Grass Silage Analysis Reports an Explanation
(Reproduced by kind permission of Frank Wright)
DM (Dry Matter)
Typical value 25 - 35 (Low 15 - 25, Normal 25 - 35, High 35 - 45)
With very wet silages, or very dry silages, the dry matter intake (DMI) may be reduced. This may be because of sheer bulk fill with wet silages or that the silage is just far too dry and the cow cannot produce enough saliva to cope. Either way, there is danger when formulating rations based on kg fresh weight intake, e.g:
- 45kg intake of a 20% DM silage = 9kg dry matter
- 45kg intake of a 25% DM silage = 11.3kg dry matter.
- Normal to High is Best
Crude Protein
Typical value 11 - 15 (Low 7 - 11, Normal 11 - 15, High 15 - 15)
The total nitrogen content of the silage multiplied by 6.25 (P = % N x 6.25) this figure may also contain any residual nitrogen fertilizer that remains in the sample. This is not a measure of available dietary protein.
If low, there will be a shortage of protein in the overall diet unless the concentrates are correctly balanced.
If high, the extra protein tends to be in the form of rapidly degradable protein and may not be utilised properly by the rumen microbes. These silages have often been associated with high blood and milk urea readings. High blood ureas have been associated with reduced fertility. Very careful balancing of the concentrates is needed.
- Normal is Best
D Value
Typical value 64 - 72 (Low 56 - 64, Normal 64 - 72, High 72 - 80)
The digestibility of the dry matter, very closely related to crop maturity at harvest and the ME value. Calibrated on in vivo studies in sheep.
- Higher the Better
ME
Typical value 10.4 - 11 (Low 9.8 - 10.3, Normal 10.4 - 11, High 11.1 - 12.8)
Metabolizable Energy is the energy value of the silage in MJ ME/Kg DM available to the animal after losses in faeces, urine and methane. This figure can be predicted from the 'D' value using the formula ME = 0.16 x 'D'value.
Very high ME values may not perform as well as they appear on paper.
- Higher the Better
pH
Typical value 64 - 72 (Low 3.4 - 3.8, Normal 3.8 - 4.2, High 4.2 - 4.6)
Low values indicate very acidic silages that may result in impaired rumen function, leading to acidosis. Rumen buffers would be advisable in such cases. High pH values in clamp silage are indicative of poor, or secondary, fermentation resulting in high levels of acetic or butyric acid in the clamp, that may be detrimental to production.
- Normal is Best
Oil
Typical value 3.0 - 4.5 (Low 1.5 - 3.0, Normal 3.0 - 4.5, High 4.5 - 6.0)
Very high levels may reduce digestibility of the ration in the rumen, reducing performance.
- Normal is Best
NH3-N of Total N
Typical value 3.0 - 6.0 (Low 0.0 - 3.0, Normal 3.0 - 6.0, High 6.0 - 9.0)
NH3-N (ammonia-nitrogen) as a percentage of Protein (total nitrogen content) listed above.
High values are associated with butyric fermentation giving rise to the ammonia smell. Excess ammonia may also contribute to an excess of urea in blood and milk.
- Lower the Better
Sugars
Typical value 2.0 - 4.0 (Low 0.0 - 2.0, Normal 2.0 - 4.0, High 4.0 - 6.0)
The sugar content of the silage sample. This will be lower than the sugar content of fresh grass due to the fermentation process.
- Higher the Better
Ash
Typical value 5.0 - 10.0 (Low 0.0 - 5.0, Normal 5.0 - 10.0, High 10.0 - 15.0)
High ash values would suggest that there has been soil contamination whilst making the silage. Soil is a rich source of iron, that locks-up copper, and also of aluminium, which may interfere with phosphorus uptake.
- Lower the Better
NDF (Neutral Detergent Fibre)
(Neutral Detergent Fibre) Dietary fibre is required to promote rumen function and development. Too much fibre can slow fermentation leading to a reduction in intakes; too little can allow fermentation to occur too rapidly, leading to acidosis.
However, the physical nature of the silage is also important and chop length will affect fermentation more than the actual level of NDF.
- Normal is Best
ADF (Acid Detergent Fibre)
Typical value 40.0 - 50.0 (Low 20.0 - 30.0, Normal 40.0 - 50.0, High 50.0 - 60.0)
The Acid Detergent Fibre value measures cellulose and lignin.
Fermentation Characteristics
VFAs (Volatile Fatty Acids)
(Volatile Fatty Acids) These comprise of acetic, propionic and butyric acids. Butyric acid is normally present only in trace amounts but can increase significantly in cases of poor clamp consolidation. Silages of this type often have a high pH and strong ammonia smell.
High VFAs not associated with butyric acid are usually dominated by acetic acid. Silages with a high proportion of acetic acid are not very stable (acetic acid is a weak acid) and this may lead to secondary fermentation.
- Normal is Best
Lactic Acid
Normally the main acid in the well preserved clamp. A low lactic result combined with a high VFA value would indicate that the clamp may not be very stable, especially when the face is exposed to air and deterioration would occur.
Lactic acid is a powerful acid and so very high levels (greater than 100g/kgDM) could give rise to acidosis type problems. In these situations, buffering is advised. These very high levels are often seen with very wet silages, since it takes more acid to
preserve the clamp.
- Normal is Best
Feed into Milk (FiM) Metabolisable Protein
MPB
Metabolisable Protein from Bypass protein. This is part of the new Feed into Milk protein system. Essential in high yielding cow diets since the rumen can only supply a limited amount of microbial protein for milk production and the proportion of MPB in the ration must increase as yield goes up.
- Higher the Better
MPN
Metabolisable Protein supply where rumen Nitrogen is limiting. This is split into two types: rapidly degradable and slowly degradable, calculated from the degradability fractions: “s”; “a”; “b”; “c”. If the forage has high “s” & “a” values, this indicates a high proportion of the protein is instantly degradable. Balancing these types of silage with instant energy can be difficult and calls for plenty of rapidly degraded dry matter, such as sugars. Too much MPN in the ration can lead to fertility problems.
- Normal is Best
MPE
Metabolisable Protein supply where rumen Energy is limiting. Like nitrogen, this is split into two types, rapidly and slowly degradable, based on the dry matter degradability fractions: “s”; “a”; “b”; “c”. Degradable dry matter comes from sugars,
starches and fibre and provides energy for the rumen bugs. This is usually the limiting factor in most rations.
- Higher the Better
Intake Prediction
This gives a guide as to the potential dry matter intake of the silage, given free choice to the silage. It does not account for the effects of other forages, concentrate feeding and milk yield.
- Higher the Better
RSV (Rumen Stability Value)
Another Feed into Milk attribute. Calculated from the NDF fraction and the Potential Acid Loading (PAL) of the silage. Low RSVs may lead to reduced rumen pH and possible acidosis.
- Higher the Better