W.M. Cerkoney¹, R.H. Pritchard² and Z.W. Wicks III³
Animal and Range Sciences Department, and Plant Science Department

Yield, lb/acre, bushel weight (BD), lb/bu, relative maturity (RELMAT), chemical composition and in vitro dry matter digestibility (IVDMD) were used to screen corn varieties (n = 20) of varying maturities grown under the stress of a shortened growing season. Corn varieties were planted in late June of 1996. Whole shell corn (WSC) and ear corn (EC) were harvested. Bushel weight was quantified on WSC. Yields for WSC, and EC were 1941 ± 706 and 2307 ± 997 lb DM/acre, respectively. Whole shell corn and EC were sorted into yield (YGP) groups. Whole shell corn yields were 2942 ± 248, 2305 ± 138, and 1292 ± 343 lb DM/acre for YGP1 through YGP3, respectively. The EC yields were 3740 ± 207, 2980 ± 208, 1897 ± 235 and 1139 ± 455 lb DM/ acre for YGP 1 through 4, respectively. For WSC, YGP 1, YGP 2 and YGP 3 produced 2529, 1956 and 1099 lb TDN/ac, respectively. Ear corn YGP 1, YGP 2, YGP 3 and YGP 4 produced 2980, 2180, 1437 and 780 lb of TDN/ac, respectively. Increased yield in corn varieties grown under the stress of a shortened growing season was attributed to an increase in starch content. Even so, digestibility and energy content were not related (P > .05) to yield or BD. Relative maturity did not influence (P > .05) chemical composition, digestibility or energy content. Results suggest that while selecting earlier maturing corn for short growing seasons improves yields, it gives no advantage to feed value. Bushel weight, yield or maturity date should not be used as single criteria when predicting feed value of corn grown under the stress of a shortened growing season.

Wet springs, drought, disease and early frost, are all factors that prevent corn, in this region, from reaching maximum yield, desired moisture content, and full maturity. When corn crops do not reach full maturity and are grown under stressful growing conditions, "soft corn" production or corn with low bushel weights is often the result. Low yielding corn generally is associated with a decrease in BD and assumed to have a lower feeding value for livestock.

However, previous research here and at the University of Nebraska concluded that corn that was below USDA Grade No. 2 corn did not have a lower feeding value than No.2 corn.

The objective of this research was to screen corn varieties grown under the stress of a shortened growing season for usefulness as livestock feed. Corn varieties were evaluated for yield, chemical composition, and digestibility. This information is intended to aid cropping decisions in years when planting is delayed.

Twenty corn varieties of varying maturity were planted late June, 1996. Corn plots were located at the South Dakota State University Agronomy research plots at Brookings. Corn varieties planted and their respective growing degree day (GDD) requirements are listed in Table 1.

Each corn variety was represented by 12 rows. Three test plots (P1, P2, and P3) consisting of two rows/plot were marked out for each variety to collect harvest yield data.

Relative maturity was calculated for each variety by dividing the actual growing degree units (GDU) that occurred by the total GDU each corn variety required for full maturity and was expressed as a percentage. Each variety was exposed to 2015 actual GDD (from planting to the first killing frost).

Fall snow storms delayed harvest until December 26 and 27, 1996. Ears were hand picked from one row of each test plot. Test plots within a variety were pooled. De Kalb 471 was lodged and lost due to the heavy snowfall. Five sample ears from each variety were collected and frozen for subsequent determination of moisture content of the EC at harvest. The remainder of the harvested ears, were dried down to approximately 12% moisture content. After drying, ear corn weights were recorded for each pooled variety sample. Each variety sample was divided into two equal aliquots. One aliquot was shelled for WSC yield and bulk density calculations.

Ear corn samples frozen at harvesting were lypholized to determine the DM content (FIELDM) at harvest. Whole shell corn and EC were analyzed for CP, starch, ether extract (EE), ADF, NDF and ash content.

The TDN content of EC and WSC was estimated based upon ADF content and also by two stage in vitro digestion.

Energy content (ME, NE_m and NE_g) was predicted from both ADF content and IVDMD derived TDN.

Variety data were compared using the General Linear Models procedure of SAS (1996) for a completely randomized design. Statistical analysis was based on assay replication not plot replication since plots 1, 2 and 3 for individual varieties were composited. Linear regression analysis was used to determine relationships among yield, bulk density (bushel weight), RELMAT, chemical composition, digestibility and energy content.

Chemical composition, yield, BD, RELMAT, IVDMD, and energy content (ME, NE_m, and NE_g) of corn grain varieties are presented in Table 2, along with, 1996 Beef NRC chemical composition and energy content values for 54 lb/bu corn.