J. agric. Engng Res. (1990)46, 129-145

Operational Research Study of Forage Conservation Systems for Cool, Humid Upland Climates. Part 2: Comparison of Hay

and Silage Systems

M. B. McGECHAN*

The improved operational research model described in Part 1 of this paper is exploited to compare the economics of hay and silage systems for climatically different areas of Scotland. Both field-dried and barn-dried hay are explored, and the silage systems studied include unwilted-clamp silage, wilted-clamp silage and baled silage. Also, high cost, high capacity systems are compared with low cost, low capacity systems for each method. Each system is costed and compared in terms of net value of forage produced.

Results of simulation and feed evaluation runs for a range of grassland areas suggest that the economics of silage as a conservation method are nearly always more favourable than hay under Scottish climatic conditions. The economics of wilted silage are much better than unwilted silage. Silage systems should be based on a precision chop forage harvester for small farms, and on big bales rather than a flail forage harvester for small farms. Only on small farms in drier East of Scotland areas are the economics of haymaking comparable to silage, and here there is little to choose between field drying and barn drying. The importance of selecting a system with sufficient capacity for the quantity of crop to be conserved is also demonstrated.

Parts of the model which could be further developed are discussed. Opportunities for further exploitation of the model in its present form have also been identified.

1. Introduction

Part 1 of this series of papers (McGechan 1) describes a new operational research (OR) or "systems" model, developed to study the economics of alternative forage conservation practices, particularly for climatically unfavourable areas. In this paper, the model is exploited to compare the main alternative methods of conservation of grass forage currently used in Northern Britain; these are field-dried hay, barn-dried hay, direct-cut clamp silage, wilted-clamp silage, and big bale silage. Typical machine combinations have been selected for each method, and the costs of ownership and use of the machines are considered. Simulations are carried out with the model to test these options for a range of sizes of dairy farm, for sites in climatically different areas of Scotland.

2. Selection of forage systems and their parameters

2.1. Alternative enterprise sizes

The range of conservation areas explored, representing different sizes of dairy farms, are listed in Table 1, together with the number of dairy cows assumed for the forage evaluation procedure (as described in Part 1, McGechanl) .

* Scottish Centre of Agricultural Engineering, Bush Estate, Penicuik, Midlothian, EH26 0PH Scotland

Received 20 January 1989; accepted in revised form 25 November 1989

129

130 STUDY OF F O R A G E C O N S E R V A T I O N SYSTEMS

Table 1

Alternative dairy enterprise sizes

Conservat ion area, ha 10 20 40 60 80 No. of dairy cows considered

in forage evaluation procedure 25 50 100 150 200

2.2. Cutting policies

For the hay systems, a single cut has been assumed, commencing after the grass D-value drops to 65%. For the silage systems, a two-cut policy has been assumed, each cut commencing after the D-value drops to 68%.

2.3. Field machine combinations

Typical combinations of field machines were selected for each of the conservation methods, as listed in Table 2. For each, except big bale silage, both a low capacity system with low cost machinery requirements, and a higher cost, high capacity system have been considered. Typically, the low cost system includes a simple drum mower, while the high cost system uses a mower conditioner with a wider cut. The low cost clamp silage systems use a flail forage harvester, while the high cost systems use a precision chop harvester set to give a median chop length of about 30 mm. In the low cost direct cut silage system, the flail forage harvester cuts and picks up in a single operat ion, so no mower is required.

2.4. Costs of mechanization systems

The annual cost of machine ownership, repairs and housing have been calculated by the method described by Witney and Saadoun, 2 using their computer program. This treatment of machinery ownership is an advance on several earlier methods of costing, such as that described by Audsley and Wheeler, a and the treatment of repair costs incorporates the work of Rotz. 4

For the current study, a distinction is made between specialist machines, which are used only for forage conservation operations, and machines which are shared with other operations on the farm. For the specialist machines, the whole annual cost is considered to be borne by forage making. For the shared machines, the hourly cost to the forage operations has been calculated from the total annual cost, assuming a total annual usage of 1000 h for tractors and 300 h for other items.

