Agronomy 375 Exam Archive
Exam 1 Key Spring 1999

  1. (7 pts.) A producer can monitor input use efficiency by following the cost of production per bushel. Maximum economic yield (maximum profit) is associated with the position where cost per bushel is minimized. A producer is justified in adding further inputs as long as the cost per bushel is continuing downward. The maximum profit position is attained when inputs are added up to the point where marginal cost equals marginal revenue ( the last dollar of input cost returns only one dollar in additional revenue).
  2. (7 pts.) Yes. Over the long run, a producer who is watching their input costs as noted in question 1 above, will be adding optimal input levels and avoiding excessive input levels or abusive practices which may prove harmful to the environment or to the long term productive potential of the soil. In addition, effective management of productive fields leaves other, less productive areas out of production and in alternative uses for which they are better suited (e.g. pasture, wildlife refuge, etc.). Example: Efficient utilization of applied N fertilizer (as influenced by the timing and rate of application) also means low levels of nitrate leaching into the groundwater. In addition, other less productive acres which would require greater inputs per bushel produced, may then be left out of production.
  3. (5 pts.) Any interaction in which response to one input is aided by the addition of sufficient levels of another input.

    Examples:

    1. Nitrogen response at increasing corn population levels.
    2. Potassium response when N and P levels are sufficient.
    3. Soybean narrow row response in yield when adequate weed control is accomplished and lodging resistance is obtained through variety selection.
    4. Planting date response when soil N, P, and K levels are sufficient for corn.
    5. Corn plant population response when irrigation is added.
  4. (5 pts.) A primary symptom is directly related (in appearance) to the cause.

    Example: Tabled roots are a primary symptom of soil compaction

    A secondary symptom is indirectly related (in appearance) to the primary cause.

    Example: Purple leaves on a young corn plant could be a primary symptom indicative of soil Phosphorous deficiency. However, this would be a secondary symptom where root growth has been restricted by soil compaction (the Phosphorous deficiency indicated by purple leaves is really a reflection of poor root access to soil phosphorous). Tabled roots would be the primary symptom of soil compaction.

    1. (6 pts.) Soil type (as denoted on soils maps and visually by color, texture, slope, drainage).

      Prior management. Records are used to indicate prior management differences within a field. These might include differences in prior subdivision, crop rotation, yield level, etc.

      Once a field's soils map, yield map, and soil test map are established, similarly productive areas with a history of equivalent soil test levels and yields can be consolidated for representative sampling as units.

    2. (4 pts.) Soils should be sampled to a depth of 8 inches for routine P and K soil test levels. Split samples (i.e. top 4 inches vs. bottom 4 inches may be collected as needed to follow up on surface pH or other stratification issues in no-till production systems.
  6. (4 pts.) GPS is an acronym for "Global Position Systems" and commonly refers to the use of satellites and differential correction using signals emanating from transmitting towers whose global position and altitude are known.

    GIS is an acronym for "Geographic Information Systems" and refers to the acquisition and spatial mapping of information referenced to accurate global position.

    VRT is an acronym for "Variable Rate Technology" and refers to the application of variable rates of a crop input as determined by accurately-mapped spatial data (such as change in soil type within a field).

    Example: GPS may be used to determine the position of a combine as it moves through a field. GIS could include the monitoring and mapping of yield data relative to discrete positions within the field. VRT could be used to vary the amount of N applied site-specifically within the field to reflect differing historical yield levels.

  7. (8 pts.) Fall zone tillage uses multiple coulters plus row markers (normally found on planters) to prepare next Spring's seedbed in bands. The result of this Fall band tillage should be greater residue incorporation and faster soil warming than would otherwise occur if the row surface was left under the nearly-complete cover of heavy corn residue until Spring planting on this poorly drained soils. Warmer early Spring soil temperature should enhance early crop growth rate and may result in higher yield potential and faster dry down at maturity.
  8. (5 pts.)
    1. Shallow rooting and poor root uptake of mineral nutrition. Poor root growth may result in slowed ion uptake (anaerobic environment represses root respiration). Shallow rooting may also produce poor access to mineral nutrition from the soil profile and drought sensitivity during the drier Summer months of July and August.
    2. b) Increased soil compaction potential.
    3. Delayed planting Delayed planting and the resulting reduced yield potential (particularly with corn).
    4. Delayed harvest and increased field losses.
    5. Late-season drought stress
    6. Non-uniform herbicide incorporation
    7. Potential delay in field access for post-emergence herbicide application
    8. Greater N losses to denitrification (anaerobic conditions encourage the gaseous loss of nitrogen through microbial activity).
    9. Increased compaction potential.
    10. Low soil temperature (slows establishment of the crop during early vegetative growth).
    11. Increased potential for flooding in low areas.
  9. (8 pts.) Economic goals by definition are intended to optimize economic return. As such these goals factor in the potential for other crop system variables such as climate, weed control, insect pest pressure, etc. to be more limiting to yield than is soil fertility in a given year.
    1. (4 pts.) (0.85) (0.50) (0.90) (0.80) (0.70) = 0.214 or 21.4 % surface residue cover.
    2. (3 pts.) The line transect method measures percent surface residue cover.

