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The Facts About Feeding Straw
Eleanor M. Kellon, VMD
The major reason most people feed straw rather than hay is to reduce the calories, but there really is not that much difference – 0.789 Mcal/lb on average versus 0.913 Mcal/lb for grass hay which is a 14% drop. (Dairy One Feed Composition Database). If you are already feeding a mature hay, or one chosen for low-sugar and starch levels, the difference is even less, since they typically run about 0.850 Mcal/lb with straw then offering only a 7% reduction in calories.
Straw isn’t necessarily safe from a sugar and starch standpoint either. Sugar as high as 6.2% has been reported and starch up to 4.3%. Straws with a large amount of grain left in the seed heads will be even higher.
There are significant differences in the fiber fractions and not in a good way. ADF and NDF are very high, making straw more difficult to ferment which may result in “hay belly”, and diarrhea, or free fecal water in older horses especially.
Protein is also severely deficient, averaging 5.3% in straw versus 10.9% in hay, necessitating protein supplementation. Mineral levels are similar, except for lower average phosphorus and magnesium, but may be less available because of binding to the higher fiber fractions.
Straws have virtually no vitamin value. Because they are difficult to ferment, they are poor support for the microorganisms which would normally produce B vitamins for the horse. To top it off, there is a higher risk of toxic nitrate levels in straw https://hereford.org/wp-content/uploads/2017/02/issue-archive/0217_Nitrates.pdf .
In summary, there is a small reduction in calories but a much higher loss of protein, vitamins, and fermentability compared to feeding hay. With all the supplementing you will have to do, including for donkeys, you won’t save any money and analyzing for nitrates is advisable. You are better off investing in slow feeder nets and feeding a nutritionally appropriate hay.