By Guest Blogger: John Goeser, PhD PAS
Director of Nutrition, Reseach, and Innovation, Rock River Laboratory, Inc.
Adjunct Asst. Professor, University of Wisconsin – Madison
Chopped whole-plant corn (corn silage) continues to gain popularity for dairy and feedlot nutrition programs. Increased yields per acre, consistency through feed out, and energy value per ton of feed relative to legume and other grass forages are factors leading to increased inclusion rates in the total mixed ration (TMR). Diet inclusion rates often exceed 75% of total forage and in some cases corn silage comprise 100% of forage.
The corn silage total digestible nutrient content (TDN) for dairy and beef cattle improves as fermentation (ensiling) progresses. Corn silage fermented for less than 90 days is often referred to as “new crop” corn silage and corresponds to recently harvested forage.
Field experience suggests the new crop of corn silage often yields less energy for gain and production than the corn silage being fed from the previous year’s harvest. Dairies feeding freshly chopped or minimally (<90 d) fermented corn silage often experience decreased performance and increased milk butterfat and protein concentrations. The reasons behind the performance changes have been debated for years and this circumstance is referred to as the “new crop slump”.
The new crop slump economic impact can be sizable. For every 1000 cows, using 2014 milk prices, each pound of milk lost corresponds to $0.20 per cow and $200 per day. Summed through the 60 to 90 days, which experience suggests is the minimum time frame necessary to alleviate the new crop slump, economic losses could add to $6000 per month for each pound in performance lost across 1000 cattle. Consider the economic impact if 3 to 5 lbs. of milk per cow are lost.
The identifying reasons behind the new crop slump have been partly debated, but recently work by Prof. Pat Hoffman and colleagues (2011) have shed light on the cause. But before uncovering the new crop slump cause we should understand the corn plant’s physiology and evolution.
Corn silage is composed of chopped stover (stalk, leaves, and tassel) and ear (grain, husk and cob) fractions. The stover makes up a substantial portion of whole-plant yield, however the ear comprises up to 60% of the silage yield and the grain portion averages 50% of total yield (Coors et al., 1994). Changes in corn silage yield within a year and from year to year are largely driven by increases in the grain fraction (Prof.J. Lauer, 2012 pers. comm.).
Historically, the stalk evolved and has been bred to support the entire plant and then act as a rigid support structure responsible for holding the ear upright until silage or grain harvest. Genetic variation influences stalk fiber (NDF) rumen digestibility, however, the stalk contains less energy per pound of feed than the leaves and ear.
The leaves are the photosynthetic factories that convert energy from sunlight and CO2 into carbohydrates (sugars) which the plant further converts into starch within the ear as the plant matures. The ear leaf (directly beneath the ear) drives a majority of sugar development for conversion to starch (Prof. J. Lauer, 2012 pers. comm.).
The corn seed is composed of a germ (embryo), endosperm (starchy energy source for the embryo), and pericarp (hydrophobic outer protection layer for the seed prior to germination).
We consider grain as an energy source. However, corn plants evolved to develop the grain as the seed for the next plant generation. Just as humans protect their offspring, corn plants evolved to produce a seed that is protected from environmental conditions. Historically, corn plant breeders have further added to grain ruggedness in order to ensure grain flow through multiple augers and handling systems without cracking, breaking or producing fines.
The kernel has adapted so that the plant’s offspring (germ) can withstand environmental conditions while protecting starch to ensure energy is available for offspring growth following germination in spring. The germ and starch granules are protected by water-repelling caps and cages; moreover, the entire grain is protected by a pericarp (outer cap) and the starch granules are encased in a grid of zein proteins referred to as prolamin proteins. Both of these water repelling protection layers are the reason for the new crop slump.
During plant maturation and as the ear develops, starch is deposited in the endosperm around the germ. As the plant continues maturing towards black layer (physiological maturity) corn kernel hardness increases (Jennings et al., 2002) and the potential new crop slump worsens. Kernel hardness increases as the hydrophobic protein matrix built with α, γ, δ, and β zein proteins surround the starch. The protein-forming characteristic is influenced by many factors including genetics and environment. Different grain endosperm types have been demonstrated to substantially impact rumen starch digestion (Lopes et al., 2009).
Floury or dent endosperms are softer and more digestible while flinty or vitreous are respectively harder and more difficult to digest (Lopes et al., 2009 and Philippeau et al., 2000). Genetic impact on grain quality is important to consider however both flinty and floury endosperm grains can be subject to the new crop slump, meaning digestion and energy value improves through fermentation
The new crop slump, caused by the factors listed above, results from inhibited rumen starch digestion. The grain pericarp and prolamin (zein) protein both impede solubilization in rumen fluid and bacterial attachment to starch, which are the first necessary steps for rumen bacteria starch digestion or enzymatic breakdown in the intestines.