The purchase prices of all the machines considered in this study are listed in Tables 3 and 4, based on information collected from various sources by McGechan. S Calculated annual and hourly costs are also listed, assuming an interest rate of 12%, investment rate of 8%, inflation rate of 4% and tax rate for allowances of 25%. From these, the machine ownership contributions to the annual and hourly costs of each operation have been calculated (Table 2).

Annual ownership costs for storage structures (Section 2.5.) have been determined by an adaptation of the machinery costing procedure. Unlike a machine where the capital allowance for tax relief is 25% of its written down value, the capital allowance for a building or structure must be calculated on the basis of straight line depreciation of 4% per year over a 25 year period, according to current UK tax legislation (Witney6). Annual costs of storage structures assuming no salvage value at the end of a 25 year life, and the above interest, investment inflation and tax rates, were calculated as 5-1% of the capital cost.

M. B. M c G E C H A N

Table 2

Alternative field mechanization systems for each conservation method

131

Hourly cost

(shared

Conservation method Total no. machines + and mechanization of men Tractor labour), system available Operation size. kW Implements £ Workrate

Low capacity field-dried hay "l 2 cutting 45 I-6 m drum mower 5-76 1.5 ha/h spreading, tedding 45 4 m universal haymaker 5-76 2-5 ha/h

Low capacity baru-dried hayJ and rowing up, baling, 45 pick-up baler plus bale sledge 7.72 5.0 t dm/h transport 45 trailer ~1 13.39 5-0 t dm/h

45 front end loader with squeeze type bale handler

High capacity field-dried hay ~1 4 cutting, 65 2.1 m power conditioner 7-17 1-75 ha/h spreading, tedding 45 4 m universal haymaker 5-76 2-5 ha/h

High capacity barn-dried hay ) and rowing up, haling 45 pick-up baler plus bale 8.19 5.0 t dm/h

accumulator transport (2 m e n ) 4 5 trailer } 13"511 5"0tdm/h

45 front end loader plus fiat ~1.14 10.0 t dm/h 8 bale fork

transport (when 3rd 45 2nd trailer man available)

Low capacity direct cut silage 3 cutting and 55 flail forage harvester and ") harvesting in one silage trailer (3 in line) t 22.01 7-0 km/h operation. (max)

transport, 45 2nd silage trailer filling silo 55 buckrake

Low capacity wilted silage 3 cutting, 45 1.6 m drum mower 5.76 1-5 ha/h windrowing (second 45 4 m windrower (haymaker) 5.76 2-5 ha/h

cut only), harvesting 55 flail forage harvester and ")

silage trailer (3 in line) I 22-01 7-0 km/h transport, 45 2nd silage trailer (max) filling silo 55 buckrake

High capacity direct-cut silage ~ 4 cutting, 65 2-1 m mower conditioner 7-17 1-75 ha/h windrowing (second 45 4 m windrower (haymaker) 5-76 2-5 ha/h

High capacity wilted silage cut only), harvesting 75 precision chop forage harvester 1 23.61 7.0 km/h

and silage trailer (3 in line) ~ (max) transport, 45 2nd silage trailer | filling silo 65 buckrake )

Big round bale silage 2 cutting 45 1-6 m drum mower 5.76 1.5 ha/h windrowing (second 45 4 m windrower (haymaker) 5.76 2-5 ha/h

cut only), baling, 65 big round baler 111.72 4,0 t dm/h transport (I man) 45 front end loader with spikes 7.69") 4,0 t dm/h

plus trailer ,~15-71 8,0t dm/h transport (when 45 trailer 2nd man available

Table 3

Tractor costs

45 kW, 2 WD 12 900 2261 2.26 55 kW, 4 WD 18 000 3148 3-15 65 kW, 4 WD 21 000 3670 3.67 75 kW, 4 WD 24 200 4226 4-23

H o u r l y cost ,

Capital cost, Annual cost, assuming lO00h Tractor size £ £ annual use, £

132 STUDY OF FORAGE CONSERVATION SYSTEMS

Table 4

Implement costs

Implement

Hourly cost (shared Capital Annual items), assuming cost, £ cost, £ 300h annual use, £

