasture pests. Here, alternative grassspecies (cocksfoot, tall fescue) may becomeincreasingly important.

Based on a number of experiments, whiteclover is expected to increase in proportion as aresult of the increase in atmospheric CO2

(Newton et al., 1994), with the size andsignificance of the increase being dependent onprevailing seasonal temperature and rainfall(Campbell et al., 1995). Overlying this will alsobe the less well understood effects of climaticvariability, increased competition fromsubtropical grasses and increased UV radiation,which are all expected to reduce cloverproportions in summer. Legumes are generallyrecognised as particularly vulnerable to UVdamage (Campbell and Grime, 1993), but littledata are available for white clover. At present, itis difficult to predict what the net effect of all thesechanges would mean for clover content.

Subtropical grassesSubtropical grasses have a higher temperatureoptimum for growth than the traditionaltemperate species ryegrass and white clover,and are also more tolerant of hot periods andless tolerant of frosts and chilling (Campbell etal., 1996b). Consequently, an increase in theabundance of subtropical grasses is anticipatedin the future, especially where there areincreases in summer temperatures and rainfall,such as the Waikato and Bay of Plenty regions.As a general rule of thumb, if the rain falls whenthe temperature is above 25°C, this tends tofavour subtropical species over temperatespecies. Increased winter rainfall may alsocontribute to subtropical grass invasion becauseof greater pugging damage to the temperatespecies, opening up pastures for subsequentinvasion in spring.

From laboratory experiments, increases inCO2 are predicted to reduce the competitive

Pasture CompositionRyegrass and white cloverThe composition of pastures is expected to besensitive to future changes brought about byincreases in UV, increases in climatic variability,and extreme climatic events (frost, hot days,

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Common name Species Life history Description

carpet grass Axonopus affinis Chase perennial Spreading, stoloniferous, light-green grass; warm,moist sites; lower-fertility, summer-dry soils; cattleand sheep pastures and some dairy pastures.

crowfoot grass Eleusine indica Gaertn. annual Tufted, spreading grass; heavily disturbed areas.

Indian doab Cynodon dactylon L. perennial Spreading, mat-forming, stoloniferous andrhizomatous; moderate - high fertility, dry to verydry sites; cattle and sheep pastures.

kikuyu Pennisetum clandestinum Chiov. perennial Spreading, stoloniferous and rhizomatous; warm,moist fertile sites but can withstand some drought;frost sensitive.

knot root bristle grass Setaria geniculata (Poir.) P. Beauv. perennial Spreading rhizomatous; moderate - high fertilitysites.

Mercer grass Paspalum distichum L. perennial Spreading, stoloniferous, rhizomatous, formingloose mat; wet sites; heavily trodden areas.

paspalum Paspalum dilatatum Poir. perennial Tufted, rhizomatous; widespread; warm, moistsites; moderate drought tolerance.

ratstail Sporobolus africanus (Poir.) perennial Tufted; sunny, dry, north-facing slopes and dry, light Robyns and Tourn. soils; cattle, sheep and coastal pastures.

smooth witchgrass Panicum dichotomiflorum Michx. annual Tufted; adventive in pastures and disturbed sites;survives drier situations; dairy and cattle pastures.

summer grass Digitaria sanguinalis (L.) Scop. annual Tufted; disturbed sites; light soils; dairy and cattlepastures.

Table 1: Characteristics of ten subtropical grasses common in New Zealand pastures.

abilities of subtropical (C4) species relative totemperate (C3) species. However, thesignificance of this effect under realistic fieldconditions is unclear and the CO2 effect may besmall in relation to the direct effects of climate.

The rate of increase in subtropical grassabundance is not expected to be linear but willvary widely from year to year. The greatestabundances will coincide with years having agreater frequency of warm and wet days, suchas experienced in the 1990-91 season and tosome extent this last 1995-96 summer. Thiscurrent variability is associated with the El Nino/Southern Oscillation index.

Many of the grasses that are expected toincrease in future are already present in pasturesin a C3-C4 transition zone, which extends fromNorthland to Manawatu and also into limitedparts around Nelson and the South Island westcoast. These grasses (Table 1) have a varietyof growth forms and include both annual andperennial types. Both perennial and annualtypes are expected to increase in the future.

These grasses present both opportunities andthreats, including:

• high warm-season growth rates and toleranceof drought and heat stress,

• tolerance of pests,

• poor cool-season growth rates and poortolerance of chilling and frost,

• low nutritive value,• high competitive effects, resulting in loss of

ryegrass and clover,• some toxicity to stock.

From what is understood of the generalbiology of these species, it is expected that theperennial species paspalum and kikuyu willincrease in range and abundance in moreproductive pastures, and ratstail, carpet grassand knot-root bristle grass in less productivepastures. The annual species (smoothwitchgrass, summer grass and crowfoot grass)are expected to become increasingly importantin intensively-used dairying areas. Some ofthese annuals are recent introductions to NewZealand. For example, smooth witchgrass wasfirst recorded in 1945 but is now widespread inpastures. Up to 5000 seeds/m2 have beenrecorded in soil cores in the Bay of Plenty (Bell,1996), indicating considerable potential for futureinvasion when conditions permit. The rates ofingress of subtropical grasses into pastures willdepend heavily on both the climatic conditionsand the prevailing grazing management applied.Disturbance of pasture cover will facilitate anaccelerated ingress of invading species,especially where there are even minor increasesin summer rainfall (Table 2).