Some have speculated that corn silage NDF digestion improves through fermentation, however research has documented that fiber digestion is not improved through ensiling (Huhtanen and Jaakkola, 1993; Darby and Lauer, 2002; Fish et al., 2010). The focus remains on starch.
Particle size and bacteria access to starch are critical factors to starch digestion. With new crop corn silage the effective particle size is larger than that of a fermented crop, meaning less starch is available to bacteria. The performance impact is very similar to that of coarsely ground corn relative to finely ground corn.
Physically breaking the pericarp (kernel processing) opens up the starchy endosperm, however the hydrophobic prolamin matrix around starch granules further impedes digestion by blocking bacteria access to starch. Regardless of processing, an intact prolamin matrix effectively creates a larger average grain particle size (Hoffman et al., 2012) causing the new crop slump.
Field experience suggests that even with 1mm kernel processing gaps and seemingly adequate processing scores, starch digestibility with fresh corn silage is less than that that of fermented silage. Huibregtse et al. (2013) found similar kernel processing scores in corn silage samples surveyed from commercial dairies, but found substantially different rumen starch digestion estimates and variability (Table 1). Corn silages sampled in fall, closer in time to harvest, were less digestible and exhibited a much wider range in digestion than silages sampled the following year after extended fermentation.
Table 1: Rumen 7h lab bench in vitro starch digestion (ivSD) differences for harvest (fall) and spring corn silages (Data adapted from Huibregtse et al.,2013)
The reason starch digestion improves with time in the silo is that prolamin proteolysis takes place during fermentation, cleaving the endosperm protein matrix, improving bacterial access to starch granules, and effectively decreasing particle size (Hoffman et al., 2011). The result is effectively smaller grain particles and greater bacteria surface area access after extended fermentation. Similar to that described previously, the decrease in effective particle size through fermentation can be analogized to beginning with an 800 micron particle size grain, then further grinding to 400 microns. Surface area and bacterial attachment increase and digestion improves.
As mentioned previously, quality traits are highly heritable (Lauer et al., 2009) meaning that seed genetic choice is important. Considerable genetic and environmental variation in kernel endosperm characteristics exists, however the digestion improvement with extended fermentation is real across genotypes (Oba and Allen, 2003 and Philippeau and Doreau, 1998).
Knowing that the new crop slump is due to starch digestibility, how much potential gain can be expected through minimizing the new crop slump? While the lab bench in vitro digestion data in Table 1 show that new crop corn silage is less digestible, the in vitro lab bench approach tends to overestimate real rumen starch digestion (Heuer et al., 2013) and does not capture the full range in digestion potential.
In vivo rumen corn silage starch digestion averages approximately 60% (data adapted from Ferraretto and Shaver, 2012). At Rock River Laboratory, commercial corn silage samples have shown between 50 and 80% starch digestion when measured using rumen incubation (in situ) techniques and assuming an 8%/h passage rate, suggesting there is a wide range in starch digestibility of commercial corn silages. Philippeau and Michalet-Doreau (1998) fermented corn for 90 d and found a nearly 6% unit improvement in rumen starch digestibility using the rumen incubation (in situ) approach. Oba and Allen (2003) found an 18% unit improvement (64.8 vs. 46.4 % of starch) in rumen starch digestibility for fermented corn grain when averaged across high- and low-starch diets.
Extrapolating from the rumen digestion measures reviewed here, rumen starch digestion for adequately fermented (>90d) corn silage is estimated at 10 % units greater than new crop. Using CNCPS v6.1 (Tylutki et al., 2008), a rumen biology based nutrition and performance model, this 10% unit improvement in corn silage rumen starch digestibility when feeding 22 lbs. DM of corn silage results in 5 lb. gain in milk production.
Plant genetics partly dictate kernel hardness at harvest. Selecting a softer kernel variety will improve starch utilization and help alleviate the new crop slump. Prolamin content and kernel hardness also increase with advancing maturity as described previously. Harvesting the corn crop at appropriate moisture (approximately 35% DM) and plant maturity (half-milk line) will help avoid excessive kernel hardness.
Ensuring kernel processing score is greater than 70% (of starch passing through a 4.75 mm screen) is critical for maximizing rumen starch digestion. To achieve 70% or greater scores, ensure kernel processor gap is set to 2mm or less and inspect rolls and bearings for wear. Excessive wear can create gaps of 3mm or greater despite machinery setting at 1 mm. Also, field speed and crop throughput (tons per hour) affect processing.
Finally, allowing corn silage to adequately ferment (>90d) will break down hydrophobic prolamin proteins and ensure efficient rumen starch digestibility provided kernels have been adequately processed. Proteolysis continues past 90 days of fermentation, however, balance the economics of feed inventory versus expected return. Research-proven inoculants and enzymes may help speed crop fermentation, improve preservation efficiency and may impact rumen starch utilization.
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