20

D A Clark, J W PennoDRC, Hamilton

n An average South Auckland dairy farmgrows 14.3t DM/ha/year, of which 10.0tDM/ha/year is eaten. Direct costs ofgrowing this pasture are $426/ha (3 c/kg DM grown, or 4.2 c/kg DM eaten).

n Uneaten pasture quickly dies anddecays, so that wastage goes unnoticed.Pasture lost through decay on highlyutilised farms is 3 t DM/ha/year (16% oftotal).

n Uneaten pasture dies and accumulatesduring late spring and summer to givepasture dead matter contents of 35-40%, even in well-utilised swards.

n Stocking rate is the key determinant ofpasture utilisation.

n Attempts to increase total farmproductivity by lowering stocking rateand feeding supplements are doomedto failure because no supplements areavailable that have substitution ratesclose to zero when cows are fully fedpasture.

n The extra money being spent on bought-in supplements, grazing off, N fertiliserand summer crops, can only return aprofit if stocking rate is increased.

Summary Introduction

Table 1 shows the estimated cost of pasture DMgrown and eaten for an average South Aucklanddairy farm in 1994/95. Pasture is obviously alow cost feed compared to any bought-insupplement. A comparison of costs of pasturegrown and pasture eaten shows that the cost ofpasture eaten can be significantly reduced byincreasing pasture utilisation.

This paper reviews some of the uniquefeatures of pasture that offer both challengesand opportunities in dairying, and uses the No 2Dairy farmlet experiment to illustrate the effectof different stocking rate and N fertiliser use onseasonal pasture dynamics.

How Do Pastures Grow?

Pasture grass plants, such as perennialryegrass, are made up of several til lersconsisting of three live leaves. Leaves originatefrom a growing point close to the soil surface.Grazing only very rarely removes this growingpoint and, therefore, the tiller can persist undergrazing. In late winter the growing point istransformed and will produce a single stem thatwill flower.

The rates of leaf appearance and death areclosely related, which accounts for the relativelyconstant number of three leaves per tiller.Consequently, increased pasture green leaf (kgDM/ha) can only occur if leaves increase in sizeand/or til ler density (number/unit area)increases. Rate of leaf appearance is much

The Importance OfUsing PastureGrown

21

Table 1: Pasture grown and the direct cost of grazing pasture on an average South Aucklanddairy farm (data from Livestock Improvement (1995) and New Zealand Dairy Board(1995)).

Milksolids (kg/ha) 738Conversion efficiency (kg DM/kg MS) x 13.6Pasture eaten (kg DM/ha) 10,037Pasture grown (kg DM/ha) 14,339 (assumes 70% eaten)

Pasture costs ($/ha)Fertiliser (incl. N) 270Pasture renovation and conservation 140 (assumes 50% of published value -Weed and pest control 16 excludes meal and grazing off costs)TOTAL COST 426

Cost of pasture eaten (c/kg DM) 4.2Cost of pasture grown (c/kg DM) 3.0

faster in spring (seven days) than in winter (30days). But because of the linkage betweenappearance and death, leaf life spans are onlyapproximately 21 days in spring, compared with90 days in winter. Even before death, leavesexport nutrients to other plant parts, so that deadleaves have a much lower nutrient content thanlive ones. Warm, moist, fertile conditionsencourage the rapid decay of dead leaves, butthe opposite environment leads to accumulationof dead matter in the sward.

This flow of leaf appearance and death isinterrupted when the growing point of a tillerproduces a flowering stem. Hormonal changesand changes in leaf placement on the stem leadto increased DM production and increased leaflife span. However, the mature stem containshigher levels of lignin and lower levels of crudeprotein and soluble carbohydrate than leaves.Therefore, the nutritive value of pasturescontaining mainly flowering stems is decreased.In addition, the growing point is now elevatedand removal by grazing or cutting results in thedeath of the tiller. Intensive grazing means thata large proportion of the largest tillers die at thesame time, and DM production may slump asnew, small, vegetative tillers struggle to establish.

The brief summary above shows the dynamicnature of both DM production and nutrientcontent in a pasture. The use of such materialto feed cows, optimise profitability, and maintainlong term ecological stability, presents a majorchallenge.

How Can We Maximise PastureEaten?

