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Grass silage feed value – a key factor influencing flock profitability

Grass silage feed value – a key factor influencing flock profitability

Grass silage feed value is the third most important factor influencing the efficiency of prime lamb production. Dr. Tim Keady, Animal and Grassland Research and Innovation Centre, Teagasc, Athenry, Co. Galway, explains why in this article.

Grass silage feed value influences ewe body condition score at lambing and thus the efficiency of the partition of energy to milk production in early lactation. Grass silage feed value also influences lamb birth weight. Previous data from Athenry clearly show that each 0.5kg increase in lamb birth weight results in an increase in weaning weight of 1.6kg, which is equivalent to a reduction in the age at slaughter of 1.5 weeks. 

Silage production is the largest harvest that occurs annually in Ireland, and it is about to commence. Herbage from 1.2 million hectares (3 million acres) will be ensiled. Whilst it may take one day to ensile herbage on most farms, the resultant silage may be utilised over a 3-5 month period. Thus, ones day’s work can have long-term consequences for meat or milk production. My aim in this article is to present the key factors that impact the production of high feed-value grass silage.


Digestibility (DMD) is the main factor influencing grass silage feed value. For high feed-value silage, the DMD should be greater than 75%. Increasing silage DMD presents the producer with two options next winter. Either maintain concentrate feed level and increase animal performance or maintain animal performance and reduce concentrate supplementation.

Factors affecting digestibility

Most factors that affect silage DMD are within the control of the producer. The factors that affect silage DMD are:

Harvest Date

Silage DMD declines by 3.3 units, on average, for each one week delay in the date of harvest, and is similar for first and second harvests. If a producer delays harvest by one week, extra concentrate supplementation will be required to maintain animal performance. This means that an additional 12kg of concentrate is required during late pregnancy to maintain the performance of pregnant ewes. For finishing beef cattle, an additional 1.2 kg/day would be required to maintain carcass gain.

The decline in DMD is due to stem elongation and the accumulation of dead material at the base of the sward canopy. When deciding when to harvest walk the swards, look at the top of the canopy for plant maturity (seed head development) and at the base for accumulation of dead leaf. Mowing too close to ground level results in ensiling low-digestibility stem and risks soil contamination, both of which reduce DMD. In broken weather do not delay harvesting for a protracted period of time with the hope of getting a wilt – it may not happen.

Crop lodging

Lodging of the grass accelerates the rate of decline in herbage DMD as harvest date is delayed. This accelerated decline is due to the accumulation of dead leaf, and stem at the base of the sward. In severely lodged crops, DMD may decline by as much as 6-9 units.

Sward type

Old permanent pastures, that contain a reasonable proportion of perennial ryegrass and are harvested at the correct stage of growth, can consistently produce a high feed-value silage, similar to that from perennial ryegrass swards.

Heading date

Comparisons of intermediate- and late-heading varieties have shown that to produce silage with the same DMD, herbage from late-heading varieties (heading date 12 June) must be ensiled no more than 8 days later than intermediate-heading (heading date 19 May) varieties despite the 24-day difference in heading date. 

If the primary growth both intermediate- and late-heading varieties were harvested at 50% ear emergence, the DMD of the resulting silage from the late-heading varieties would have 7 units lower DMD.


Wilting herbage pre-ensiling reduces effluent production, improves ensilability, reduces weight of material for transport during ensiling and feed out, and reduces straw requirement for bedding of livestock.

A rapid wilt is desirable. The rate of water loss during wilting is primarily related to solar radiation (sunshine) and swath density. Herbage in auto-swaths (two swaths placed into one) has a much higher density than herbage that is tedded out. Reducing sward density increases herbage drying rate. For example, in the prevailing weather conditions in one study, to increase herbage dry matter concentration from 16% to 25% required 65, 30 and 14 hours, respectively, for herbage that was left in auto-swaths, single swaths  or tedded out (to cover the total ground area), immediately post mowing.

Prolonged wilting reduces DMD - by up to 2 percentage units per 24-hour wilting period in extreme cases.  If wilting, ensure to avoid soil contamination by accurately adjusting the tedders and rakes.

Animal performance effects

From the mean of 11 comparisons, using dairy cows (Table1), it was concluded that rapid wilting prior to ensiling increased milk yield, and the concentrations of fat and protein resulting in an increase of 6% in fat plus protein yield, but at a cost of increasing silage intake by 16%. Delayed wilting will reduce DMD and thus result in no benefit in animal performance.

Table 1: Effect of rapid wilting on dairy cow performance

 Unwilted treatmentWilted treatment
Silage DM intake (kg/day) 10.2 11.9
Milk yield (kg/day) 20.7 21.2
Milk composition (%)    
Fat 4.52 4.66
Protein 3.27 3.38
(Patterson et al 1996, 1998)

Ensiling in showery weather

Often ground conditions are good but occasional showers are not conducive to wilting. In these conditions, some producers fear that herbage will be difficult to ensile and are tempted to delay harvest, thus reducing DMD, in the expectation of better weather.  

Results from a study, using dairy cows (Table 2), where grass was ensiled at 19.0% or 13.7% dry matter (following water application) showed that herbage dry matter percentage at ensiling had no effect on silage fermentation or on the silage intake or performance of lactating dairy cows. Therefore, if the grass is ready to ensile when dry, it is also ready when wet. 

Table 2: Effect of grass dry matter at ensiling on dairy cow performance

 Grass dry matter (%)Grass dry matter (%)
  13.7 19.0
Milk yield (kg/day) 20.1 20.0
Fat plus protein yield (kg) 1.47 1.46
(Keady et al 2002)

Silage additives

Animal performance is the most important measure of the efficacy of a silage additive since producers are paid for animal product, and not for the measured preservation quality of the silage. 

It is important to apply additives at the correct rate.  For example, if the dry matter concentration of the herbage is increased from 18 to 25%, the fresh weight of grass will be reduced by 40%. So this must be reflected in the rate of application. 

There have been many comparisons of additives with respect to animal performance. A review of the published studies shows that:

  • Effective inoculants, used under a wide range of ensiling conditions, increased the performance of dairy cows and finishing beef cattle.
  • Effective inoculants can substantially improve animal performance without necessarily altering fermentation quality.
  • Formic acid applied under difficult ensiling conditions improves animal performance.
  • Molasses, sulphuric acid and enzyme-based additives improved silage fermentation, but had no significant effect on animal performance.



  • Silage DMD declines by 3.3 units for every one week delay in harvest.
  • Decide on harvest date following inspection of the sward for presence of seed heads and decaying material at the base.


  • Ensile within 36 hours.
  • Ted immediately after mowing.
  • Avoid soil contamination.
  • Target 25–30% dry matter concentration.


  • Choose based on proven ability to increase animal performance.
  • Effective inoculants under a wide range of conditions, or formic acid under difficult conditions, increase animal performance.