1.6 m drum mower 1500 181 2-1 m mower conditioner 5200 597 4 m two-row rotary haymaker/windrower 2500 297 Pick-up baler 5000 560 1.87 Bale sledge 250 28 0.09 Flat 8 bale accumulator 1500 168 0-56 Big round baler 9500 1064 3.55 Trailer for bales 2000 260 0-87 Front end loader 2500 280 0.93 Squeeze-type bale handler 200 22 0.07 Flat 8 bale fork 500 56 0.19 Spikes for big bales 350 39 0-13 Forage harvester, flail 4000 462 Forage harvester, precision chop 7200 805 Tipping trailer with silage sides 3500 390 1.31 Buckrake 900 100 0,33

2.5. Storage structures and their costs

The size of the hay storage barn, the hay storage drier, the silage clamp or concrete platform for storing baled silage varies according to the conservation area (Table 5). Since the size of a storage structure cannot vary with variations in forage yield between years, these sizes are based on average forage yields.

Capital costs of storage structures are based on information collected by McGechan. 7 Hay is stored in a "Dutch" type barn with a concrete floor, with a capital cost of

£40 m -2 of floor area. For barn drying a wire mesh porous floor and sides and an electric fan must be added, increasing the cost to £75 m -z of floor area; this figure includes only a half share of the cost of the fan on the assumption that it has other uses on the farm. The low cost clamp silage systems have an earth banked clamp with a concrete floor costing £11 t -1 of wet silage, while the high cost systems have a clamp with concrete sides and floor costing £27 t -1. Baled silage is stored on a concrete platform costing £15 t -1.

An effluent tank is required for all the clamp silage systems, but its size must be greater for the direct cut systems. Effluent production has been estimated from data reviewed by McGechan7 and the tank size assumes that it will be emptied 15 times during the storage period. It has been assumed to cost £1200 for the first 100001, increasing by £96 per 10001 for larger sizes.

2.6. Other costs

Costs of £3-50 h -1 for labour, £0.13 1-1 for tractor fuel and £0-05 kW -1 for electricity (for barn drying) have been assumed. The use of a formic acid based additive at its standard application rate (2.51 of 85% solution t -1 of fresh silage) has been assumed for both the direct-cut silage systems; this costs £0.531-1 . Clamp silos are covered with polythene costing £0.15 t -~ of wet silage. Bags for big bale silage cost £1.15 each and hold 0-65 t of wet silage.

M. B. McGECHAN 133

Table 5 Costing of forage storage structures

Conservation area, ha 10 20 40 60 80

Typical annual quantity of forage 40 80 160 240 320 produced, t dm

Field-dried floor area, m 2 70 120 200 280 350 hay capital cost, £ 2800 4800 8000 11 200 14 000 Barn-dried floor area, m 2 120 200 300 400 500 hay structure

capital cost, £ 6600 11 000 16 500 22 000 27 500 air flow, m 3 h - l 27.6 46 69 92 115 fan cost, £ 6390 9890 13 590 17 890 21 590 total capital cost

with half share of fan, £ 9800 15 800 23 300 30 800 38 300

Clamp silage annual quantity of silage, t wet 200 400 800 1200 1600

silo capital cost, earth walled, concrete floor, £ 2400 4500 8900 13 000 17 000

silo capital cost, concrete walls and floor, £ 6000 11 300 21 600 30 000 38 000

annual cost of polythene, £ 38 75 150 225 300

effluent production, I wilted 8000 16 000 32 000 42 000 64 000 direct cut 32 000 64 000 128 000 192 000 250 000

effluent tank capital cost, £ wilted 1200 1200 1200 1200 1200 direct cut 1200 1200 1200 1470 1880

