*Bill Mahanna, Ph.D., Dipl ACAN, is a collaborative faculty member at Iowa State University and a board-certified nutritionist for DuPont Pioneer based in Johnston, Iowa. To expedite answers to questions concerning this article or to submit ideas for future articles, please direct inquiries to Feedstuffs, Bottom Line of Nutrition, 5810 W. 78th St., Suite 200, Bloomington, Minn. 55439, or email firstname.lastname@example.org.
LAST February, I had discussed laboratory methods to estimate starch digestibility, including the increasingly popular seven-hour in vitro ruminal starch digestibility (Feedstuffs, Feb. 11).
The various factors that influence differences in starch digestibility were a hot topic at the July joint meetings of the American Dairy Science Assn. and the American Society of Animal Science and reinforced why this test is becoming a standard request for many nutritionists.
Field study protocol
Seglar et al. (2013) presented a field study examining how weight and nutrient composition of corn kernels changed during kernel maturation with five commercial hybrids grown at two separate locations in Wisconsin in 2011 and six commercial hybrids grown in the same plot locations in 2012 (Dane and Waupun, Wis.). Grain from each was harvested at three maturities: one-half milk line, black layer and full maturity (grain harvest).
Soil fertility was high at Dane and moderate at the Waupun location. In 2011, the rainfall was adequate at both locations, but the Dane location was very dry late in the season. The trial locations were dry and unseasonably hot in 2012, with seasonal temperatures and growing degree unit accumulations above average for both locations.
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.
Mean kernel weight was calculated, and the nutrient composition of replicate sets of each of the treatment groups was assayed at Dairyland Laboratories Inc. for total starch, crude protein, neutral detergent fiber, acid detergent fiber, crude fat (ether extract), ash, prolamin and seven-hour in vitro starch digestion by incubating 4 mm of ground grain with rumen fluid.
Kernel specific gravity (absolute density) was determined with a gas pycnometer.
Correlations between prolamin concentration and seven-hour ruminal starch digestibility were also determined.
Kernel weights and nutrient compositions were averaged across years and hybrids for grain harvested at the three maturities.
Both year and hybrid altered (P < 0.01) the kernel composition for all nutrients except neutral detergent fiber and ash, indicating that both environment and genetics can alter the composition.
As kernels matured from the half milk line to black layer, kernel weight increased an average of 24% and starch by 27%, suggesting that premature harvest of corn silage dramatically reduces its starch content (Figure 1).
The seven-hour starch digestibility tended to increase only very slightly, whereas the prolamin content tended to decrease as kernels matured.
Changes in nutrient composition with harvest maturity were surprisingly small, although protein and ash percentages declined with advancing maturity due to greater dilution by deposited starch (Figure 2).
Prolamin concentrations for each hybrid, each year and at each harvest are shown in Figure 3.
Kernel prolamin concentration ranged from 3.5% to 5.5% of dry weight across the three harvest maturities but differed with the growing season and soil fertility. The prolamin concentration increased consistently as protein content increased in the grain (R-square = 0.60). Kernels from the growing location with higher nitrogen fertility had greater (P < 0.05) concentrations of both protein and prolamin.
Prolamin concentrations were greater in 2012 (solid lines), which presumably was associated with heat and drought compared to 2011 (dashed lines). Although the prolamin concentration differed with each year, no increase in prolamin content was apparent with kernel maturation between the half milk line and the full grain maturity. Specific gravity (proxy for test weight) was correlated with prolamin content in 2011 but not in 2012.
The relationship of starch disappearance during seven hours of incubation with rumen fluid (in vitro) and the prolamin content of grain samples from each maturity and location are shown in Figure 4.
Figure 5 shows a similar relationship between in vitro starch disappearance and the prolamin:starch ratio of kernels.
Seven-hour starch digestion was not correlated consistently with either prolamin concentration (R-squares ranging from 0.10 to 0.32) or prolamin:starch ratios (R-squares ranging from 0.07 to 0.38) within or across years or locations. In some cases, seven-hour starch digestion decreased as prolamin increased, but in other cases, it increased.
The inconsistency of the in vitro starch disappearance response to prolamin concentration or the prolamin:starch ratio indicated that neither assay reliably predicted in vitro starch digestion of individual samples based on grain samples obtained in this study (Owens, 2013).
Figure 6 illustrates that the environment during different years and harvest maturities had a 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 (Owens, 2013).
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 or lower kernel density, (2) a lower prolamin content or (3) a lower prolamin:starch ratio.
The relationships across all samples were calculated between seven-hour starch digestibility and each of these three indices or their combination. Of these three factors, kernel density was the single factor related most closely to seven-hour starch digestibility (R-squares = 0.24, 0.15 and 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 (Owens, 2013).
The Bottom Line
This study of North American commercial corn hybrids grown in different years at two different locations and harvested at three maturities indicated that neither the kernel density, prolamin content nor prolamin:starch ratio of kernels reliably predicted seven-hour starch digestibility across the hybrids, harvest date and maturities tested.
Instead, silage producers should rely on direct laboratory analysis for starch digestibility rather than predicting it from other proxy measurements.
Selecting specific commercial hybrids based on historical measurements of starch availability is also a questionable practice due to the very large impact the growing environment has on ruminal starch digestion.
Owens, F. 2013. Personal communication.
Seglar, W.J., M. Pauli, A. Patterson, L. Nuzback and F.N. Owens. 2013. Influence of maize kernel maturity on chemical characteristics, prolamin content and in vitro starch digestion. J. Anim. Sci. Vol. 91, E-Suppl. 2/J. Dairy Sci. Vol. 96, Poster T88, E-Suppl. 1, p 32.