Key Forage Quality Metrics for Corn Silage

Introduction

Whole-plant corn silage is a major component of dairy cattle diets, and forage quality is a key factor for the economic success of dairy systems. Forage quality can be influenced by several factors including nutritional and physical characteristics of plants, as well as rumen digestion kinetics—all of which can be related to fiber composition and digestibility in dairy cows. Plant cell walls are formed by structural carbohydrates, often referred to as fibrous carbohydrates, and are constituted primarily of cellulose and hemicellulose. These carbohydrates may be linked to lignin, a polymer negatively associated with fiber digestibility (Cherney et al., 1991). As plants mature, neutral detergent fiber (NDF) and lignin concentrations increase, whereas crude protein (CP) decreases, potentially contributing to reduced fiber digestibility and dry matter intake in ruminants (Bosch, 1991).

In rumen digestion kinetics, fiber digestibility results from the competition between digestion and passage rates (Krämer et al., 2013). Degradation rate (Kd) represents the proportion of nutrients that disappear at a given moment in time, whereas passage rate (Kp) represents the proportion of nutrients that flows from the rumen to the intestine. The nutrient composition of forages can directly or indirectly affect Kd and Kp in ruminants. Although information on rumen digestion kinetics is valuable for helping nutritionists improve diet formulation, determining Kd and Kp requires resource-intensive techniques in research trials. Therefore, nutritionists rely on nutrient composition analyses to assess the impact of feeding different forages and their potential effects on dairy cow performance.

Our objective was to review and describe the main variables reported in nutritional analysis of corn silage that can be easily accessed by nutritionists and farmers, providing a guide for the interpretation and evaluation of corn silage quality in dairy cow nutrition. For this discussion, data from the 2025 Professional Dairy Managers of Pennsylvania (PDMP) corn hybrid evaluation program were used.

Forage quality metrics: Characterization and Interpretation

Hybrid

A hybrid is the genetic material of corn or another forage crop obtained by crossing two parent lines to improve agronomic and nutritional traits. In corn silage production, hybrids are evaluated for nutrient composition, fiber, and starch digestibility, which determine corn silage feeding value for dairy cows. Hybrid selection is one of the most critical decisions a farmer needs to make as hybrid genetics (and its interactions with management and environment) can influence whole-plant corn silage quality (Diepersloot et al., 2021).

Relative Maturity (days to maturity)

Relative maturity (RM) reflects the degree of maturity of corn hybrids and the degree-days of development, which basically indicates how long a hybrid takes to mature. Hybrids with higher RM tend to provide greater yield (tons/ac), although they may present challenges with excessive DM concentration in silage if harvested too late. As maturity increases, starch concentration increases; however, the digestibility of starch and fiber can decrease (Peyrat et al., 2016). In 2025, the average (± SD) RM in corn silages in Pennsylvania was 107 ± 8.2 days.

Dry Matter (DM, % as-is)

Dry matter represents the proportion of nutrients that are not water and provides the basis for comparing feeds. When DM content is too low (e.g., <32%), excessive moisture can be detrimental to fermentation quality and to nutrient preservation in the silo. The ensiling of low DM forages may increase nutrient losses due to undesirable fermentation and runoff (i.e., silage leachate). Additionally, wet forages tend to have poor aerobic stability. Excessive DM (> 38%) makes packing difficult, increasing spoilage risk and fermentation losses. Overall, the optimal harvest window for corn silage is 32–38% DM. In 2025, the average (± SD) DM concentration in corn silages harvested in Pennsylvania was 35.0 ± 1.86%.

Crude Protein (CP, % DM)

Crude protein is determined based on the total nitrogen content of feedstuffs (N × 6.25), assuming that protein in feeds contain 16% nitrogen. Corn silage typically contains 6.5-8.5% CP. Although corn silage has a relatively low CP concentration, its contribution to the total dietary CP supply may be relevant as dairy diets become corn silage-based. In 2025, the average (± SD) CP concentration in corn silages harvested in Pennsylvania was 7.5 ± 0.23% DM.

