Under The Maize Silage Cover: Why Every New Maize Silage Stock Is A New Feed

This article is based on a hypothetical evaluation designed to illustrate what typically happens when a dairy operation moves from one maize silage bunker to another without a full nutritional reformulation. The evaluation specifically considers a transition from Bunker A, containing a 32% dry matter maize silage, to Bunker B, containing a more mature 38% dry matter maize silage. The intention is to demonstrate, using realistic ration data, how common on-farm practices create predictable nutritional outcomes.

While dry matter concentration is the most visible difference between the two silages, Table 1 shows that the underlying nutritional shifts are far more significant. The 38% DM silage contains substantially more NDF (414 vs 382 g/kg DM), lower sugar content, reduced organic matter digestibility and a lower energy value (925 vs 968 VEM/kg DM). Although starch concentration is slightly higher, the overall fermentability of the silage declines, resulting in slower rumen fermentation and reduced microbial efficiency. These changes directly affect rumen fill dynamics and voluntary feed intake.

Table 1. Key nutrient differences between maize silage bunkers used in the hypothetical evaluation.

 
Table 2 summarises the four rations evaluated in this scenario. Ration A represents the original balanced baseline ration formulated around the 32% DM maize silage. Cows consume approximately 23.3 kg of dry matter per day at an energy density of 1 027 VEM/kg DM, resulting in a total VEM intake of 23 929 VEM per cow per day. Fibre, protein and fermentable organic matter are well aligned, with NDF at 338 g/kg DM, crude protein at 162 g/kg DM and SFOS at 483 g/kg DM. This combination supports stable rumen function and consistent milk production.

  Table 2. Summary of hypothetical ration responses when transitioning between maize silage bunkers

The first customary practice examined is a direct one-to-one replacement of maize silage on an as-fed basis. In Ration B, the 38% DM silage replaces the original maize silage, kilogram for kilogram. The calculated dry matter intake increases to 24.6 kg per cow per day, despite a reduction in energy density to 1 009 VEM/kg DM, resulting in an apparent increase of approximately 892 VEM per cow per day compared to the baseline ration. However, the ration structure changes significantly. NDF increases to 352 g/kg DM, crude protein declines to 157 g/kg DM, and SFOS drops to 470 g/kg DM.

These shifts result in a bulkier, slower-fermenting ration. Biologically, rumen fill increases, passage rate slows and eating speed declines. As a result, cows rarely realise the calculated increase in intake, feed refusals increase and effective energy intake declines. Although the calculated VEM supply suggests a higher energy intake, this does not translate into improved production. In practice, milk yield typically remains unchanged or declines by approximately 0.5 to 1.5 litres per cow per day. This highlights the disconnect between calculated and realised energy intake.

When refusals become evident, a second practice often follows:
reducing total ration delivery. This response is illustrated by Ration
C. Although calculated dry matter intake returns to approximately
23.4 kg per cow per day, nutrient density remains unchanged from Ration B. Energy concentration remains at 1 009 VEM/kg DM, and NDF stays elevated at 352 g/kg DM. Consequently, the total daily VEM supplied is approximately 318 VEM lower than in the baseline ration. Using a practical conversion where 460 VEM supports approximately 1 litre of milk, this represents a predicted milk loss of around 0.7 litres per cow per day.

Cows are now constrained both physically by fibre load and nutritionally by reduced energy availability, and milk yield commonly declines further.
 
A third practice is often perceived as technically sound: adjusting only the maize silage dry matter after a rapid on-farm test to restore intake. In Ration D, dry matter intake is successfully corrected to approximately 23.2 kg per cow per day. However, energy density remains lower at 1 014 VEM/kg DM, and NDF remains elevated at 348 g/ kg DM. Despite improved intake control, cows still receive approximately 419 VEM less per day than in Ration A. This equates to a predicted milk loss of close to 0.9 litres per cow per day, even though intake targets appear to have been met.

The biological explanation for these responses is well established. Small increases in dietary NDF, particularly when fibre digestibility declines, increase rumen fill and limit voluntary intake. At the same time, reductions in fermentable organic matter reduce microbial protein synthesis, lowering metabolisable protein supply and milk output. These mechanisms originate under the maize silage cover, but their consequences are expressed at the feed bunk, in the milk tank and ultimately in farm profitability.

From a production perspective, even relatively small energy deficits of 300 to 400 VEM per cow per day translate into measurable milk losses of approximately 0.7 to 1.0 litres per cow per day. When combined with intake limitations, these losses are often greater than expected and frequently only become visible once reflected in a declining milk yield.

Under the silage cover lies either controlled performance or silent losses. Progressive dairy producers recognise that when transitioning to a new maize silage bunker, the ration must be reformulated accordingly; otherwise, the cows will make that adjustment themselves.

Every new maize silage stock is a new feed, so to ensure good, sound feeding of maize silage and to prevent any pit falls get practical tips and support from your local De Heus Technical Advisor and keep your cows healthy and productive. - https:// www.deheus.co.za/meet-our-team/.