Growing Season, Genetics Affect Feeding Value of BMR Corn Silage
Attention to the strengths of each individual BMR hybrid planted will go a long way toward minimizing possible negative effects of the growing season.
Written by Bill Mahanna and Bill Powel-Smith*
Starch Content, Digestibility
Corn silage, both conventional and brown mid-rib (BMR), provides two significant energy sources: energy from starch and energy from digestible fiber.
The trend towards growing and feeding more high-quality BMR silage has come into sharp focus in recent years as the seemingly endless trough of low milk prices and loss of technologies like recombinant bovine somatotropin have left dairies straining to find profitability.
The promise of more milk from higher-forage diets built around BMR silage has been realized by many dairy operations that incorporated BMR silage into their cropping and feeding strategies.
The challenge with BMR silage is that it does not always deliver as expected. As dairy producers move between growing seasons or hybrids, nutritionists sometimes become frustrated with figuring out how to dial in diets to get the expected results. In the end, individual hybrids are often blamed for a lower-than-expected milk response, causing dairy producers and crop growers to needlessly limit their hybrid options.
When it comes to selecting corn silage genetics, university and seed company plots prove that there are minimal genetic differences — three to four percentage points — between (non-BMR) hybrids regarding digestibility of neutral detergent fiber (NDF). This is the reason several seed companies have decided to commercialize BMR genetics.
Most growers and nutritionists now realize that the large variation in NDF digestibility (NDFD) observed from farm to farm and season to season is primarily the result of environmental factors such as precipitation and heat units.
In general, dry conditions during stalk development enhance fiber digestibility, and wet conditions, while improving whole plant yield (taller plants), tend to reduce fiber digestibility (Feedstuffs, June 14, 2010).
Figure 1 shows data from Michigan State University silage plots harvested in the relatively wet growing season of 2006 compared to the same hybrids harvested from the same plot in the relatively dry growing season of 2007 (D. Bolinger, personal communication, 2007). Hybrids averaged 6.5 points higher in 24-hour NDFD in the drought year. It was interesting to note that, as expected, the highest NDFD in both seasons was a BMR hybrid, but BMR genetics are not immune to the effect of growing environment.
Figure 1. Growing environment effect on the same hybrids grown in Michigan State University silage plots in 2006 (wet year) vs. 2007 (drought year). (Click here (JPG 147 KB) or on the image above for a larger view.)
Nutritionists are beginning to realize that while diets are designed at the bunker, corn silage yield and quality are determined in the field.
In 2017, Pioneer dairy specialists and agronomists conducted field studies at more than 100 locations in eastern Wisconsin.
Corn plants were evaluated during the growing season for emergence and plant health, with representative plants sampled at harvest maturity (one-half to three-quarters kernel milk line). Some of the harvest-stage plant stalks were analyzed without the ear in order to investigate digestible fiber fractions without the influence of starch from the ear, given that the stover and leaf account for about 71% of the NDF of the corn plant (F.N. Owens, personal communication, 2016).
More than 500 samples were sent to an independent laboratory for analysis, and the following two key findings begin to explain the variability in the nutritional profile of various conventional and BMR silages:
- In the fields studied, almost 20% of plants were late emerging (by 12-72 hours). Later-emerging plants were as much as 50% lower in starch and 25% higher in undigestible NDF (uNDF) than earlier-emerging plants. Tillage practices and planter performance are two significant reasons for uneven plant emergence. Selecting hybrids with exceptional “stress emergence” is an obvious first step when considering a BMR hybrid.
- Plants that run short on nitrogen during the ear fill stage are known to cannibalize nutrients from the stalk to fuel ear development. Cannibalization is expressed as deterioration of the lower stalk and leaves. Stalks and leaves dry out and appear brown and dead. This damaged and deteriorated plant material can lower plant NDFD by as much as one percentage unit per day. The damaged plant material is also frequently the site of molds and possibly mycotoxins.