Big bale annual quantity silage of silage, t wet 171 432 684 1026 1369

no. of bales 263 527 1052 1580 2100 bag cost (total), £ 300 600 1210 1810 2420 concrete base area, m 2 75 150 300 450 600 concrete base

capital cost, £ 1130 2250 4500 6800 9000

2.7. Workrates

Workrates for each operation, selected from literature sources as reviewed by McGechan s are listed in Table 2. The slightly lower implied forward speed for a mower conditioner compared with a drum mower reflects the trend observed in mower workrate data from several of the reviewed sources. For forage harvesting in heavy crops, lower workrates than those listed have been calculated from a formula presented in Part 1 [Eqns (19) and (20)]. This represents the limit imposed by the available tractor power in relation to chop length, crop yield, silage dry matter content, weight of machines (tractor/harvester/trailer) in a three in line combination (assumed to be 15-2 t), and angle of uphill slope (assumed to be 3 ° representing a typical upland Scottish farm). For the high cost hay systems and for big bale silage, two alternative workrates have been assumed for transporting bales, depending on the number of men available; the lower workrate represents transport with only one tractor/trailer, but this rate can be doubled by introducing a second tractor/trailer when manpower is released from other operations.

134 STUDY OF F ORAGE C O N S E R V A T I O N SYSTEMS

2.8. Tractor fuel consumption

Tractor fuel consumption was assumed to be 0-3441 (kWh) -~, representing a thermal efficiency of 29.1%. This is the same as that assumed by Corrall et al., 8 and is similar to the average value found in tractor tests summarized by RASE. 9 For mowing, tedding, windrowing, baling and transport operations, the average tractor power consumption was taken to be 40% of its rated power, assumed by Corrall et al. 8 This includes allowances for both the unproductive time in the field (such as headland turns), and the fact that excess tractor power is usually available for these operations. For forage harvesting, the actual power consumption was calculated from the workrate formula [Part 1, Eqns (19) and (20)], assuming a field efficiency of 66% for a three in line machine combination; the no load power during unproductive time in the field was calculated from the same equations, but with the yield set to zero.

2.9. Swath treatments and drying rates

Coefficients for the swath drying equation incorporating Penman's formula based on experimental data collected at SCAE (Glasbey and McGechan, 1° Lamond et al. ~1) are listed in Part 1, Table 1. For hay drying it has been assumed that the swath is spread immediately after cutting and tedded at intervals thereafter until it reaches a moisture content of 40% (w.b.); barn-dried hay is then rowed up (raked), baled and carted to the barn, while field-dried hay is left undisturbed until it reaches a moisture content of 20% (w.b.) before rowing up, baling and carting. Wilted silage has been assumed to be left undisturbed in its mower windrow between cutting and picking up. Wilted clamp silage is wilted to a 25% dry matter content, and bale silage to 30% dry matter content.

2.10. Weather data

Files of hourly rainfall and potential evaporation according to Penman's formula were prepared from hourly meteorological data from two Scottish sites. Dyce (Aberdeen airport) is in a typical East of Scotland area of mixed arable, beef and dairy farming, while Prestwick Airport is on the edge of the mainly dairy farming area of South-West Scotland. The necessary solar radiation data were available for Dyce but for Prestwick radiation was estimated from daily sunshine hours by a procedure described by McGechan and Glasbey. 12 The data cover the months of May-October for the ten years 1974-1983.

For the grass growth sub-model, daily weather data covering the whole year were required. For Prestwick Airport these were not available, so data from nearby Auchincruive were used.

3. Results of simulation and feed evaluation runs

3.1. Forage dry matter production and losses

Simulation runs show quantities of grass dry matter cut from 40 ha, and how they are reduced by losses in each category to give the quantity of forage dry matter produced, as illustrated in Fig. 1. For the silage systems, these quantities are the sums of two cuts. The hay systems exhibit high field losses and low storage losses, while for silage systems the situation is reversed. Quantities of forage dry matter produced from a range of cutting areas are illustrated in Fig. 2.

