Grape marc: Friend or foe?
Dr Jillian Kelly, District Veterinarian
There comes a time in every drought when people start to consider buying Grape Marc. That time is here - District Vets and Livestock Officers are getting lots of enquiries about Grape Marc and feeding it to stock.
Grape marc is a by-product of the wine making process and at the moment is being sourced from wineries as a raw product. It is the seeds and skins of the grapes and is often low cost or free, so seems attractive in comparison to other feeds. There are some issues though, and its use is of concern if the product is not fully understood by the person feeding it to their livestock.
If you are contemplating feeding grape marc then there are some things that you need to know:
- Grape marc poses a high risk for chemical residues. Some work done in Victoria in 2015 showed that from 24 samples tested, 24 had some form of chemical residue and that 11 of these had 3 or more. So it is wise to ensure you get a commodity vendor declaration when purchasing so that you know what it has been sprayed with.
- The dry matter (DM) can vary hugely. Some samples tested have come back as low 20% Dry Matter and other as high as 70% DM. This means that the rest of the product is water. It is expensive to transport water across the state, and you need to bear this in mind when calculating quantities. 1kg of hay = 2kg of grape marc on an as fed basis because of the water content if it is only 50% dry matter.
- The energy and protein vary hugely. The energy has been tested at lower than 3 MJ/kg DM (cardboard is about 4-5MJ/kg DM)and as high as 13MJ/kg DM. The Crude Protein (CP) varies from 5.5%-18% CP and the average is about 12.2% CP.
- Most of the energy comes from the oil content of the grape marc - I have tested samples as high 17% oil with an average of 9%. Animals can only tolerate 6% oil in their diet – yet another reason why Grape Marc cannot make up the whole diet. It is also worth bearing in mind if you are feeding cotton seed which has an oil content of about 16-21%.
- Grape marc is high in tannins which bind much of the available protein, making it unavailable to the livestock.
- In the raw product, whole seeds can be indigestible but in the ensiled product is not as much of an issue.
- A diet too high in grape marc has caused impaction in cattle so it is important to feed it with another high quality feed.
- Incorrectly stored grape marc poses a risk of developing mould. Only a few moulds will cause stock deaths, with most causing a suppression of appetite and therefore poor performance.
- It may contain some residual alcohol so there is also a risk of alcohol poisoning of stock.
In summary, if you have or are going to buy grape marc, ensure you get commodity vendor declaration and a feed test. Ensure that you know how much water is in the sample that you are buying, not one of the earlier tests. Work out the real cost of the grape marc by doing your sums on a dry matter basis and always feed it with something else, never on its own. Raw grape marc has long been used in the feedlot industry as a roughage source, it can be included in a ration up to a maximum of 20% but most of the time it is only 10% of the mix as a roughage source. Be careful and ensure you have plenty of other good quality feed in the diet as well!