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Competency Area 7: Forage Harvesting Factors

PO 31. Describe the stage of development when corn is ready to harvest as silage.
A. Know the basic procedures for evaluating forage quality, including fiber digestibility,
B. Know the ideal forage quality at harvest (NDFD, ADF, NDF, NEL, TDN, etc.)

A. Know the basic procedures for evaluating forage quality, including fiber digestibility
Corn silage harvest should commence when whole plant moisture is at about 68% for bunker silos and 65% for tower silos. Corn silage yields increase and quality decreases as moisture decreases from 70 to 60%. A good compromise for yield and quality of corn silage is in the 63-68% moisture range (32- 37% dry matter). Harvesting above 70% moisture will result in effluent losses from a bunker silo and above 65% will result in effluent losses form a tower silo. Harvesting at less than 58% moisture will result in fermentation problems because of lack of moisture; water may have to be added to the silo.
Corn silage moisture can range from 58-65% (35-42% dry matter) at the half-milk line stage of the grain so the milk line should only be used as a guide on when to begin corn silage harvest. In years with dry and warm conditions in August and early September, corn silage moisture can be at the 65-68% moisture range (32-35% dry matter) at the dent stage of corn development (R5).

Corn silage moisture is in the 68% range from 35-40 days after silking if corn silage harvest typically occurs in early September harvest, the 40-45 day range if it typically occurs in mid-September, and the 45-50 day range if it typically occurs in late September. Hybrid selection should be based on these harvest goals. Another guide to use: corn silage moisture is usually in the 68% range at about 800 GDD after silking.

B. Know the ideal forage quality at harvest (NDFD, ADF, NDF, NEL, TDN, etc.).
Unlike perennial forages, there is no real set of guidelines for the ideal corn silage quality at harvest. Generally, NDF should be in the low 40% range, CP should be at about 8%, NDFD (30 hour) should be in the 55% range, and starch should be in the low 30% range. Climatic conditions during the growing season, however, can greatly influence these values (wet year typically results in higher NDF and lower NDFD values, a dry year can reduce starch values, a high-yielding year can dilute or lower CP values, a low-yielding year can concentrate or increase CP values, etc.).

In addition, management practices such as hybrid selection, planting density, and harvest management further influence these values. Some hybrids will produce very high yields with high NDF values; other hybrids will have low yields with very high NDFD values (i.e. brown midrib hybrids); some hybrids will have high starch values; whereas other hybrids will have lower starch values. Higher planting density typically results in lower CP values (dilution effect because of higher yields) and lower starch and higher NDF values, especially in dry years.

Timing of harvest also greatly influences forage quality. NDFD digestibility decreases as whole plant moisture decreases but starch values increase because of more grain development. Likewise, NDF values decrease between 70 and 60% moisture because of the increase in the grain portion in the silage. Also, CP values also decrease because of the dilution effect (yields are increasing).