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Amount of disturbance Amount of water added (mm/week)added (per m 2)

0 3 8 15 30

0 .2 .3 .3 .5 .313 .2 .5 .4 .4 .425 .4 .5 .4 .5 .650 .7 .7 .8 1.5 1.2

100 1.7 3.1 3.5 4.4 4.7

Water x disturbance effect significant P<0.01.

Table 2: Effects of combinations of additional water and disturbance on summer grassseedling shoot mass (g) in a Manawatu dairy pasture in summer (White, Campbelland Kemp, unpublished data). The existing ryegrass/white clover pasture wasdisturbed in early summer by scuffing 150 mm diameter circular areas and waterwas applied by irrigation.

Pasture Nutritive Value

The anticipated changes in climate are expectedto produce both positive and negative effectson pasture nutritive value, through increases inclover content, decreases in digestibility, andincreases in subtropical grass abundance. Onbalance, the prevailing direction of change isexpected to be negative. A reduction in foragequality with increasing proportions of subtropicalgrasses is expected to be a dominant influence.These grasses have lower protein and higherfibre contents than ryegrass (Jackson et al.,1996) and, consequently, have lower organicmatter digestibility (Table 3). A higher incidenceof droughts and hot periods, producing death ofshoots and plants, would also reduce pasturenutritive value. An increase in CO2 is alsoexpected to increase the carbon to nitrogen ratioin forage, reducing nutritive value.

Common name Crude protein Neutral Organic matter(% DM) detergent fibre digestibility

(% DM) (% OM)

crowfoot grass 8 64 61kikuyu 17 58 67paspalum 16 62 64smooth witchgrass 10 56 68summer grass 16 48 74perennial ryegrass 23 38 84

Table 3: Nutritive value of five common subtropical grasses and ryegrass (Jackson et al.,1996).

Implications for Future Pastureand Livestock Management

The outcome of the climatic changes onproduction, composition and nutritive value willbe strongly determined by the way pasturerenewal and grazing management programmesare implemented on farms (Gifford, Campbelland Howden, 1996). It is anticipated thatchanges in management may be used to offsetpotential losses, provided that the changes andpossible threats can be adequately anticipated.In this respect, more quantitative data arerequired on rates of change in pasture andnutritive value in response to climate.

The recent IPCC analysis of agriculture(IPCC, 1995) observes that adaptation (suchas changes in forage plants and crop varieties,

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improved water management and irrigationsystems, and changes in planting schedules andtillage practices) will be important in limitingnegative effects and taking advantage ofbeneficial changes in climate. The extent ofadaptation will depend very much on theaffordability of such measures, access to know-how and technology, the rate of climate change,and biophysical constraints such as wateravailability, soil characteristics and plantgenetics. In New Zealand, it is anticipated thatthe rate of adaptation may be high, especially indairy production systems.

Changes are therefore to be anticipated infarm management. Some of the changes willbe direct changes in livestock, throughalterations in the amount and seasonaldistribution of feed quality and quantity, andanimal and plant pests and diseases. Otherchanges will arise through direct pro-activeadaptation responses to capitalise onopportunities and minimise potential threats ofclimate change, with options including:

• more extensive use of irrigation in dairying,

• increased pasture renewal and use ofimproved species,

• adjustment of management practices toaccommodate the presence of subtropicalspecies,

• improvement of ryegrass and white clover tofit a C3-C4 transition zone where subtropicalspecies are also present, and possiblyimprovement and use of subtropical foragespecies,

• greater use of alternative summer species(e.g. cocksfoot, tall fescue) in pastures,

• development of forage plants with greatertolerance of UV radiation,

• increased fertiliser applications to takeadvantage of better climatic conditions forpasture growth,

• increases in stock numbers and exportvolume of milk fat, lamb, beef meat and wool,(however, significant farm managementchanges would be necessary),

• movement of dairying south, sheep farmingto eastern and southern parts of both islands,and beef cattle to northern and western partsof both islands.

Changes in the economics of farming are alsoexpected to result from climate change effectson offshore pastures and crops in other parts ofthe world. For example, anticipated climatechange effects on grain prices and livestockfarming efficiencies in the United States andEurope would be likely to alter production costs,trading volumes and world prices for livestockproducts.

An important local tool for farming in NewZealand in the future may be the use offorecasting of problem years for subtropicalgrasses or droughts. This would be based onunderstanding of climatic patterns and climaticconditions (such as higher summer temperaturesand rainfall), and the sensitivity of pastures toloss of temperate species and invasion ofsubtropical grasses. Such an approach wouldallow farmers to make more use of forecastingin making management decisions, rather thanas at present where decisions are basedprimarily on historical experience.

Acknowledgements

Thanks to Harry Clark and Paul Newton forhelpful comments on this paper and to ToddWhite for use of unpublished data. This researchwas supported by the Foundation for Research,Science and Technology under Contract NumberC10535. This research contributes at the coreresearch level to the Global Change andTerrestrial Ecosystems project of theInternational Geosphere Biosphere Programme.

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