Agronomy 375 Exam Archive
Exam 3 Key Spring 1997

1. (4 pts.) Young larvae (will be actively feeding for some time to come) on older plants (growing point approaches the soil surface before V6 and is above the soil surface at and beyond V6, so older plants are at greater risk of serious economic injury).

2. (4 pts.)
   1. Plant (extended leaf) height. Note that corn plants are protected by a naturally produced feeding deterrent prior to an extended leaf height of at least 16 inches so no treatment would be indicated for plants younger than this stage.
   2. Percent of the plants which show infestation (evidence of feeding, or egg masses or live larvae present).
   3. Crop growth stage (e.g. late whorl).
   4. Number of live larvae per plant

3. (14 pts.)
   1. V6
   2. V12
   3. V17
   4. Mid-ear kernels with a milky white fluid content.
      Mid-ear seed coats are yellow.
      Silks are have just become red to brown.
   5. 5 weeks (35 days) between silking and R5
   6. 40-45%
   7. Black layer is formed at the tip of kernels taken from the center of the ears.

4. (8 pts.) Excessive rainfall in the springtime often restricts planting at a critical time. However, this spring maximum provides a useful recharge of the soil's water holding capacity. Mid-season days (e.g. July, August) are generally quite dry and may result in drought stress during the critical period of pollination. Fall precipitation may delay harvest.

   Mid-season clear skies may result in stress during pollination but they also contribute the abundant, uninterrupted solar radiation necessary to produce the high corn yields typical of the U.S. Midwest.

   Low temperatures early and late may restrict season length and therefore yield potential. Extreme high temperatures mid-season may result in stress during the critical pollination period. However, corn is a tropical grass species and grows most rapidly at high temperatures if it is well-watered.

5. (2 pts.) Approximately (2.1 inches per foot) (4 feet) = 8.4 inches of plant available water.

6. (4 pts.)
   1. Seed within 2 weeks after the Hessian Fly Free date.
   2. The range of Hessian Fly Free Dates for Indiana is September 22 in the North through October 9 in the South. (September 27).
7. (6 pts.)
   1. 25 to 30 plants/square foot
   2. 144 in.2/ 8 in. = 18 inches of row length/square foot
   3. (3 heads per plant) X (15 plants per square foot) X (1.6) = 72 bushels per acre

8. (6 pts.)
   1. 40 + [(1.75) (90 Bu/Acre - 50)] = 107.5 Total Lbs. N/Acre

      Subtract N Applied At Seeding = 107.5 - 20 = 87.5 Lbs. N topdressed in spring

   2. (0.63 Lbs. P2O5/Bu) (90 Bu/Acre) = 56.7 Lbs. P2O5 / Acre
   3. [(.37 Lbs. K20 / Bu) ( 90 Bu/Acre)] + 20 = 53.3 Lbs. K20 / Acre

9. (6 pts.) As dormancy is broken, while tillering (prior to jointing)
   1. N available when needed for growth as it begins in the Spring.
   2. Least foliar burn potential as exposure is limited to the first leaves.
   3. Avoids stem breakage as only leaf tissue is exposed.
   4. Maximum ability to compensate for damage done by wheel traffic or through leaf burn as tillering and leaf development continue until jointing.

10. (3 pts.) Uniform stand establishment for this Winter annual crop generally suppresses Summer annual weed pressure and is generally sufficient as the primary weed control method. (Fall seedbed preparation has also generally contributed to weed suppression.)

    Post emergence herbicide application is rarely necessary as a rescue. Such applications are generally targeted at perennials or at annuals which are growing in areas of thin wheat stand establishment.

11. (6 pts.) (60 Bu/Acre) (0.80 Lbs. P2O5/Bu) = 48 Lbs. P2O5/Acre

    [(60 Bu/Acre) (1.40 Lbs. K20/Bu)] + 20 = 104 Lbs. K20/Acre

12. (6 pts.)
    1. Determinate plants will be shorter than Indeterminate plants of the same relative maturity because they cease vegetative growth at the time of flowering while indeterminate plants continue to grow until the plant has flowered all the way to the top, using the pattern from bottom to top.
    2. Indeterminate plants have a maximum of one to three small pods at the uppermost node while Determinate plants have a terminal raceme or cluster of pods. Indeterminate plants continue vegetative growth while flowering progresses up the main stem, hence the uppermost pods are developmentally younger and smaller. Because of the more nearly-uniform flowering pattern of Determinate soybeans, pod number and size are more nearly uniform from top to bottom on the plant.

13. (5 pts.)
    1. Increases
    2. Increases
    3. Increases
    4. Decreases
    5. Decreases

14. (8 pts.) The advantage for narrow rows (vs. wide rows) is greatest at later planting dates as delayed planting means fewer days for vegetative growth (photoperiod determines flowering date) and a more compact plant results. These compact plants need to be in very close row orientation in order to effectively capture as much light as possible during R4-R7 (seed development).

15. (10 pts.)
    1. R2 = A flower at one of the top two fully developed nodes on the main stem.
    2. R4 = A pod > or equal to 3/4 inch (2 cm) long at one of the top four fully developed main stem nodes.
    3. A seed > or equal to 1/8 inch (3 mm) long in a pod at one of the top four fully developed main stem nodes.
    4. A green seed which fills the seed cavity in a pod at one of the top four fully developed main stem nodes.
    5. Pod of mature color anywhere on the plant's main stem.

16. (8 pts.)
    1. Harvest at high moisture and dry artificially: This procedure gains approximately 1 week over the normal wheat harvest schedule as harvest occurs at as high a moisture as 25%. Drying cost is low as air temperature is high at this time of the year, and there is little pressure on the drying systems themselves as corn harvest occurs much later. Potential yield gain approximately 5 bu/acre.
    2. Windrow the wheat at 30 to 40% moisture and allow it to dry in the field while soybean germination is under way. This procedure gains approximately 2 weeks over normal harvest. Potential yield gain approximately 10 bu/acre.
    3. Chop the wheat as silage (wheatlage) at approximately the milk stage, thereby gaining about 3 to 4 weeks over normal wheat harvest. Potential gain is approximately 15 to 20 bushels/acre.
    4. Interseed into standing wheat at approximately the heading stage. This strategy gains approximately 4 or more weeks over the normal wheat harvest date. Potential yield gain approximately 20 plus bushels per acre.

(5 pts.) BONUS Soybeans are the more drought tolerant crop.

1. Soybeans sustain their photosynthetic rate under a more negative leaf water potential (e.g. down to - 11 bars vs. down to -4 bars for corn).
2. Soybeans flower over a much longer period and are therefore able to compensate for stress during one part of flowering by retaining a higher percentage of flowers, pods, and seeds as they are formed later in the season when the stress is likely to be lessened (soybeans produce a much larger number of flowers than will ever be realized as pods).
3. Soybeans are a self-pollinating crop and are therefore less vulnerable to drought stress than corn which is open-pollinated.