Conflicting results are observed when the different mechanization system sizes are

M. B. McGECHAN 135

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136 S T U D Y O F F O R A G E C O N S E R V A T I O N S Y S T E M S

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Fig. 2. Forage dry matter production for a range of farm size. Mean of lO years; [5, field-dried; +, barn-dried hay; ~ , direct-cut clamp silage; A, wilted clamp silage; x, big bale silage; - - , low cost

systems; high cost systems

compared for each conservation method. With the high cost systems similar or larger quantities of forage are produced as hay compared to silage, while for the low cost systems the situation is reversed. The reason for this can be understood by considering the losses. Larger total cut grass yields are achieved with a two cut silage system rather than a single cut hay system. However, dry matter losses in storage with silage are even higher than field losses during haymaking with the high cost mechanization systems, giving higher net forage production as hay in most cases. In contrast, with the low cost

M. B. McGECHAN 137

hay systems there is insufficient capacity to adequately cope with the required throughput except with small conservation areas; consequently a substantial quantity of material is totally spoiled and the silage systems become superior.

Forage dry matter production is higher for direct-cut silage than for wilted silage, and for barn-dried hay than for field-dried hay, due to the lower level of field losses in each case; these differences are most marked for the low cost systems.

Approximate D values when reaching the store are 59% for field-dried hay, 61% for barn-dried hay, 66% for direct-cut silage, 65% for wilted clamp silage and 64% for baled silage.

3.2. Net forage value The importance of the forage evaluation sub-model used in this study is that it

attributes a value to a batch of forage as a component of an animal production system. Hence, the net value of forage is a much more useful parameter to determine than forage dry matter production. Net value is the gross value determined by the forage evaluation sub-model (Part 1, Section 8 and Fig. 1, McGechanl), less the costs of producing the forage. In practice, the relative merits of the different forage conservation methods in terms of forage net value differ considerably from those in terms of forage dry matter production. In particular, direct cutting with a flail forage harvester in a single operation (the low cost direct cut silage system) gives one of the highest levels of forage dry matter production but the lowest forage net value. This arises mainly because of the very low value of forage intake with very wet silage.

For each system, gross forage values, together with the production costs in each category which are deducted to give the net forage value, are illustrated for a 40ha conservation cut in Fig. 3. Net forage values for a range of conservation areas are illustrated in Fig. 4, and their coefficients of variation (CV) arising due to variations in weather between years are illustrated in Fig. 5.

3.2.1. Comparison of hay systems Barn-dried hay is of higher quality with a higher gross value than field-dried hay.

However, when the additional costs of electricity and storage facilities are considered, the net values of hay produced by the two methods are similar. The only deviation from this pattern occurs for small conservation areas at Dyce, where field-dried hay has a higher net value than barn-dried hay.

Comparing the low cost and high cost systems for each method, the high cost systems give forage with a higher net value, except for the smallest conservation area where there is little to choose between the two systems.

3.2.2. Comparison of silage systems In every case, wilted silage gives forage with a higher net value than unwilted silage.

The high cost wilted silage system gives the highest net value with large conservation areas. With smaller conservation areas, baled silage has the highest net value. At the smallest area, the low cost wilted-clamp silage system gives a higher net value than the high cost wilted system, but both are inferior to baled silage.

3.2.3. Comparison of hay and silage systems in different areas At Dyce there is little to choose between the high cost field-dried hay system and big

bale silage for the smaller conservation areas, but for large areas wilted silage has the highest net value.

138 S T U D Y OF F O R A G E C O N S E R V A T I O N S Y S T E M S

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~ -'-gross forage value;

polythene cost;

storage structure cost;

additive or drying energy cost;

mechanisation cost, fixed;

mechanisation cost, hourly;

net forage value

M. B. M c G E C H A N 139

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Fig. 4. Forage net value for a range of farm size. Mean of i0 years, r-I, field-dried hay; +, barn-dried hay; ~ , direct-cut clamp silage; &, wilted clamp silage; x , big bale silage; , low

cost systems; , high cost systems

At Prestwick, silage is always superior to hay. High cost wilted silage has the highest net value for large conservation areas, while baled silage has the highest value for smaller areas.

