QuestionQuestion

Farm Management Case Study
(from Hillier and Lieberman's Introduction to Operations Research)
The Ploughman family farm owns and operates a 640-acre farm that has been in the family for several generations. The Ploughman's always have had to work hard to make a decent living from the farm and have had to endure some occasional difficult years. Stories about earlier generations overcoming hardships due to droughts, floods, etc., are an important part of the family history. However, the Ploughman's enjoy their self-reliant lifestyle and gain considerable satisfaction from continuing the family tradition of successfully living off the land during an era when many family farms are being abandoned or taken over by large agricultural corporations.
John Ploughman is the current manager of the farm while his wife Eunice runs the house and manages the farm's finances. John's father, Grandpa Ploughman, lives with them and still puts in many hours working on the farm. John and Eunice's oldest children, Frank, Phyllis, and Carl, also are given heavy chores before and after school.
The entire family can produce a total of 4,000 person-hours worth of labor during the winter and spring months and 4,500 person-hours during the summer and fall. If any of these person-hours are not needed, Frank, Phyllis, and Carl will use them to work on a neighboring farm for $5 per hour during the winter and spring months and $5.50 per hour during the summer and fall.
The farm supports two types of livestock: dairy cows and laying hens, as well as three crops: soybeans, corn, and wheat. (All three are cash crops, but the corn also is a feed crop for the cows and the wheat also is used for chicken feed.) The crops are harvested during the late summer and fall. During the winter months, John, Eunice, and Grandpa make a decision about the mix of livestock and crops for the coming year. Currently, the family has just completed a particularly successful harvest which has provided an investment fund of $20,000 that can be used to purchase more livestock. (Other money is available for ongoing expenses, including the next planting of crops.)
The family currently has 30 cows valued at $35,000 and 2,000 hens valued at $5,000.
They wish to keep all this livestock and perhaps purchase more. Each new cow would cost $1,500, and each new hen would cost $3.
Over a year's time, the value of a herd of cows will decrease by about 10 percent and the value of a flock of hens will decrease by about 25 percent due to aging.
Each cow will require 2 acres of land for grazing and 10 person-hours of work per month, while producing a net annual cash income of $850 for the family. The corresponding figures for each hen are: no significant acreage, 0.05 person hours per month, and an annual net cash income of $4.25. The chicken house can accommodate a maximum of 5,000 hens, and the size of the barn limits the herd to a maximum of 42 cows.
For each acre planted in each of the three crops, the following table gives the number of person-hours of work that will be required during the first and second halves of the year, as well as a rough estimate of the crop's net value (in either income or savings in purchasing feed for the livestock).
Data per Acre Planted
                                                          Soybeans    Corn      Wheat
Winter and spring, person-hours          1.0             0.9          0.6
Summer and fall, person-hours             1.4             1.2          0.7
Net Value                                              $70            $60          $40
To provide much of the feed for the livestock, John wants to plant at least 1 acre of corn for each cow in the coming year's herd and at least 0.05 acre of wheat for each hen in the coming year's flock.
John, Eunice, and Grandpa now are discussing how much acreage should be planted in each of the crops and how many cows and hens to have for the coming year.
Their objective is to maximize the family's monetary worth at the end of the coming year (the sum of the net income from the livestock for the coming year plus the net value of the crops for the coming year plus what remains from the investment fund plus the value of the livestock at the end of the coming year plus any income from working on a neighboring farm, minus living expenses of $40,000 for the year).
(a) Identify the components of a linear programming model for this problem.
(b) Formulate this model.
(c) Obtain an optimal solution and generate the additional output provided for post- optimality analysis (e.g., the Sensitivity Report). What does the model predict regarding the family's monetary worth at the end of the coming year?
(d) Find the allowable range to stay optimal for the net value per acre planted for each of the three crops.
The above estimates of the net value per acre planted in each of the three crops assumes good weather conditions. Adverse weather conditions would harm the crops and greatly reduce the resulting value. The scenarios particularly feared by the family are a drought, a flood, an early frost, both a drought and an early frost, and both a flood and an early frost. The estimated net values for the year under these scenarios are as follows:
                                           Net Value per      Net Value per      Net Value per
                                             Acre Planted       Acre Planted       Acre Planted
Scenario                               Soybeans                Corn                   Wheat
Drought                                        -$10                     -$15                      $0
Flood                                             $15                      $20                      $10
Early frost                                     $50                      $40                      $30