McMeekan (1961) concluded “that no morepowerful force exists for good or evil than thecontrol of stocking rate in grassland farming”. Ifmaximum pasture is to be eaten, the choice ofstocking rate is critical. However, optimumstocking rate will differ for every farm becauseof soil type, fertility status, topography, andclimate, as well as cow factors such as breedand breeding worth. The efficiency with whichpasture is converted into milk can be expressedas:

Milksolids produced x Pasture eatenPasture eaten Pasture grown

or Feed conversion efficiency x Pasture harvesting efficiency

(from Holmes and Macmillan 1982)

An increased stocking rate increases pastureharvesting efficiency. Feed conversion efficiencycan be increased by increasing per cow intake,and hence milksolids (MS) yield, resulting in alower maintenance cost per unit of milksolids.However, attempts to do this experimentallynearly always result in a decline in pastureharvesting efficiency that is larger than theincreased feed conversion efficiency.

Substitution

Attempts to improve feed conversion efficiencyon pasture by supplementary feeding invariablydecrease total efficiency unless stocking rate is

22

Table 2: Summary of annual pasture, intake and milksolids yield for Farmlet 1 (3.4 cows/ha,no N fertiliser), Farmlet 3 (3.4 cow/ha, 400 kg N/ha/year) and Farmlet 5 (4.4 cows/ha, 400 kg N/ha/year).

Farmlet1 3 5

Pasture grown (t DM/ha) 18.6 21.7 23.6Pasture eaten (t DM/cow) 4.44 4.74 4.22Pasture eaten (t DM/ha) 15.1 16.1 18.6Pasture decay (t DM/ha) 3.00 3.23 3.54Conservation (t DM/ha) 0 1.80 0.57Change in pasture cover (t DM/ha) 0.5 0.6 0.9Milksolids yield (kg/cow) 327 369 315Milksolids yield (kg/ha) 1094 1235 1389Feed Conversion Efficiency (kg MS/t DM eaten) 72.5 76.7 74.7Pasture Harvesting Efficiency (t DM eaten/t DM grown) 0.81 0.74 0.79Total efficiency (kg MS/t DM grown) 58.8 56.9 58.8

increased. This occurs because cowsconsuming supplements reduce their intake ofpasture, and hence decrease pasture harvestingefficiency (see Figure 1). At pasture intakesabove 13 kg DM/cow/day, substitution ratesclose to one occur when pasture silage is fed.This means that pasture equivalent to the silagefed will be ungrazed and, in all probability,eventually wasted. Unless silage is of very highquality, per cow milksolids yield will decline whensubstitution rates are close to one.

When pasture intakes are below 10 kg DM/cow/day, substitution rates of 0.3-0.6 arecommonly recorded. This implies that thesupplement is making a significant contributionto the cows’ diet, but also that pasture is beingspared. This may be exactly the responserequired in autumn when pasture is being savedfor winter and condition score of cows improved.However, in spring, the pasture saved is likely tocontribute to unwanted surpluses.

Figure 1: Effect of base pasture intake onsubstitution rate for cows fedsilage (from Phillips, 1988).

Pasture Dynamics at No 2 Dairy1995-96

Pasture cover, accumulation rate, and cow intakemeasurements were measured weekly in the No2 Dairy experiment (Penno 1996, thisproceedings). Data from Farmlets 1 (3.4 cows/ha, no N fertiliser), Farmlet 3 (3.4 cows/ha, 400kg N/ha/year) and Farmlet 5 (4.4 cows/ha, 400kg N/ha/year) were combined with pasture deathand decay rates based on published data forperennial ryegrass-white clover pastures (Hunt,1972; Korte and Sheath, 1978) in a spreadsheetmodel. This model was used to estimate thedynamic changes in live and dead matter contentof the pasture through the year, undercontrasting stocking rate and N fertiliser options,in the No 2 Dairy Farmlet experiment. GreenDM was assumed to contain 12.5 MJ ME/kg DMin winter and spring, 10.7 MJ ME/kg DM insummer, and 12 MJ ME/kg DM in autumn; deadDM was assumed to contain 7 MJ ME/kg DM(Hoogendoorn and Holmes 1992). A summaryof the annual pasture, intake and milksolid yielddata is given in Table 2.

