Agronomy 375 Exam Archive
Exam 3 Key Spring 2000
100 points possible
- (8 pts.)
- Milk line positioned one-half way between the kernel crown and tip (kernels sampled from mid- ear).
- Kernels first demonstrate yellow color. Milky kernel content. Silks first dark red.
- Dents visible at the middle of the ear. No free moisture may be expressed from the kernels.
- Node 5.
- (8 pts.) Soybeans are the more drought tolerant crop.
- 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).
- Soybeans flower over a much longer period (e.g. as long as 4 to 6 weeks for Soybeans vs. approximately 1 week for Corn). Soybeans are therefore able to compensate for stress during one part of flowering by retaining a higher percentage of flowers, pods, and seeds at times of lower stress.
- Soybeans produce a much larger number of flowers than will ever be realized as pods).
- Soybeans are a self-pollinating crop and are therefore less vulnerable to drought stress than corn which is open-pollinated.
- (10 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.
- (4 pts.)
- Seed within 2 weeks after the Hessian Fly Free date for a given location.
- The range of Hessian Fly Free Dates for Indiana is September 22 in the North through October 9 in the South.
- (6 pts.)
- 25 to 30 plants/square foot
- 144 in.2/ 6 in. = 24 inches of row length/square foot
- (2 heads per plant) X (20 plants per square foot) X (1.6) = 64 bushels per acre
- (6 pts.)
- 40 + [(1.75) (85 Bu/Acre - 50)] = 101 Total Lbs. N/Acre
- N Applied At Seeding - 15
=86 Lbs. N topdressed in spring
- (0.63 Lbs. P2O5/Bu) (85 Bu/Acre) = 53.6 Lbs. P2O5 / Acre
- [(.37 Lbs. K20 / Bu) ( 85 Bu/Acre)] + 20 = 51.5 Lbs. K20 / Acre
- (6 pts.) As dormancy is broken, while tillering (prior to jointing)
- N available when needed for growth as it begins in the Spring.
- Least foliar burn potential as exposure is limited to the first leaves.
- Avoids stem breakage as only leaf tissue is exposed.
- Maximum ability to compensate for damage done by wheel traffic or through leaf burn as tillering and leaf development continue until jointing.
- (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.
- (6 pts.) (60 Bu/Acre) (0.80 Lbs. P2O5/Bu) = 48 Lbs. P2O5/Acre
[(60 Bu/Acre) (1.40 Lbs. K2O/Bu)] + 20 = 104 Lbs. K2O/Acre
- (5 pts.) No fertilizer N is recommended for soy beans as they are capable of meeting the majority of their N requirement through symbiotic nitrogen fixation. Root infection by Rhizobium japonicum bacteria results in root nodules which make atmospheric nitrogen available for plant growth in exchange for sugars produced by the plant. Mineralized nitrogen and residual N present in a medium textured soil will be sufficient to sustain the crop during the first three to four weeks while the crop is being established and the root nodules begin to function.
Applied nitrogen generally will reduce the amount of nitrogen fixed symbiotically in direct proportion to the amount applied. The exception would be in the case of very sandy soils where available N levels are low and very small amounts of applied N may be useful to help with the establishment of crop growth until the root nodules can become effective.
- (6 pts.)
- Indeterminate plants are generally taller than determinate plants of comparable maturity. Indeterminate plants continue to grow vegetatively for a time after flowering and pod initiation has begun (up to 6 weeks). In contrast, Determinate plants flower over a relatively brief period.
- 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.
- (5 pts.)
- (7 pts.) The leaf canopy needs to be closed by the time of flowering / pod initiation in order to efficiently harvest all available light during seed development (R4 to R7) and optimize yield potential.
The advantage for narrow rows (vs. wide rows) is greatest in any situation where plants are likely to be compact during R4 to R7. Examples are as follows;
Late 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). An extreme example of delayed planting is to double crop soybeans after wheat harvest.
Relatively early maturing varieties as their abbreviated vegetative duration n leads to less height and greater urgency for narrow/drill rows.
Adapted varieties grown at a northern latitude (abbreviated vegetative duration leads to shorter stature and greater drill row response).
Compact genotypes (e.g. determinate varieties adapted to northern latitudes) will benefit most from the closer proximity of drill rows.
Soybean production in un-irrigated dryland areas may show a strong advantage for drill row spacing as plants grown in that environment are likely to be relatively short in stature. Reduced plant population levels are recommended in extreme dry conditions.
- (10 pts.)
- R2 = A flower at one of the top two developed nodes on the main stem.
- R3 = A pod > or equal to 3/16 inch (5mm) long at one of the top four fully developed main stem nodes.
- R4 = A pod > or equal to 3/4 inch (2 cm) long at one of the top four fully developed main stem nodes.
- R5 = A seed > or equal to 1/8 inch (3mm) long in a pod at one of the top four fully developed main stem nodes.
- R6 = A green seed which fills the seed cavity in a pod at one of the top four fully developed main stem nodes.
- (6 pts.)
- 6 seeds per foot of row. Approximately 104,500 seeds per acre
- 2 seeds per foot of row. Approximately 174,000 seeds per acre
- Soybeans planted in drill rows are much more likely to have difficulty emerging uniformly through a crust since there are only approximately 2 seeds per foot of row length in such a system. In 30 inch row spacing, 6 seeds per foot or row germinate and emerge resulting in a fissure over the row, effectively helping each other through the crust and increasing the uniformity with which the crop will emerge. Only two emerging plants per foot of drill row will have a much more difficult time emerging uniformly through the crust.
- (4 pts.)
- 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.
- 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.
- 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.
- 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.
BONUS (5 pts.) A plant at V2 will have leaf margins no longer touching at the third trifoliolate node but still touching at nodes above that point. Said another way, cotyledonary node, unifoliolate node, and the first trifoliolate nodes are said to be developed while the nodes above these are not yet developed.
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