Drought and early frost                -$15                      -$20                     -$10
Flood and early frost                     $10                        $10                      $5
(e) Find an optimal solution under each scenario after making the necessary adjustments to the linear programming model formulated in part (b). In each case, what is the prediction regarding the family's monetary worth at the end of the year?
(f) For the optimal solution obtained under each of the six scenarios [including the good weather scenario considered in parts (a) and (d)], calculate what the family's monetary worth would be at the end of the year if each of the other five scenarios occur instead. In your judgment, which solution provides the best balance between yielding a large monetary worth under good weather conditions and avoiding an overly small monetary worth under adverse weather conditions.
Grandpa has researched what the weather conditions were in past years as far back as weather records have been kept, and obtained the following data.
Scenario                            Frequency
Good weather                           40%
Drought                                     20%
Flood                                        10%
Early frost                                  15%
Drought and early frost             10%
Flood and early frost                   5%
With these data, the family has decided to use the following approach to making its planting and livestock decisions. Rather than the optimistic approach of assuming that good weather will prevail [as done in parts (a) to (d)], the average net value under all weather conditions will be used for each crop (weighing the net values under the various scenarios by the frequencies in the above table).
(g) Modify the linear programming model formulated in part (b) to fit this new approach.
(h) Repeat part (c) for this modified model.
(i) Use a shadow price obtained in part (h) to analyze whether it would be worthwhile for the family to obtain a bank loan with a 10 percent interest rate to purchase more livestock now beyond what can be obtained with the $20,000 investment fund.
(j) For each of the three crops, use the post-optimality analysis information obtained
in part (h) to identify how much latitude for error is available in estimating the net value per acre planted for that crop without changing the optimal solution. Which two net values need to be estimated most carefully? If both estimates are incorrect simultaneously, how close do the estimates need to be to guarantee that the optimal solution will not change?

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Let p_1,p_2 "and" p_3 be the acres of land that will be planted for soybean, corn and wheat, respectively. Let q_1 "and" q_2 be the number of additional cows and hens, respectively, that the farm should buy. Note that the q_i's must be both integers while the p_i's need not be integers.

At the end the of the year, the farm has 30+q_1 cows and 2000+q_2 hens which generated a net cash income of

850(30+q_1 )+4.25(2000+q_2 )=34000+850q_1+4.25q_2

The crops, on the other hand, the crop’s net value (either in net income or savings in purchasing feed for the livestock) will be

70p_1+60p_2+40p_3.

At the end of the year, after incorporating the aging of the livestock, the value of the livestock will be

0.9(35000+1500q_1 )+0.75(5000+3q_2 )=35250+1350q_1+2.25q_2

Next, we calculate the income of the entire family from working outside their farm if they will have unused person-hours. During the winter and spring time, the total hours that they will work in the their farm will be

p_1+0.9p_2+0.6p_3+10(6)(30+q_1 )+0.05(6)(2000+q_2 )=p_1+0.9p_2+0.6p_3+60q_1+0.3q_2+2400

While during summer and fall, the total hours that they will work in their farm will be

1.4p_1+1.2p_2+0.7p_3+10(6)(30+q_1 )+0.05(6)(2000+q_2 )=1.4p_1+1.2p_2+0.7p_3+60q_1+0.3q_2+2400

Thus provided that these total hours do not exceed the available total hours for the season, then their total income from working from outside their farm will be

5(4000-p_1-0.9p_2-0.6p_3-60q_1-0.3q_2-2400)+5.5(4500-1.4p_1-1.2p_2-0.7p_3-60q_1-0.3q_2-2400)=19550-12.7p_1-11.1p_2-6.85p_3-630q_1-3.15q_2

Incorporating now the cost of the new livestock and the living expenses per year, then the net worth P of the family at the end of the year will be

P=20000-1500q_1-3q_2-40000+34000+850q_1+4.25q_2+70p_1+60p_2+40p_3+35250+1350q_1+2.25q_2+19550-12.7p_1-11.1p_2-6.85p_3-630q_1-3.15q_2
=68800+70q_1+0.35q_2+57.3p_1+48.9p_2+33.15p_3...

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