Lignin (% DM)

Lignin benefits the plants by providing structural support, water impermeability, and protection from insects. However, lignin is a polymer considered completely indigestible in the rumen, acting as a physical barrier that interferes with microbial colonization and fiber degradation (Buxton and Redfearn, 1997). As plants mature, lignin content increases, reducing fiber digestibility and energy availability. Corn silage lignin content generally ranges from 2% to over 6% in mature forages. Varieties with reduced lignin (e.g., brown midrib 3 mutant; BM3) often exhibit greater NDF digestibility than conventional hybrids (e.g., 55.9 vs. 46.5% NDF digestibility, respectively; data from Oba and Allen, 2000), which may support increased intake and higher milk yield in high-producing dairy cows. Across Pennsylvania in 2025, the mean (± SD) CP concentration of corn silage hybrids was 2.2 ± 0.15% DM.

Ash (% DM)

Ash represents the mineral fraction of forages, measured as the inorganic residue left after burning a sample at high temperatures (approximately 500–600 °C for 8 to 24 hours). It serves as an indirect indicator of forage quality, as elevated ash values often suggest soil contamination, which may negatively affect silage fermentation. Corn silage has approximately 5.0% ash, but samples may have greater ash concentration (e.g., ~10.0%) due to harvesting practices and weather conditions (e.g., heavy rainfall). In 2025, the average (± SD) ash concentration in corn silages harvested in Pennsylvania was 2.9 ± 0.28% DM.

Starch (% DM)

Starch is one of the primary energy sources in corn silage. Although starch concentration is an important indicator of forage quality, concentration alone does not necessarily translate into high nutritional value. Adequate kernel processing is required to ensure ruminal microbes can access starch granules and ferment them effectively. Typical corn silages contain between 30 and 40% starch on a DM basis. In 2025, the average (± SD) starch concentration in corn silages harvested across Pennsylvania was 34.8 ± 3.67% DM.

Starch Digestibility (% Starch or % DM)

In vitro starch digestibility (IVSD) measures the extent to which starch can be digested under laboratory conditions. Commercial laboratories commonly determine IVSD using either a 4-h incubation with substrate ground to 1 mm or a 7-h incubation with substrate ground to 4 mm, both designed to simulate ruminal starch digestion. Higher IVSD values indicate greater starch availability and energy supply to the animal, which can translate into increased milk production and improved feed efficiency. Corn silage starch digestibility is strongly influenced by the ensiling process. Substantial increases in IVSD are typically observed during the first 0–60 days after ensiling, with several studies reporting continued improvements in starch digestibility for more than 120 days as proteolysis of the protein matrix surrounding starch granules progresses (Raffrenato et al., 2018; Cueva et al., 2023). In 2025, the average (± SD) 7-h IVSD concentration in corn silages harvested across Pennsylvania was 69.2 ± 0.88% starch.

Total Fatty Acids (TFA; % DM)

Fat in feedstuffs can be characterized by their fatty acid (FA) composition. Most FA in corn silage are concentrated in the kernels and consist primarily of unsaturated FA, mainly linoleic acid (C18:2; 35–55% TFA), oleic acid (C18:1; 20–28% TFA), and palmitic acid (C16:0; 15–17% TFA). Corn silage typically contains between 1.5 and 4.0% FA (DM basis). Although corn silage and corn grain contain relatively low concentrations of TFA, the intake of unsaturated FA can be substantial due to their high inclusion rates in dairy diets. In 2025, the average (± SD) TFA concentration in corn silages harvested across Pennsylvania was 2.3 ± 0.21% DM.

