An operational research study of the alternatives to the combine harvester


Cereals & Oilseeds
Project code:
01 July 1987 - 30 June 1989
AHDB Cereals & Oilseeds.
AHDB sector cost:
£57,504 From HGCA (Project No. 0027/2/87)
Project leader:
E Audsley NIAE



About this project


This report studies the profitability of different systems of harvesting cereal grain and straw. They aim to beat the universally used combine harvester system by exploiting one or more of the disadvantages of the combine. A combine is very expensive and can only work if the grain and straw are sufficiently dry. Grain yield is lost by respiration while waiting for the crop to be 'combine ripe', by shedding losses, by cutter-bar header losses and by threshing losses. The feed value of straw in the chaff and leaf fragments is mostly lost. Methods of exploiting these disadvantages to produce a better method of cereal harvesting are:

a) Grain Stripper Header which reduces the amount of straw passing through the combine and allows harvesting to be carried out in damper conditions.

b) Harvest the grain and straw when dry with a forage harvester and separate them at the farmstead. This reduces grain and straw losses. In addition the harvester is much cheaper but expensive separation equipment is needed at the farmstead.

c) Harvest the grain and straw and preserve with urea; Feed to cattle. This also removes the need for a dry crop at harvest and replaces expensive separation equipment with preservation.

d) As proposal C but use for industrial purposes. Separation is now needed but its use can be spread over the year. A use for the straw must be found.

The feed value of straw depends on the amount of the different fractions harvested. In barley, leaf has the best metabolisable energy content followed by chaff and nodes; in wheat, leaf is best followed by nodes, and chaff is similar to stem. In calculating feed value, the effect of degradability in the rumen is taken into account. Wheatchaff is worth only £5/ha and amounts to 0.7t/ha. However with the lost fraction of leaf, the unbaled fraction of wheat straw is worth £34/ha, amounts to 1.7t/ha and increases straw yield by 33%. The unbaled fraction in barley is worth £62/ha, amounts to 0.7t/ha and would increase straw yield by 20%. The benefit of harvesting lost parts of wheat straw is due to more of the same, not a better product. The benefit in barley straw includes this effect but the leaf fraction represents a good feed on a par with hay.

Using partial budgets to compare harvesting systems ignores the complex knock-on effects of the end of harvest being earlier or later. A whole farm model is used to calculate these effects. The results can be used in comparing low and high temperature drying, assessing the cost of unreliability due to the age of combine, assessing the optimum combine speed or assessing the value of new developments such as in - combine controls and the grain stripper header. The results are shown to be not affected by the soil type.

The knock-on benefit or cost of changes to the harvest system in terms which can be used in partial budgeting exercises is

+/earlier -/later Possible Cause

Start of harvest 1 week £11/ha £10/ha Grain drying

2 weeks £18/ha £19/ha

Workrate 10% £52/h £68/h Reliability

Workrate in 10% £64/h - Grain stripper

cereals 20% £57/h -

A computer simulation of the harvesting process was adapted to provide a comprehensive analysis of harvesting with both the stripper and cutter-bar headers. This calculates the total cost of harvesting including machine capital and operating costs and shedding, header and threshing losses. The crops considered were principally barley, wheat, and oats. Six hypothetical farms were considered featuring the extremes of crop area and yields in order to highlight the situations which are best for the stripper or otherwise. Three of the farms have equal areas of wheat and barley, which is a normal distribution of crops. The three other farms have all standing wheat, which is the situation in which the stripper should have least advantage over the cutter-bar, though unusual in practice.

On farms with an equal area of wheat and barley, the distribution most usually found in the UK, the stripper header was better than the cutter-bar header. On large moderate yielding farms where the system includes barley, there are clear unquestionable advantages in the stripper system. Although the results are less clear on high yielding wheat farms, the faster workrate can result in a more favourable cost profile if the farmer takes full advantage of finishing earlier by burning or incorporating the straw, providing this takes no extra labour.

Threshing and header losses are an important component of the cost comparison between stripping and cutter-bar headers. Conventionally one would expect a direct relationship between separating loss and straw throughput. The relatively little experimental evidence with the stripper suggests that is not strictly true, especially in wheat. Although the capacity of a combine and stripper header is higher in wheat crops, this is not proportional to the reduction in straw throughput. This information also has consequences for conventional advice on the height of cut with a cutter-bar. Further experiments are needed to clarify this.

Forward speed has an effect on header losses with the stripper header. Speeds in excess of 7 km/h should be used to keep these at a low level. Header losses in standing versus laid crops also need further study with both types of header.

Three alternative whole crop harvesting proposals are analysed using a whole farm model on a 250 ha combinable crops farm and a 200 ha grass-cereal farm (half cereal). Land type was shown not to affect the comparisons. The proposal for whole crop harvesting with drying and separation at the farmstead, leading to conventional dry grain for sale, produces a saving which allows a capital expenditure on the farmstead equipment of over £70,000. Estimates of the likely equipment needed suggest negligible difference in profit in the end on the cereal farm but a profit increase of £50/cereal ha on the mixed cereal/livestock farm.

The proposal for whole crop cereal silage preserved with urea and fed to cattle, is insufficiently profitable to pay for the increased transport costs on the cereal farm. On the mixed cereal/livestock farm profit increases by £50/ha, even after allowing for the purchase of bedding. Additional livestock will be needed.

The proposal for whole crop cereal preserved with urea and used for industrial purposes, is insufficiently profitable to pay for separation costs on the cereal farm. On the livestock farm profit is increased by £45/ha.

The results clearly show that the value of whole crop harvesting comes from the saving of the combine harvester on a relatively small cereal area on which a forage harvester is already available plus the availability of adjacent livestock. Transport costs are the major disadvantage. A 25% error in estimates of value was sufficient to eliminate the savings. In particular reducing the value of whole crop cereal silage from £80.8/t to £77.2/t.

Lack of information compounds the problem of variability when making comparisons. Predictive systematic estimates of straw intake by cattle are almost negligible in spite of a considerable quantity of experimental measurements. The value of straw or whole crop silage depends on the feed value and intake of straw. A change in value of only £3/t can make or break the profitability. Grain losses when combining are only well studied for wheat, barley and oats. More information is needed on shedding, header and threshing losses in crops such as oilseed rape, beans and peas. In all cases the need is for data suitable for system analysis rather than just comparative experiments.