Fibre and Energy in Corn Silage

M.L. SWIFT
Abbotsford Veterinary Clinic, Abbotsford, British Columbia

Corn silage provides a palatable source of energy, protein and minerals for ruminant animals. However, like all forages, corn silage is inherently variable in nutrient value due to hybrid, climatic conditions, maturity upon harvest and conservation methods as shown in Table 1.

Table 1.  Nutrient values for 83 samples of corn harvested at different maturities in the lower Fraser Valley region of British Columbia. (Range of values in brackets)

Energy

Corn silage is included in ruminant rations primarily as a source of energy. The starch in corn grain accounts for approximately 45% of the energy value of corn silage. Table 1 shows that immature corn silage (defined as DM less than 25%) contains approximately 10% units less starch than more mature corn silage (DM more than 25%). Microbial digestion of cellulose and hemicellulose (NDF fraction) in the rumen contributes a further 25% to the energy value of corn silage. The remaining 30% of energy in corn silage comes from sugars, pectin, organic acids, crude protein and crude fat. Traditionally the energy content of feedstuffs has been expressed in terms of TDN (total digestible nutrients) or NE (net energy of lactation or gain). Values for TDN and NE of corn silage and other forages are computed using regression equations based on acid detergent fibre (ADF) content (Table 2 ) which broadly represents the content of cellulose and lignin contained within the plant.

Table 2. Prediction equations for energy content of corn silage based on fibre analysis.

Although this approach accounts for the increasing grain content (ADF decreases, energy increases) and increasing fibre content (ADF increases, energy decreases) as the corn plant matures, it does not account for variation in starch or fibre digestibility.

Over the past decade, there has been increasing interest in improving nutrient value of silage corn hybrids by increasing digestibility of its fibre, in particular, the NDF fraction. Recent studies confirm that digestibility of the NDF fraction affects dry matter intake and milk production (1). In these studies, a 1-unit improvement in NDF digestibility increased dry matter intake by 0.17 kg (0.37 lb) per day and production of milk (4% fat corrected) by 0.25 kg (0.55 lb) per day.

Table 3 provides an interesting comparison between two corn hybrids, one selected for grain the other for silage production. Digestibility of NDF in the stalks and leaves was approximately 10% greater in the silage hybrid than in the grain hybrid. Since these plant fractions represent over 40% of the plant structure, NDF digestibility was 3.4% greater for the silage than for the grain hybrid.

Table 3.  Comparison of corn hybrids developed for grain and silage: plant components and their concentrations of crude protein, neutral detergent fibre (NDF) and digestibility of NDF (IVNDF).

Recent work showed a large range in the NDF digestibility of corn stover in hybrids tested at two sites in British Columbia (Table 4). Digestibility of NDF was tested as the proportion of the NDF that disappeared after 30 h of incubation in rumen fluid. The differences among hybrids shown in Table 4 are reflected in the data presented in Table 1 for immature (DM < 25%) and normal (DM >25%) corn silage samples. Silage harvested early showed an eight percentage point spread between lowest and highest NDF digestibility (51 to 59%). As the corn silage was allowed to mature, the differences between hybrids became increasingly evident as the variation in NDF digestibility increased to fifteen percentage points, ranging from 44 to 59%. Interestingly, there was no relationship between NDF digestibility and the content of DM or NDF in the sample.

Table 4.  Values for NDF and NDF digestibility in stover from hybrids grown in coastal BC in 2001

In light of the work showing the effect of NDF digestibility on dry matter intake and subsequent production, producers are urged to incorporate nutritional quality assessments into their decision-making processes in choosing hybrids for corn silage production. Recently, computer software has been introduced which balances rations for ruminants based on digestibility and dry matter intake. One example is the program associated with the Nutrient Requirements of Dairy Cattle (2) in which the TDN or energy value of corn silage is computed as the sum of digestible nonfibre carbohydrates (e.g. starch), digestible crude protein, digestible fatty acids and digestible NDF.

Minerals:  The calcium to phosphorus ratio in corn silage is approximately 1:1 as shown in Table 1. However there is a large range in mineral content of corn silage. The low potassium content (<2%) makes corn silage ideal for incorporation into diets for dry cows less than 21 days (close-up) from parturition.