Neutral Detergent Fiber (NDF, % DM)

Neutral detergent fiber is a nutritional fraction determined after the solubilization of plant pectins, proteins, sugars, and lipids using a neutral detergent solution. As a result, NDF represents primarily the structural carbohydrates of the plant—cellulose, hemicellulose, and lignin. In most forages, an inverse relationship between NDF and starch concentration is typically observed, as structural carbohydrates increase while non-structural carbohydrates decrease. Neutral detergent fiber is commonly analyzed as amylase-treated NDF (aNDF) to remove residual starch that may contaminate the fiber fraction, particularly in high-starch feeds such as corn silage. For greater accuracy, NDF is often expressed on an organic matter basis (aNDFom) to correct the residue for ash contamination. During the analytical procedure, a neutral detergent solution and heat-stable α-amylase remove the readily digestible components, leaving primarily cellulose, hemicellulose, lignin, and indigestible nitrogenous compounds. As a gravimetric estimate of total insoluble fiber, NDF is inversely related to DMI. In 2025, the average (± SD) NDF concentration in corn silages in Pennsylvania was 34.6 ± 2.26% DM.

Neutral Detergent Fiber Digestibility (NDFD, %NDF or %DM)

Digestibility of NDF is typically determined using either in vitro or in situ methods. Among these, in vitro 30-h NDF digestibility (NDFD30) is often used because it approximates the average rumen retention time of forages in high producing dairy cows. Forages with greater NDFD provide more available energy to the animal, stimulate dry matter intake, and are generally associated with increased milk yield. Typical NDFD30 in corn silages range from 40 to 60% NDF (Hoffman et al., 2006). In 2025, the average (± SD) NDFD concentration in corn silages in Pennsylvania was 58.2 ± 2.66% NDF.

Undigestible Neutral Detergent Fiber (uNDF, % DM)

Undigestible neutral detergent fiber (uNDF) is the fraction of NDF that is not digested in the rumen. This fraction is largely influenced by lignin concentration, plant maturity, forage species, and environmental conditions during plant growth. Undigestible NDF is typically determined using in vitro fermentation for 240 hours (uNDF240). Typical uNDF concentrations in corn silages range from 7 to 11% DM. In 2025, the average (± SD) uNDFom concentration in corn silages in Pennsylvania was 9.9 ± 0.72% DM.

Fresh yield, DM yield, and Organic Matter (OM) Yield (tons/ac)

Corn silage fresh and DM yields represent corn silage productivity (DM yield, tons/ac = fresh yield, tons/ac × DM, %). Organic matter is calculated as DM minus ash content and represents the non-ash fraction of forages that contains all energy-yielding nutrients. Organic matter yield reflects the amount of organic nutrients produced per unit of land and can be calculated using the following equation: OM yield = DM yield × (100 – Ash, %), where OM yield is typically expressed as tons/ac. Low OM yield often indicates poor agronomic performance or elevated ash contamination, both of which reduce the amount of organic nutrients available in the harvested forage.  In 2025, the average (± SD) fresh yield, DM, and OM yields were 19.3 ± 2.76, 6.8 ± 1.17 and 6.6 ± 0.95 tons/ac, respectively.

Organic Matter Digestibility Index (OMD or OMDI, % DM) and Digestible OM yield (DOM yield, tons/ac)

Organic matter digestibility index (OMDI) is a forage quality index that reflects the proportion of silage OM that is potentially digestible by the animal. This index integrates several nutritional variables, including CP, fat, NDF, starch, NDFD, and IVSD digestibility (Hristov et al., 2023). Plant maturity plays a critical role in OMDI because it affects both nutrient accumulation and digestibility. In 2025, the average (± SD) OMDI and DOM yield in corn silages harvested in Pennsylvania were 74.3 ± 0.58% DM and 4.3 ± 0.58 tons/ac, respectively.

¹RM = relative maturity; DM = dry matter; CP = crude protein; TFA = total fatty acids; NDF = neutral detergent fiber; NDFD30 = 30-h in vitro NDF digestibility; uNDF240 = 240-h undigestible NDF; ADF= acid detergent fiber; IVSD= 7-h, 4 mm in vitro starch digestibility; OM= organic matter; DOM= digestible OM; OMDI = OM digestibility index.

References

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