3.2.4. Comparison of coefficients of variation The ideal forage system will have not only a high average net value, but also a low

variation in net value between years, as indicated by its CV. The CV of forage net value is greater for the hay systems than for the silage systems,

except for the smaller conservation areas. The CV for the hay systems arises almost entirely due to variations in weather during field drying, whereas that for the silage

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Fig. 5. Coefficient of variation of forage net value for a range of farm sizes. Mean of lO years. D, field-dried hay; +, barn-dried hay; ~ , direct-cut clamp silage; A, wilted clamp silage; ×, big bale

silage; - - , low cost systems; , high cost systems

systems also includes a contribution from yield variations arising due to water shortage at the second cut in some years; hence the variation in net forage value arising due to weather during drying or wilting is much greater for hay than for silage. Values of CV are markedly higher for the high cost wilted silage system than for the high cost direct cut system. For big bale silage they are only slightly higher than for direct cut silage, and lower than for the wilted system based on a precision chop forage harvester. This may arise because the baling workrate (4.0 ha h -1) is higher than that for the forage harvester (typically about 1.5 ha h-1), so baled silage can be more rapidly protected from the weather once it has wilted to the target dry matter content. In some cases, the CV for field-dried hay is greater than that for barn-dried hay, as would be expected, but in other

M. B. MeGECHAN 141

cases it is not. The CV for either low cost hay system is particularly high. Corresponding CV values are considerably higher at Prestwick than at Dyce.

Consideration of the CV of forage net value emphasizes the benefits of conservation as silage rather than as hay. However, it contributes little to the choice between alternative hay or alternative silage systems, apart from emphasizing the importance of selecting a system with adequate capacity for the area of grass to be conserved.

4. Interpretation of results in terms of climatic variables during field wilting or drying

4.1. Potential evaporation to reach target moisture contents

Values of the Penman potential evaporation (PE) calculated from the swath drying equation [Part I, Eqn (7)] to achieve the required target moisture content under rain free conditions for each conservation method are listed in Table 6. Lower values are shown for the high cost systems with a mower conditioner which gives faster drying rates than the drum mower in the low cost systems. Drying rates have been adjusted for a dry matter yield of 5.0 t ha -~, close to the average value found in a simulation for hay and first cut silage, using the adjustment curves given in Part 1, Fig. 4.

4.2. Daily potential evaporation

Histograms of daily total Penman PE subdivided into rainy and rain free days for Prestwick and Dyce are shown in Fig. 6. These are for the 10 week period commencing on 18 May, which covers all the haymaking and first cut silage making periods, and second cut silage making in some years. High values tend to occur on rain free days.

4.3. Days infield

Histograms of the number of days swaths remain in the field, from the simulation for each conservation method, are illustrated in Fig. 7. For simplicity, days have been counted as overnights.

Numbers of rain free wilting days shown in Fig. 7 for each conservation method are broadly as would be expected from the values of Penman PE shown in Table 6 and Fig. 6. Wilting periods are extended when swaths are rained on. A higher proportion of forage is conserved without being rained on with the silage systems compared to the hay systems.

4.4. Weather parameters at different sites

Mean values of some climatic and evaporative parameters at the two sites, again for the 10 week period commencing on 18 May, are listed in Table 7. Although rainfall is higher

Table 6

Potential evaporation (PE) required to reach target moisture content u d e r rain free conditions

Required Penman PE

Target m.c. w.b. , Drum Mower Conservation method Swath treatment % mower conditioner

Wilted clamp silage windrow, undisturbed 75 6-3 5.2 Baled silage windrow, undisturbed 70 9-7 - - Barn-dried hay spread, tedded 40 12.0 10-1 Field-dried hay spread, tedded 20 20.8 17.5

142 STUDY OF F O R A G E C O N S E R V A T I O N SYSTEMS

0 -

o

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Z

200-

180 160 ~4o: ~2o: 100- 80- 60-- 40-- 20--

200 18o I

$ 16o: O .

-~ 140 ~" 120: o ~oo~ .E ~, 8o: t ~

-~ 60 ~ "5 4o~ 6 z 2o:

o

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Dyce

~x~ kq& ×\x:

. \ \ \ . . . . " \ \ " / /A / / / >>>. >-//,

" / /~ / " / / A , , ,

1-2 2-3 3-4 4 - 5

Daily total Penman

Prestwick

0-1 1- 2 4-5 Daily total Penman

5-6 6-7 PE, mm

~'7

x x x "

, \ \ ,

\ \ \ \ \ \ \

, \ \ \

/ / / " / / t

2'-3

Fig. 6. Daily total potential evaporation (PE).

