Corn Kernel Characteristics: Effects of Hybrid, Harvest Maturity and Growing Location

This field study examined how weight and nutrient composition of corn kernels changed during kernel maturation among multiple commercial silage hybrids grown at two different locations. Correlations between prolamin concentration and 7-hour ruminal starch digestibility also were determined with five commercial Pioneer hybrids grown at two separate locations in Wisconsin in 2011 and six commercial hybrids grown in the same plot locations in 2012. Grain from each was harvested at 3 maturities: ½ milk line, black layer, and full maturity (grain harvest). The dried grain samples were assayed at Dairyland Laboratories for prolamin, 7-hour in vitro starch digestion, composition, and specific gravity (density) with a gas pycnometer. Relationships were appraised by regression and GLM.

The experimental design was a split/split-plot arrangement with five Pioneer^® brand products (33F88 [HXX,LL,RR2], P0891XR, P0448XR, 35F48AM1™, P0115AM1™) planted in 2011 at two Wisconsin locations: 1) south-central near the town of Dane and 2) east-central Wisconsin near Waupun. Soil fertility was high at Dane and moderate at the Waupun location. In 2011 the rainfall was adequate for both locations but the Dane location was very dry late in the season. In 2012, six Pioneer brand products planted at the same trial locations used in 2011. P0891XR, 35F48AM1™, and P0448XR were planted in both 2011 and 2012 while P1339XR, P1376XR, and P0705XR were planted only in 2012. Trial locations were dry and unseasonably hot in 2012 with seasonal temperatures and GDU accumulations above average for both locations.

In 2011, intact ears were frozen using dry ice, shipped frozen to the Pioneer Livestock Nutrition Center (PLNC) near Polk City, IA where ears were dried at a cool temperature (<50C). Kernels were hand shelled from each of the 300 dried ears so that total kernel weight and kernel number from each ear could be determined. In 2012, the ears were hand shelled at the plot sites and kernels were frozen with dry ice for transport back to PLNC. Mean kernel weight was calculated and nutrient composition of replicate sets of each of the treatment groups was determined by wet chemistry procedures at a commercial laboratory. Assays included: total starch, crude protein, neutral detergent fiber (NDF), acid detergent fiber (ADF), crude fat (ether extract), ash, prolamin, and 7-hour in vitro starch digestion by incubating 4mm ground grain with rumen fluid. Kernel specific gravity (absolute density) was determined with a gas pycnometer by Pioneer. Relationships among measured components were appraised by regression and GLM.

Kernel weights and nutrient compositions averaged across years and hybrids for grain harvested at the three maturities are presented in Figure 1 and Figure 2. Both kernel and starch weights increased markedly (24 to 27%) as kernel maturity advanced from half milk line to black line maturity (Figure 1). The 7-hour (STRD) tended to increase only very slightly whereas prolamin content tended to decrease as kernels matured. Changes in nutrient composition with harvest maturity were surprisingly small though protein and ash percentages declined with advancing maturity due to greater dilution by deposited starch (Figure 2).

Prolamin concentrations for each hybrid each year at each harvest are shown in Figure 3. Prolamin concentrations were greater in 2012 (solid lines) presumably associated with heat and drought compared to 2011 (dashed lines). Although prolamin concentration differed with year, no increase in prolamin content was apparent with kernel maturation between ½ milk line and the full grain maturity.

Several laboratory procedures are available to estimate ruminal digestion of starch. The most common method is to measure the amount of starch that disappears when ground grain is incubated with rumen fluid for 7 hours in vitro. With ground corn grain, some past studies have indicated that floury (less vitreous) hybrids and hybrids with lower grain density and lower prolamin content are more extensively digested in the rumen and total digestive tract. Some have proposed that prolamin content or the prolamin to starch ratio of a corn grain sample can be used to predict the extent of ruminal and total tract digestion of starch. 