The Table shows stalk (whole plant minus ear) data from two different BMR fields. One field had healthy plant stalks, and the other field contained diseased stalks. The effect of late-season plant health and disease resistance on NDFD and uNDF is a key factor affecting fiber digestibility that may be overlooked if assessing only the silage coming out of the bunker/pile. It also has a major impact on how these two BMR fields would be complemented in any ration formulation.
A 2014 field study by the Miner Institute (Cotanch, 2015) reinforced these findings, showing that frosted and fungal-infected corn leaves had six percentage points lower 30-hour NDFD and five points higher uNDF compared to green, healthy leaves.
Starch Content, Digestibility
It only makes sense that individual field growing conditions and hybrid stalk and leaf health would affect corn silage starch content. Nutritionists routinely account for starch content differences in ration formulations. However, the cause of unrealized herd performance expectations could be related to ruminal starch digestibility.
It has been well documented that older BMR genetics have lower ruminal starch digestion (Lim et al., 2011; Grant, 2017). More recent BMR hybrids entering the market have shown seven-hour ruminal starch digestion equal to or greater than conventional hybrids while claiming to have more floury pedigrees (F.N. Owens, personal communication, 2015).
The combination of higher starch in healthy plants and ruminal starch digestion more like conventional hybrids introduces another variable for nutritionists to consider in high-inclusion BMR diets. BMR silage increases in starch digestion during fermented storage, just like conventional hybrids do.
This may make high-starch, high-ruminally fermented BMR hybrids less able to couple successfully with supplemental grain sources like wet snaplage (particularly late winter/summer) compared to BMR genetics with lower ruminal starch digestion.
Diseased hybrids in the middle six rows are surrounded by more disease-resistant hybrids.
Figure 2 shows Fermentrics reports summarizing the challenge when feeding BMR corn silage due to differences in hybrid genetics, the growing environment and late-season plant health.
The diseased plant in the lower right has less starch, a slower starch digestion rate (17% per hour) and a slower rate of fiber digestion (5.2% per hour) compared to the healthy plant sample profiled in the upper left, with 8% points more starch, a much faster rate of ruminal starch digestion (33% per hour) and a slightly faster NDF digestion rate (5.9% per hour).
These two BMR silages would need to be complemented in completely different ways during diet formulation.
Figure 2. Reports for two BMR corn silages with distinctly different fiber and starch profiles due to a combination of genetics, growing environment and late-season plant health. (Click here (JPG 135 KB) or on the image above for a larger view.)
The adage that “diets are designed at the bunker, but corn silage yield and quality is determined in the field” should reinforce with silage growers and nutritionists that the BMR silage fed to high-producing cows is a function of the individual hybrid genetics, disease resistance, growing conditions and harvest maturity (affecting both NDFD and starch content).
No matter which BMR hybrid a grower plants, attention to the strengths of the individual hybrid, along with planting accuracy and fertility, will go a long way toward minimizing possible negative effects of the growing season. Once harvested, sophisticated laboratory profiling of the resulting silage will help nutritionists better understand the genetic and environmental impact on fiber and starch levels and digestion rates.
Cotanch, K. 2015. Effect of frost damage and fungal growth of corn leaf NDFD. Miner Institute Farm Report. March.
Grant, R. 2017. Bringing together fiber and starch digestibility in corn silage. Proceedings of 2017 Cornell Nutrition Conference.
Lim, J.M., M.C. Santos, J.P. Riguera, M.C. Der Bedrosian, K.E. Nesto and L. Kung. 2011. The nutritive value of mature corn silage from BMR, non-BMR and a 50:50 mix ensiled for varying lengths of time. J. Dairy Sci. 94(E-Suppl. 1):216(abstr.).
*Bill Mahanna (Ph.D., Dipl. ACAN) is a board-certified nutritionist for DuPont Pioneer based in Johnston, Iowa, and Bill Powel-Smith is a DuPont Pioneer dairy specialist based in Wisconsin.
Used by permission from the March 5, 2018, issue of Feedstuffs.