, \ \ \

\ \ \ "

z z z .

<( (

/ / / , / / / / / / , / . / / . " / / i 3-4

Mean of i0 rain

I I

5-6 6-7 >7 PE. m m

years. I~, rain free days; ~, days with

~oo~

6 0

4 o

2 0

~ o .

Field dried hay Dyce

Barn dried hay Dyce

~. lOOj Field dried hay I Barn dried hay J k~ Wilted Big bale silage

~ 80] Prestwick / Prestwick ; ~ i ¢ l a m p ; 1 I ~ silage I P r i s iw i ck

0 1 2 3 4 5 6 7 8 9>-100 1 2 3 4 5 6 7 8 9>-100 1 2 3 4 5 6 0 1 2 3 4 5 6 7 No.of nights in field No, of nights in field No. of nights in field No. of nights in field

Fig. 7. Length of field drying or wilting period. Mean of 10 years. [~, rain free forage; 9, rained on forage

M. B. McGECHAN 143

Table 7

Mean daily climatic and evaporative parameters at two sites, period 18 May-12 July

Site

Parameter Dyce Prestwick

Rainfall, mm 1.7 1.8 Penman PE, mm (overall) 3.2 3-4 Penman PE, mm (rain free days only) 3-8 4.2 Daytime temperature (0900-1800 h), °C

(overall) 14.6 15.3 (rain free days only) 15.5 16-5

Equilibrium m.c.d.b., fraction (overall) 0.208 0-202 (rain free days only) 0-165 0.149

Probability of rainfall in 24 h period, fraction 0.50 0.52

at Prestwick than at Dyce, temperature and PE are also higher, so it is not immediately apparent which site should be more favourable for field drying.

5. Comments about the value of the model at its current stage of development

The study described in this paper is an illustration of how some pertinent questions about the relative economics of different forage conservation systems can be addressed using an OR model.

The model can be regarded as being at an intermediate stage of development, since there are refinements which will be made when experimental and modelling work on some of the physical processes have been completed. The model could be used in a formal sensitivity study to identify the importance of further research in detailed areas, but this has not yet been done. Nevertheless, some of the strengths and weaknesses of the model have already become apparent, leading to varying levels of confidence about different conclusions drawn from this study.

5.1. Forage assessment procedure

The ideal forage conservation system produces the maximum quantity of forage from a given area, with the highest quality, and at the lowest cost. There is a wide range of possible methods of evaluating forage, each putting a different relative weight to quantity, quality and cost. The procedure adopted in this study waz chosen because it takes account of most of the important factors in a simple manner. However, it is clear that quantity of material in a batch of forage has a large influence on its value.

5.2. Crop growth sub-model

Early simulation runs using the model with weather data from four Scottish sites, showed a wider variation in 10 year mean grass yields between sites than observed in yield trials (McGechanla); this led to some uncertainty about the validity of the crop growth sub-model. Thereafter the sites at Prestwick and Dyce were chosen for comparisons of conservation in the West against the East of Scotland, since simulated grass yields were slightly higher at Prestwick than at Dyce, as found in practice. It is anticipated that the

144 S T U D Y O F F O R A G E C O N S E R V A T I O N S Y S T E M S

current empirical crop growth sub-model will eventually be replaced by a mechanistic growth model, which will more accurately represent varying grass growth in different years and at different sites.

5.3. Forage intake

The assumed value of forage dry matter intake has a large influence on the economics of a forage system assessed in this study. Values measured in forage intake experiments have been very variable, and the factors which influence intake are not all fully understood. Nevertheless, there is a very clear trend towards higher intakes with wilted silage than with unwilted, very wet material, with a high ammonia content. The conclusions about the superior economics of wilted silage systems can therefore be treated with confidence.

