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Nutrition – for Oct. 5, 2009

David P. Price is a consulting nutritionist specializing in feedlot and range cattle

Last month we discussed how straw can be used as a substitute for hay in feedlot rations. In that situation it requires some formulation changes, but otherwise presents no major challenges or problems.

Unfortunately, that is not the case for use on pasture. The primary reason being that in a feedlot all we’re looking for is some fibre. In a range situation we’re looking for nutritive value. Straw has neither; not energy, protein, vitamins nor minerals.

What’s misleading is that a laboratory analysis will show a modicum of TDN (energy). Actually, that’s not a real analysis, but a mathematical calculation. Most laboratories will take crude protein and crude fibre and plug them into a prediction equation designed for good-quality feeds, and then project a feed value.

It’s important to realize “crude” protein is just that. It’s actually an analysis for nitrogen (which is a component of protein). A straw analysis will report three to five per cent crude protein, but actually the nitrogen extracted comes from the lignon in the fibre, not protein. Straw has virtually no digestible protein.

In the case of straw (as well as most low-quality roughages), it is best to use book values rather than laboratory analysis (or at least do not be influenced by the calculated feed values most laboratories report). Book values from reputable sources should come from actual feeding experiments (rather than analysis). The book value for net energy for gain for wheat straw is zero.

It is important to realize a net energy for gain of zero, does not mean cows will simply not gain weight. They will lose weight.

The book value for net energy for maintenance is 44 (megacalories/hundredweight). But again, that does not mean cows can maintain their weight on straw. The book value for TDN is about 44 per cent. But even a book value TDN for straw is misleading since Total Digestible Nutrients means just that. It is an experiment, calculating what went in the front end of a cow versus what came out the back. Roughages have what is known as the “heat increment,” which is the energy released as heat by the rumen micro-organisms. While this heat is useful during the winter, it is not energy the animal can use for body maintenance or productive purposes. It is, for all practical purposes, an energy loss.

The bottom line is that straw can be used in a winter feeding program, but it requires a substantial amount of supplementation with high-quality feeds. Theoretically, grain could be used as the energy source, but managing grain in a pasture feeding situation carries significant risk (of acidosis). More practical alternatives are grain byproducts such as dried distillers grains (DDG), corn gluten or wheat midds. These products contain significant energy, but are relatively low in starch, and therefore do not carry nearly as high a risk of acidosis. (The energy is primarily derived from highly digestible fibre in the grain seed coat.)

Having said that, DDG and corn gluten contain a significant amount of sulphur, which can be toxic if consumption is not controlled. (Wheat midds do not carry the risk of sulphur toxicity.)

All three of these byproducts are also high in protein and phosphorus and thus can make up for the other deficiencies in straw. DDG normally runs about 30 per cent crude protein (CP); corn gluten 20-22 per cent CP; and wheat midds 15-16 per cent CP. If byproducts are used in conjunction with straw, a (custom) free-choice mineral containing trace minerals and vitamin A would be necessary. The word “custom” was used as most commercial minerals will be high in phosphorus. Since grain byproducts are also high in phosphorus; a conflict is created. Since phosphorus is an expensive mineral, considerable cost savings can be attained through a custom mineral. Beyond that, if steer calves are present and consuming the byproducts and mineral, the excess phosphorus poses a risk of waterbelly (urinary calculi).

Ammoniation. The feeding value of straw can be improved through ammoniation, although the process is not a panacea. Extension publications will warn against the ammoniation of hay, as if the hay contains free sugars and carbohydrates; they will combine with the ammonia to form a toxic compound, resulting in what has come to be known as “crazy calf syndrome.” What you won’t read in the literature is that if straw is ammoniated during the heat of the summer, straw itself can form toxic compounds. Compounds that can apparently cause birth defects. Apparently the chemical reaction is temperature sensitive, as ammoniation during cool, fall temperatures (when most straw is ammoniated) causes no problem.

Ammoniation breaks down the bonds of the lignified fibre in straw and significantly increases the digestibility. Likewise some of the nitrogen is retained in the straw in loose bonds which function as a source of non-protein nitrogen. Anecdotally we often hear that cattle are maintained exclusively on ammoniated straw, but my experience has been they also receive significant amounts of distiller’s or brewer’s grains. In short, I would not want to suggest anyone could substitute ammoniated straw for hay. It could form the primary feed, but I would not be comfortable recommending any kind of straw feeding, without providing some sort of additional concentrated supplement.

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