1. The object of this study is the optimization of the grain harvesting operations by minimizing the total harvest costs under weather conditions prevailing in the centre of the Netherlands. The sequential grain harvesting operations consist of: combining-loading of grain wagons-transport-unloading-ventilated storage-drying-dry storage.
The capacities required of the sequential operations are mainly determined by the grain moisture content and therefore by the weather. This especially concerns the capacities of combining, drying and ventilated storage. As the weather is the key factor and varies from year to year, the criterion becomes the minimization of the average annual total harvest costs for a given cropping program over a large number of years. The total harvest costs comprise the field losses of the mature crop and the costs of personnel and harvesting equipment.
Also the harvesting organization has been studied; a system was developed to provide minimum costs of the separate components: combining, transport, drying and storage with a view to the minimum costs of the system as a whole. Selecting a system with minimum costs can only be carried out with a view to the organization of a particular farm and taking into account the conditions prevailing on that farm. Therefore, the results found have been used to minimize the total harvest costs for a 20,000 ha grain farm under the actual weather occurring during the harvest periods of the years 1931-1967.
2. The farm is a part of the reclamation enterprise of the Yssel Lake polders Development Authority which is responsible for the integral development of the polders in the Yssel Lake.
Owing to the high moisture content of the mud, the land is not ready for the planned land use immediately after reclamation. Hence a period of at least five years of farming by the Authority is required to transform the mud into a good soil. The crops grown on this temporary farm are: colza, barley, oats, wheat, alfalfa and flax. The first four of these crops (14,000 ha) are managed with farm personnel and equipment; alfalfa and flax are managed by others.
The harvesting of the grain is done with combines, transport equipment and transshipment plants. The transshipment plants dry the grain to a moisture content which is sufficiently low to permit safe shipment to storage installations elsewhere in the country. Based on the kernel moisture content the following classification for wheat can be made:a
. moisture content>28%: crop too wet for combiningb
. moisture content 17-28 %: drying necessary after threshing and before storage; the following intermediate situations can occur:b1
, moisture content 19-28%: temporary storage possible with ventilation, drying necessary before shipmentb2
moisture content 17-19%: shipping possible but drying necessary before storagec
. moisture content < 17%: grain can be shipped and stored without drying.
In the case of colza, barley and oats similar situations occur but at different moisture contents.
3. The order of maturing of the crops grown is colza, barley and wheat; the oats mature practically simultaneously with the wheat. The maturity date (combine ripeness) of the crops is chiefly determined by the weather during the growing season. Using trial field data, both the average and standard deviation of the maturity dates have been determined. The average maturity dates of colza, barley, oats and wheat are July 22, August 7, August 17 and August 17 respectively. Consequently the average available periods for colza and barley are 17 and 10 days respectively. The available periods for oats and wheat are determined by the date before which the work to be done next must be finished and the available capacity. The work consisting mainly of tillage operations and the sowing of wheat has to be finished before December 7. Based on the available capacity for these fall operations efforts are made to finish harvesting by the middle of September or at least October 1.
Other factors affecting the available period and therefore the harvesting capacity required are the field losses and the deterioration of grain quality in the mature crop. Sources of field losses are: shatter losses, dry matter losses and combine losses (caused by cutterbar and reel). Investigations in the Lake Yssel polders show that the daily field losses increase exponentially; the following average daily field losses per ha occur during 36 days after combine ripeness: barley 16 kg, oats 25 kg, wheat 15 kg. No data are known for colza.
Little is known about the losses caused by reduction in grain quality; it is shown that on this farm they can probably be neglected in comparison with the field losses.
4. During three seasons (1964-1967), the kernel moisture contents of colza, barley, oats and wheat, as affected by the weather have been investigated. Grain samples were taken hourly from the graintanks of a group of combines. The following meteorological factors have been measured: rainfall, relative humidity, temperature, wind velocity and circumglobal radiation.
Empirical exponential relationships have been established between: drying of the kernel and the circumglobal radiation rise in kernel moisture content and the square root of the product of amount and duration of the precipitation rise in kernel moisture content (by dew) and the length of the night
In 1967 the kernel moisture contents have been calculated with the aid of meteorological data and the relationships found. Comparison with the measured kernel moisture contents showed that they corresponded fairly well. With meteorological data from De Bell and Wageningen in the years 1931-1967 the kernel moisture contents have been computed for the harvest periods of those years. Then the available combine hours were computed. The average total number of available combine hours from July 22 to September 16 is 309 (σ± 55). During roughly half of this time colza and grain in the lowest moisture range (for wheat < 19%) can be combined; artificial drying is then not strictly necessary. In the other half of the time the moisture content is such that drying is necessary.
