What do silage and hay test figures mean?

18 October 2016

A feed analysis is essential for understanding the nutritive value of the fodder silage for feeding solely or in mixed feed situations and how successful the harvesting and ensiling process has been. It will also highlight what may need to be done next season to improve the quality of the fodder.

Points to remember

Firstly, the sample used in a feed test should be representative of what is in the silage/hay stack or bales. That is, a strict number of core samples should be taken to represent the batch, not just one to two bales and not a couple of grab samples from a stack.

Secondly, sample the actual silage or hay not the parent forage. Sampling the parent forage will only provide a guide to the potential quality of the fodder. To be truly representative of what is in the paddock it must be sampled to strict guidelines, otherwise the result could be misleading or confusing. Be aware that later sampling of the fodder itself (as silage or hay) may give even higher values than that of the parent forage due to poor sampling techniques. The nutritive values of the fodder should always be lower than that of the parent forage.

Hay can be sampled a few weeks after it is baled (dry matter content will alter a few percentage units) and silage needs at least six to eight weeks (to complete fermentation) before being tested. The truest and most accurate sampling should be done just before or at feed out.

Moisture content (percentage of fresh material)

This is a measure of the amount of water in the sample when received by a research laboratory. Some moisture will be lost from the sample in transport and a lot if left for too long in a hot vehicle at home before postage. Treat the sample carefully.

Dry matter (DM) (percentage of fresh material)

This is a measure of the amount of dry matter in the sample once the moisture has been dried off via a particular type of oven. For example:

100 – DM% = Moisture content (%), and vice versa.

Silage is usually referred to in terms of DM content and moisture content is used for hay.

All other feed analyses are expressed on a DM basis to allow comparisons. These comparisons may be the amount of DM per unit or batch of different feeds and allows comparison on a cost per unit weight initially, but ultimately should allow comparison on a cost per nutritive value basis such as cents per megajoule of metabolisable energy or dollar per kilogram crude protein. For example to calculate the DM of a batch, a 350 tonne stack of silage at 35 per cent DM contains:

350 x 35 ÷ 100 = 122.5 t DM

Digestibility (DDM as percentage of DM):

This is an estimate of the digestibility of a fodder and can be measured in three ways. For the purpose of this discussion, digestibility of the DM itself is the simplest to understand.

A silage or hay of 60 per cent digestibility means 60 per cent of its DM is used by the animal for maintenance and production, the rest is wasted in excrement. Obviously, a feed of high digestibility such as one with a 75 per cent digestibility of the dry matter (DMD), therefore 25 per cent waste, is a much better feed than one of 60 per cent and has much quicker digestion through the animal and higher animal production.

Crude Protein (CP as percentage of DM)

The nitrogen component of the sample is measured and then multiplied by 6.25 to represent the CP content. This figure represents both true protein and non protein nitrogen and although both can be used by animals to a large degree, unusually high levels of non protein nitrogen is often used inefficiently or wasted. The CP also does not indicate heat damage of proteins in silages and hay which reduces its availability to the microflora in the rumen so also reducing animal production.

A feed having a CP of about 16 to 18 per cent is sufficient to meet high levels of milk production. Remember it is the CP content of the whole diet that is important so a low CP hay can be offset by a high CP silage or hay.

Metabolisable Energy (ME)

This is the energy part of the feed that is available to animals for heat production, maintenance and meat/wool/milk production. Most laboratories express energy as megajoules (of ME) per kilogram of dry matter (MJ/kg DM or MJ ME/kg DM).

Many Victorian laboratories have associated with specific American Laboratories to meet the demand by private consultants and some feed suppliers for a wider range of feed analyses. Their reports express energy as Total Digestible Nutrients (percentage TDN) and MJ/kg of Net Energy of Lactation (NEL), Net Energy of Maintenance (NEM).

Direct measurement of ME is very expensive, requiring ‘calorimeter chambers’ that capture everything going into and out of the animals including heat and gases.

Therefore, the digestibility of the DM or organic matter (OM) is determined and then predictive equations are used to estimate the ME content from the digestibility value. Some laboratories in Australia and most in the US, use Acid Detergent Fibre (ADF) to calculate the DDM.

In a balanced diet, feed intake and animal production increases with increasing ME. Fodder must have ME values greater than 10 for high rates of animal production.

Fibre

If all the protein and energy components such as soluble sugars and carbohydrates, vitamins and minerals were removed from the fodder, the major component left will be fibre. This material helps to hold the plants up and although some of it can be digested to some extent, most won’t be digested at all. Fibre is often measured using two methods.

Neutral Detergent Fibre (NDF as percentage of DM)

The NDF is an estimate of the percentage of total cell wall content of forage or other feeds. Normally, lower NDF translates to higher animal intake.

Acid Detergent Fibre (ADF as percentage of DM)

The ADF estimates the portion of the feed that the animals cannot digest. The higher the ADF, the lower the DDM (and ME) and so lower the potential intake. For example, a high ADF feed could be very ‘stemmy’ hay compared to leafy hay.

Ammonia-Nitrogen (Ammonia-N as percentage of total N)

This provides a measure of the fermentation quality of silage. This test is done by the slow wet chemistry method, hence the higher cost and slower turnaround speed.

If silage has undergone a poor fermentation, the protein component of the feed is broken down. The poorer the fermentation, the greater this breakdown. Well fermented silage will have ammonia-N values under five per cent of total N and if very poor, this will be greater than about 10 per cent. This test is an optional extra and must be asked for.

Silage acidity (pH)

This test also provides an indication of the extent of fermentation and is influenced by the DM and sugar content at ensiling and the type of fermentation. Stack silage should have a pH of about 4.0 while baled silages, being drier, will be over about 4.8, although the pH test is less reliable at baled silage DM contents.

Other analyses

Other tests are available from most laboratories such as water soluble carbohydrate, acid detergent insoluble nitrogen and mineral analyses.

Table1: Analyses for a range of nutritive values of silage

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