Table 2 shows that where N fertiliser wasused, but stocking rate not increased (Farmlet3), pasture grown was increased by 3.1 t DM/hacompared with Farmlet 1. But pasture eatenper cow increased by only 0.3 t DM, hencepasture eaten per ha by only 1.0 t DM. Intensivepasture monitoring allowed surpluses to beidentified quickly and conserved as silage so thatless material was available to decay later in theseason. However, it is unlikely that the amountof silage made will be required within the system,

-1

0

1

2

0 2 4 6 8 10 12 14

Sub

stitu

tion

rate

(kg

pas

ture

D

M/k

g si

lage

DM

)

Pasture intake (kg DM/cow/day)

23

and net returns from its sale would not matchthe return made if converted directly to milk. Themarginal efficiency of the extra DM grown fromN fertiliser is 45.5 kg MS/t DM grown, comparedwith overall efficiencies of 58.8 and 56.9 kg MS/t DM grown for Farmlets 1 and 3.

Where N fertiliser was used, but stocking ratewas increased (Farmlet 5), pasture grown wasincreased by 5.0 t DM/ha, compared with Farmlet1, but pasture eaten per cow decreased by 0.22t/DM. However, the increased stocking ratemeant an increase of 3.5 t DM/ha in pastureeaten. Pasture surplus conserved as silage inspring was only 0.57 t DM/ha, but pasture coverover the year increased by 0.9 t DM/ha, whichsuggests that loss through subsequent decaycould be underestimated in this system. Themarginal efficiency of the extra DM grown fromN fertiliser is 59.2 kg MS/t DM grown.

rapidly and, within two months, most of theaccumulated dead matter has decayed. Duringthis period it is possible that the decay rate ofdead matter exceeds that of new green DMgrowth, so that negative accumulation rates arerecorded. From Table 2, the pasture lost fromdecay as a proportion of that grown is 16.1, 14.9and 15.0% for Farmlets 1, 3 and 5 respectively.

Pasture Loss

Predicted pasture growth and decay are shownin Figures 2 a, b and c for Farmlets 1, 3 and 5respectively. Predicted pasture growth rateswere -1.1%, -0.9% and +8.8% of measuredannual pasture accumulation rates. Differencesbetween predicted pasture grown and measuredpasture accumulation on a monthly basis wereoften significant. This is to be expected becausepredicted pasture growth is attempting toestimate new green DM growth, while pastureaccumulation is measuring the balance betweennew green DM grown and DM disappearanceresulting from decay of dead matter.

In Figures 2 a, b and c, the difference betweenthe pasture growth and decay curves representspasture accumulation. Pasture death isproportional to pasture growth, as explained inthe introductory section and, in spring, deadmatter is assumed to disappear within onemonth. This means that although substantialamounts of DM may be lost through death anddecay, there is little accumulation of dead matterin the swards. In summer, death rates acceleratebut decay is effectively halted in dry conditionsas earthworm activity ceases and microbialactivity declines. This combination leads to rapidaccumulation of dead matter in the pasture.

With the onset of warm, moist conditions inlate summer-autumn, decay processes increase

Figure 2: Predicted seasonal changes inpasture growth and decay ratesfor Farmlets 1 (Fig. 2a), 3(Fig. 2b)and 2(Fig. 2c) of No 2 Dairyexperiment for 1995-96.

0

20

40

60

80

100

120

140

Jun

Aug Oct

Dec

Feb

Apr

Decay Grow thFig 2b

(kg

DM

/ha/

day)

0

20

40

60

80

100

Jun

Aug Oct

Dec

Feb

Apr

Decay Grow thFig 2a

(kg

DM

/ha/

day)

0

20

40

60

80

100

120

Jun

Aug Oct

Dec

Feb

Apr

Decay Grow thFig 2c

(kg

DM

/ha/

day)

Figure 3 shows the annual pattern of deadmatter content in the pasture for the threefarmlets. Even in highly stocked, well managedpastures, the dead matter content in Februarycan reach nearly 40%. The effect of this ongreen DM allowance per cow is shown in Figure4. Green pasture allowance peaked at 32-46

24

kg DM/cow/day in November. At cow intakes of17 kg DM/day this implies that 37-53% of greenDM must be eaten at each grazing. FromNovember, green DM allowance declinesthrough summer and autumn.

Figure 3: Predicted seasonal changes indead matter content of pasture onFarmlets 1, 3, and 5 of No 2 Dairyexperiment for 1995-96.

05

10152025303540

Jun

Aug Oct

Dec

Feb

Apr

Farmlet 1 Farmlet 3 Farmlet 5

%

Figure 4: Predicted seasonal changes ingreen pasture allowance onFarmlets 1, 3, and 5 of No 2 Dairyexperiment for 1995-96.