5. The available time for combining wheat is defined as that part of the time between 9 a.m. and 7 p.m. when the kernel moisture content is below 28% less time when rain is falling. It is assumed that a similar definition holds for barley, oats and colza, with the exception that for colza the maximum moisture content is 18%.
The effective time is the time during which the combine is cutting and threshing; combine capacity during this time, the effective capacity is expressed in kg h -1
. Time measurements showed that, due to different time losses, the effective time is approximately 60% of the net working time and 50% of the available time between Monday 9 a.m. and Saturday 4. p.m.
The effective combine capacity is affected by the straw moisture content, the permitted level of separating losses and the grain straw ratio. For wheat these relationships have been established, it is assumed that these relationships also hold for colza, barley and oats. The effective capacities of combines with a 5.4-m cutter bar have been measured in the crops under average conditions.
6. In view of the prevailing limitations and conditions the choice of the transport equipment is restricted to grain wagons pulled by crawler and wheeled tractors in the field and on the road respectively. Within the limits set by soil trafficability, available tractors and regulations governing agricultural vehicles on the road a grain wagon discharging at the bottom with a volume of 8 m 3
has been developed. These wagons are used in pairs. To minimize the loading time they are loaded on one of the headlands by six combines working in one group.
The conveying capacity of the plants is of considerable importance for the organization of the transport. The investigations show that an efficient transport organization is only possible when the conveying capacity is at least equal to the total net combine capacity.
For the calculation of the required numbers of wheeled tractors and wagons an adapted formula of TISCHLER is used.
7. The costs of the harvest components are based on the farm records and where there are inadequate data on the most accurate estimates possible. All costs are based on 1967 price levels, overhead is not included. Two possibilities for drying and storage of the grain are considered: transshipment plants and storage plants.
A complication is due to the regular movement of the farm through the polders. This means that the average distance to the transshipment plants gradually increases, so that after a given time these have to be moved or the transport capacity would have to be increased. Under certain limitations the total of the costs of grain transport and of the removal of the centres is at a minimum for three transshipment centres which are removed once every 15 years. The average theoretical transport distance is then 11 km.
8. Simulation has been used for the minimization of the total harvest costs. The independent variables taken into account are: the number of combines, the method of operation of the combines (cutting with high or low stubble in wheat), the drying capacity and the ventilated storage capacity.
Other variables as the cropping plan and the maturity dates are introduced as constants. A time limit has been placed on harvesting of each crop, after this date the crop will not be harvested and is considered a partial or total loss. The introduction of the time limit makes it possible to take into account some not yet quantified factors which will tend to increase the costs. The most important time limit, that for wheat, is fixed at October 1. To show the influence of some factors which may change in the future, three levels of field losses and labour costs are introduced: low, medium and high. The following conclusions are drawn from the computations (field losses and labour costs medium).
The ventilated storage capacity required is approximately 8,000 m 3
, it is fairly irrespective of the combining and drying capacities. Adding the 4,000 m' of dry storage capacity fixes the total storage capacity of the transshipment plants at 12,000 m 3
At present price levels it is advantageous to cut the wheat at a low stubble. Only if the extra costs involved with high cutting are less than f 36 per ha the high cutting will be preferable.
In the minimum cost range combining with permitted separating losses at 0.5%. per ha is preferable to combining with losses at 2%, per ha. The decrease in the costs of combining at 2% loss is not sufficient to balance the rise in value of the separating losses.
The minimum total harvest costs are obtained when for each 5.4-m combine are available a drying capacity of one ton h -1
, a ventilated storage capacity of 100 m 3
and a dry storage capacity of 50 m 3
. One combine can then handle approximately 175 ha per harvest season. The harvest will then on the average be completed on September 10.
From the point of view of the utilization of the harvesting capacity the ratio of the acreages sown with colza, barley and wheat plus oats accordingly must be 1 : 0.6 : 1.6.
To show the effect of the period studied the average total harvest costs are computed for two periods, viz. 1931-1949 and 1949-1967. In the first period the minimum of the costs is obtained with one 5.4-m combine on 215 ha, in the second period the minimum is obtained with one 5.4-m combine on 167 ha due to the less favourable weather during harvesting.
Comparison of the actual progress of the harvesting operations with the simulated progress shows that the operational model may be applied for simulation of the harvesting operations under actual weather conditions.