The relationship of starch disappearance during 7-hours of incubation with rumen fluid (in vitro) and the prolamin content of grain samples from each maturity, and location is shown in Figure 4. Figure 5 shows a similar relationship between in vitro starch disappearance and the prolamin:starch ratio of kernels. In vitro starch digestion was not consistently correlated either within or across years or locations with either the prolamin concentration of kernels or with the prolamin:starch ratio of kernels.

As an example, starch disappearance increased as prolamin concentration increased for samples from East-Central Wisconsin in 2011 (R2 = 0.32; light blue) but decreased in 2012 (R2 = 0.24; dark red). In vitro starch disappearance of kernels from South-Central Wisconsin tended to increase with prolamin concentration and the prolamin:starch ratio both years but the relationship was not strong (R2 = 0.10 and 0.11 for 2011 in dark blue and 2012 in lighter red). The inconsistency in the in vitro starch disappearance response to prolamin concentration or the prolamin:starch ratio indicates that neither assay reliably predicted in vitro starch digestion of individual samples based on grain samples obtained across these hybrids grown at two locations and harvested at three different kernel maturities.

The range for in vitro starch digestion measured for the 8 commercial hybrids tested across the two locations and two years is shown in Figure 6 and values for different maturities of individual hybrids in different years are shown in Figure 7. The range in ruminal starch digestion within a hybrid was quite large with no hybrid consistently being high or low. Figure 7 illustrates that the environment during different years (conditions for grain development and harvest timing) and harvest maturity had much greater impact on starch availability than hybrid choice. Consequently, selecting a specific hybrid based on ruminal starch digestibility measured from previous years is unlikely to have a significant or consistent effect on the starch availability of grain produced by that hybrid in future years even at the same location. This refutes the contention that selecting one hybrid over another can consistently alter starch availability when grown in different years or across various environmental conditions.

Several reviews have indicated that starch digestibility is greater for samples of corn grain that have specific kernel characteristics such as 1) more floury endosperm (e.g. softer texture, lower kernel density), 2) lower prolamin content or 3) lower prolamin:starch ratio. The relationships across all samples between 7-hour STRD to each of these three indices or to their combination were calculated. Of these three factors, kernel density was the single factor related most closely to 7 hour STRD (R2 = 0.24, 0.15; 0.22, respectively). The size of these values indicate that less than 25% of the total variation observed in starch disappearance can be attributed to any of these factors either singly or when combined. This variability is illustrated in Figure 7.

Nevertheless, Figure 8 shows that within each maturity and also across all maturities (the dashed line), samples with a higher kernel density (more vitreous) when ground through a 4 mm screen had less starch disappearing during incubation with rumen fluid for 7 hours. This confirms the concept that more vitreous samples when dry ground generally have starch that is less extensively digested in the rumen. However, the variability in 7-hour STRD at any specific kernel density was very large (standard deviation = 4.5). Based on this imprecision, if a sample had a kernel density of 1.29 (as shown in the vertical blue line), one could be very confident (95% probability) that starch disappearance would fall somewhere between 65% and 83%. Note that all but 7 of the 90 samples in this study also fell within this same range. Had hybrids not typically grown in the US (high floury hybrids or high flint hybrids) with kernel densities far outside this range been included in this study, differences in predicted starch digestion among samples would likely have been larger. But among these 8 commercial North American hybrids grown 2 different years at 2 different locations and harvested at three maturities, kernel density or other measurements that have been proposed to predict starch digestibility proved very imprecise for predicting 7-hour starch digestion. Consequently, assaying samples for density, prolamin content, or prolamin:starch ratio would not prove worthwhile for selecting specific hybrids to grow for livestock feed.

Note: This research was accepted as a poster (T88 - Influence of maize kernel maturity on chemical characteristics, prolamin content, and in vitro starch digestion. W. J. Seglar*, M. Pauli, A. Patterson, L. Nuzback, and F. N. Owens, Pioneer, Johnston, IA.) at the July, 2013 joint meetings of the American Dairy Science Association and American Society of Animal Science. The full abstract can be sourced at: J. Anim. Sci. Vol. 91, E-Suppl. 2/J. Dairy Sci. Vol. 96, E-Suppl. 1, page 32.