    The measurement is conducted by observing the percent of regularly spaced points (e.g. the foot interval marks on a measuring tape or evenly spaced knots on a knotted rope) which are in direct visual contact with surface residue on a line laid on the soil surface at a 45 degree angle to the previous crop's row direction.

  11. (10 pts.) No-till systems will typically demonstrate higher bulk density (initial tillage lessens bulk density for the first few weeks of the season), lower temperature (insulating blanket of residue, residue reflects warming sunlight away), and higher moisture (less evaporative loss under residue - also cooler when damp) near the surface (where young roots are forming) vs. a conventionally-tilled system.

    Because of these prevailing conditions, no-till systems generally produce less total root growth and a more shallow placement of root growth when compared with a conventional tillage/planting system.

  12. (10 pts.) All are inter-related as they influence soil temperature and thereby early season crop growth. Any factor which results in low soil temperature during germination and establishment can result in slow crop development and a reduction in crop yield potential. Such factors would include: heavy crop residue (e.g. corn), poor drainage, nearly level soils, and location at northern latitudes. Combinations of these factors can determine the fit or adaptation of a tillage system in a given environment (e.g. no-till is poorly adapted to poorly-drained soils in continuous corn at a northern location).
  13. (4 pts.)
    1. Reduced yield
    2. Decreased height (expressed as irregular height as affected and unaffected plants differ in height).
    3. Delayed maturity
    4. Tabled (horizontal) roots
    5. Visible nutrient deficiency symptoms.
    6. Heightened drought stress.
    7. Increased sensitivity to herbicide injury
    8. Slowed infiltration of water (surface ponding, evidence of poor drainage such as mottled color in the upper soil profile, or slow crop residue decomposition).
    9. High bulk density as reflected by resistance to soil probe, strain gauge or a knife
    10. Surface crusting indicates shallow compaction resulting from heavy rainfall. Seedling emergence is slowed by crust.
    11. Seedling roots restricted to seed slot by sidewall compaction (tomahawk roots).
  14. (6 pts.)
    1. Cultural Control: Example - Row Width (narrow rows mean more rapid leaf canopy closure and greater control of annual weeds if they are suppressed until the canopy can close).
    2. Mechanical: Example - Row Cultivation (effective against annual weeds prior to leaf canopy closure).

      The combination of Cultural and Mechanical weed control strategies along with Chemical control strategies (e.g. Post Emergence herbicide application on a rescue basis as cost effective control for "escapes") leads to the greatest cost effectiveness in weed control for crop production systems.
  15. (4 pts.)
    1. PPI herbicide incorporation is to be uniform in the top 1 to 3 inches of the soil.
    2. Herbicide incorporation is generally one-half to two-thirds of the depth of tillage with a tandem disc.

BONUS (5 pts.) As tillage is reduced a producer is likely to save on fuel, equipment, and labor while spending more on chemical weed control. N expense may be equivalent since equivalent N form and rate can be used for alternative systems. Net savings are generally associated with reduced tillage systems with these savings most evident in the widest comparison (e.g. no-till versus conventional moldboard plow). Lesser differences exist between intermediate tillage alternatives (e.g. standard chisel plow versus moldboard plow).

Where there is a significant yield difference between alternative systems, the greatest impact on profit will come from the from the revenue side of the ledger rather than from cost savings. A relatively small yield advantage for one system over another can more than compensate for modest cost differences. Where there is no consistent yield advantage (e.g. on well drained soils in a corn and soybean rotation) cost savings become the major factor in system comparisons.

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