5.4. Barn-dried hay

The conclusion about the economics of barn-dried hay being very similar to that of field-dried hay is based on only one particular barn drying system as currently represented in the forage conservation model. There is a wide range of other possible barn-dried hay systems, some of which could be more favourable; their economics could be further explored using the model with adaptations of the barn drying sub-model.

6. Comparison with farm practice

For a whole system model as used in this study, it is impossible to carry out a model validation exercise by comparing results with experimental data. However, it is possible to compare the findings of the study with conservation practices as they have evolved over the years in the practical Scottish farming situation. In fact, forage does now tend to be conserved mainly as silage, using a precision chop harvester on large farms and a big bale system on small farms. Haymaking has declined, surviving on some very small farms and on moderate sized farms in some of the drier Eastern areas. This situation is broadly as would be expected from the model study. Where farming practice may differ is in terms of the quantity of very wet silage produced with little or no wilting, for which the model indicates that the economics are poor. This suggests a need for further model investigations, into some of the details of alternative silage making practices designed to achieve a successful wilt or to mitigate penalties which arise when wilting cannot be achieved.

7. Conclusions

This study suggests that in Scotland grass forage should be conserved as silage rather than hay in most situations. There are substantial benefits from choosing a wilted silage system rather than direct-cut silage. For large conservation areas, this should be based on a precision chop forage harvester; for small areas a big bale system should be chosen rather than a system based on a flail forage harvester. Only for small farms in drier East of Scotland areas can haymaking be considered as a possible alternative to big bale silage. Even here, there is a greater risk of poor forage in bad years with hay compared to silage. If haymaking is chosen, there is little to choose between a field dried system and a barn dried system, but for either it is particularly important that it is of sufficiently high capacity for the area of grass being conserved.

The study has demonstrated the value of the model for exploring the economics of

M. B, McGECHAN 145

forage conservation systems, potential for its further exploitation in its present form, and areas in which it could be further developed.

References

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2 Wilney, B. D.; Saadonn, T. Present annual costs of farm machinery ownership. The Agricultural Engineer 1989, 44:3-11

a Audsley, E.; Wheeler, S. The annual cost of machinery calculated using actual cash flows. Journal of Agricultural Engineering Research 1978, 23:189-201

4 Rolz, C. A. A standard model for repair costs of agricultural machinery, Applied Engineering in Agriculture 1987, 3 :3 -9

5 McGechan, M. B. Mechanisation aspects of forage conservation models--A review of alternative systems and their parameters. Departmental Note SIN/462, Scottish Institute of Agricultural Engineering, Penicuik, January 1986, (unpublished)

• Witney, B. D. Choosing and using farm machines. Longman, 1988 7 McGechan, M. B. A review of losses arising during conservation of grass forage. Part 2, storage

losses. Journal of Agricultural Engineering Research 1990, 45:1-30 s Con'all, A. J.; Neal, H. D., St. C.; Wilkinson, J. M. Silage in milk production; a simulation

model to study the economic impact of management decisions in the production and use of silage in a dairy enterprise. Technical Report 29, Grassland Research Institute, Hurley, January 1983 (unpublished)

9 RASE. Agricultural Tractors 1985. Royal Agricultural Society of England, Stoneleigh, 1985 lo Glasbey, C. A.; McGechan, M. B. A relationship between weather and drying rates in grass

swaths. Departmental Note SIN/480, Scottish Institute of Agricultural Engineering, Penicuik, 1986 (unpublished)

" Lamond, W. J.; Spencer, H. B.; Glasbey, C. A.; Haughey, D. P. Field wilting and drying of grass in cool moist climate. Research and Development in Agriculture 1988, 5(1): 23-28

12 McGeehan, M. B.; Glasbey, C. A. Estimates of solar radiation based on other meteorological parameters for use in simulations of swath drying. Departmental Note 5, Scottish Centre of Agricultural Engineering, Penicuik, 1988

la McGechan, M. B. Further development of a crop growth sub-model for a forage conservation system model. Departmental Note, Scottish Centre of Agricultural Engineering, Penicuik, 1988