1.0

11.0

21.0

31.0

41.0

51.0

Aug

Sep Oct

Nov

Dec Jan

Feb

Mar

Apr

Farmlet 1 Farmlet 3 Farmlet 5

(kg

DM

/cow

/day

)

8.0

9.0

10.0

11.0

12.0

13.0

Jun

Aug Oct

Dec

Feb

Apr

Farmlet 1 Farmlet 3 Farmlet 5

(MJ

ME

/kg

DM

This effect on cow nutrition of the fall in greenDM allowance offered is further compounded bythe decline in the ME content of the pasture(Figure 5). This decline is predominantly due toincreased dead matter content, rather than adecline in the quality of green pasture.

Figure 5: Predicted seasonal changes inmetabolisable energy content ofpasture on Farmlets 1, 3, and 5 ofNo 2 Dairy experiment for 1995-96.

Implications

The data from the No 2 Dairy experimentreiterate the importance of the balance betweenstocking rate and pasture grown for high andprofitable milksolids output. The recentemphasis on improved cow nutrition (Edwardsand Parker, 1994) has often failed to considerwhole farm implications. A decrease in stockingrate will allow increased pasture allowance percow. However, the dynamic nature of pasturegrowth and decay processes mean that eithersurplus pasture must be conserved, withassociated losses of both DM and ME content;or, pasture dead matter content increases withinitial decline of pasture ME content, followedby DM loss as pasture decays.

Short-term feed deficits should be met bymanagement decisions on culling, or by the useof N fertiliser or cost-effective supplements,rather than by decreased stocking rate. Ifstocking rate is increased, the likelihood ofsurpluses occurring is decreased, but theyshould be conserved when they occur becauseat present payout levels silage supplementation,especially at the end of lactation, will be profitable(Clark 1994).

Smaller surpluses can be removed bytopping. There is some evidence that toppinghas beneficial effects on MS yield (Bryant 1982).Topping should be done immediately beforegrazing. This will allow the topped material tobe readily eaten, and will remove developing orexisting reproductive stems, and hence reducecurrent pasture growth rates. If topping is doneafter grazing, topped material may simplyincrease the dead matter content rather soonerthan would have occurred naturally.

ConclusionPasture remains the cheapest form of total feedfor the New Zealand dairy cow. Profitabledairying requires that base stocking rate has thepotential to use 85% of annual pasture growth.Nitrogen fertiliser and supplements should be

25

used to support a high base stocking rate.Attempts to increase per cow milksolids yield tohigh levels by reducing stocking rate and usinghigh quality rations balanced for nutrients willonly succeed at the expense of wasted pasture.Research should urgently address ways toincrease per cow intake from a pasture-only diet,or from pasture plus supplements. However, nosupplements are yet known that have asubstitution rate close to zero when used withad libitum high quality pasture.

Korte CJ, Sheath GW, 1978. Herbage dry matterproduction: the balance between growthand death. Proceedings of the NewZealand Grasslands Association 40: 152-161.

Livestock Improvement, 1995. Dairy Statistics1994-1995.

McMeekan CP, 1961. Pros and cons of highstocking rate. Proceedings of the RuakuraFarmers’ Conference 184.

Phillips CJC, 1988. The use of conserved forageas a supplement for grazing dairy cows.Grass and Forage Science 43: 215-230.

References

Bryant AM, 1982. Effects of mowing before orafter grazing on milk production. In DairyProduction From Pasture. Eds. MacmillanKL, Taufa VK. New Zealand Society ofAnimal Production. Pp 381-382.

Clark DA, 1994. Silage for milk production.Proceedings of the Ruakura Farmers’Conference 45: 41-46.

Edwards NJ, Parker WJ, 1994. Increasing percow milksolids in a pasture-based dairysystem by manipulating the diet: a review.Proceedings of the New Zealand Societyof Animal Production 54: 267-273.

Holmes CW, Macmillan KL, 1982. Nutritionalmanagement of the dairy herd grazing onpasture. In Dairy production from pasture.Eds. Macmillan KL, Taufa VK. NewZealand Society of Animal Production. Pp244-274.

Hoogendoorn CJ, Holmes, CW Chu ACP, 1992.Some effects of herbage composition, asinfluenced by previous grazingmanagement, on milk production by cowsgrazing on ryegrass/white clover pastures.2. Milk production in late spring/summer:effects of grazing intensity during thepreceding spring period. Grass andForage Science 47: 316-325.

Hunt WF, 1971. Leaf death and decompositionduring pasture regrowth. New ZealandJournal of Agricultural Research 14: 208-218.