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High feed value grass silage

The foundation to ewe productivity and profitability is high feed value grass silage. The majority of ewes housed during mid and late pregnancy are offered grass silage. Thus grass silage feed value is the foundation of a nutrition plan. Sheep researcher Tim Keady gives information and advice.

Main messages:

Digestibility (DMD) is the most factor influencing silage feed value

Each 1 week delay in harvest

  • decreases  DMD by 3.3 percentage units
  • to sustain lamb birth weight pregnant ewes require an additional 12.5 kg concentrate in late pregnancy

Producing high feed value grass silage:

  1. Apply adequate levels of nitrogen fertiliser [100 units/acre (120 kg/ha) for first cut; 80 units/acre (100 kg/ha) for second cut]. If nitrogen has already been applied for grazing assume that approximately 30% of what was applied for the previous grazing is still available. If applying slurry assume 5 units per 1000 gallons of undiluted slurry applied.
  2. Base phosphorus and potassium (fertiliser or slurry) application on soil analysis.
  3. Harvest after a 6 to 7 week regrowth interval and after inspection of the sward. On inspection evaluate the level of seed head emergence and the accumulation of dead material at the base of the sward.
  4. If wilting aim for a rapid wilt and ensile after 24 to 36 hours by spreading the herbage at mowing or ted out shortly after mowing.
  5. Do not jeopardize DMD by delaying harvest with the hope of getting a wilt in showery conditions
  6. Achieve anaerobic conditions post ensiling by sealing rapidly.

Apply 6 layers of wrap on bales to be offered to sheep to reduce the risk of listeria


The majority of ewes which are housed during mid and late pregnancy are offered grass silage as the basal forage. Thus grass silage feed value is the foundation of the nutrition plan implemented during the indoor feeding period. It impacts on ewe performance, lamb birth weight, colostrum availability, lamb survival, labour requirement concentrate feed requirements all of which impact on flock profitability Grass silage feed value is a reflection of its digestibility (DMD) and intake characteristics. Digestibility is the major factor influencing metabolizable energy concentration and intake characteristics.

Silage DMD

Each 5 unit increase in the DMD of silage offered to

  • ewes in mid pregnancy increases ewe weight post lambing by 6.5 kg and lamb birth weight by 0.26 kg. Results from previous studies at Athenry show an increase in lamb birth weight of 0.26 kg results in an increase in weaning weight of 0.8 kg equivalent to the response achieve from feeding each lamb 5.5 kg concentrate to weaning. For a ewe rearing twins this is equivalent to 11 kg concentrate offered to her lambs. The objective should be to produce silage with 75% DMD.
  • Finishing beef cattle increases carcass gain by 0.12 kg daily
  • lactating dairy cows increases milk yield and protein concentration by 1.65 kg/day and 0.44 %, respectively.

When producing silage that will be offered to ewes during mid and late pregnancy, aim for a DMD of 75%.

Major factors affecting DMD  

The following are the main factors which affect DMD:

  • Harvest date: Date of harvest is the main factor affecting silage DMD. Silage DMD declines by 0.48 units for each 1 day delay in harvest. Consequently DMD declines by 3.3 units for each 1 week delay in harvest. The rate of decline in DMD from the primary regrowth is similar to that for the primary growth. Therefore for each 1 week delay in harvesting of grass for ensilage, to sustain lamb birth weight an additional 12.7 kg concentrate must be offered to pregnant ewes during late pregnancy.
  • Wilting: Prolonged wilting decreases silage DMD. Rates of loss in DMD vary from 0.6 to 2.2 percentage units for each day (24 hours) of wilting.
  • Crop lodging: Lodging of the grass crops accelerates the rate of decline in herbage DMD due to the accumulation of dead leaf and stem at the base of the sward. Herbage DMD may decline by as much as 9 percentage units per week in severely lodged crops.
  • Sward type: Perennial ryegrass varieties are classified according to heading date. To obtain silages with similar DMD’s, herbage from swards with late heading varieties needs to be ensiled within 8 days of that from intermediate heading varieties.

It is assumed that silage produced from old permanent pastures has a lower DMD than silage produced from perennial ryegrass swards. However the negative impact of old permanent pasture on silage DMD is dependent on botanical composition. If old permanent pastures are harvested at the correct stage of growth silage with a high feed value can be consistently produced. 


Wilting herbage prior to ensiling has many advantages including reduced effluent production, improved ensilability characteristics, reduced quantities of material for transport and reduced straw requirement for bedding livestock. Whilst wilting increases forage intake the impact on improving animal performance is questionable. A rapid wilt is desirable to minimize the decline in DMD.  The rate of water loss during wilting is primarily related to solar radiation and the weight, per unit area (density), of herbage in a swath.  Management practices have a big impact on herbage drying rate. Studies have shown that to increase herbage DM from 16 to 25 % required 65, 30 and 14 hours, respectively, for herbage that was mown in auto-swaths (6 m width of herbage in one swath), single swaths (3 m width of herbage in one swath) or tedded out, to cover the total ground area, immediately post mowing, respectively.

Many producers delay harvesting in showery weather conditions, with the intention of getting dry weather for wilting.  However, in a prolonged period of showery weather crop DMD is declining, whilst there may be opportunities to harvest and ensile as direct cut (unwilted).  Studies have shown that relative to wilting, ensiling herbage direct cut (unwilted) during showery conditions has no negative impact on animal performance.

Chop length

Whilst chop length has no effect on silage intake or performance of beef and dairy cattle, chop length affects the intake characteristics when offered to pregnant ewes. Reducing chop length increase silage intake and subsequently ewe performance.

Additive management

When applying additives it is important to apply them at the correct rate, taking account of changes in the moisture concentration in the herbage being ensiled.  For example, if the dry matter concentration of the herbage is increased from 18 to 25 %, the fresh weight of grass will be reduced from 29.5 to 21 t/ha consequently reducing additive requirement per hectare by 30%.

Animal performance is the most important measure of the efficacy of a silage additive as producers are paid for animal product, and not for the preservation quality of silage as measured by conventional laboratory analysis. Many studies have been undertaken to evaluate different classes of additives on animal performance (beef cattle and dairy cattle) and concluded that formic acid under difficult conditions and an effective inoculant under a wide range of ensiling condition increased animal performance.