Feeding for designer manure?

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Macro minerals (sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg) are important nutrients in dairy rations that must be fed at an appropriate level to optimize animal production and health. Overfeeding leads to increased excretion in feces and urine, and over-application can lead to excessive concentrations in soils which impact crop production.

In a paper (Impact of Macro Mineral Nutrition (Na, K, Mg, Ca) of Dairy Cows on Nutrient Excretion) presented at the 2014 Idaho Nutrient Management Conference on March 6, University of Idaho Extension dairy specialist Rick Norell and fellow researcher Mireille Chahine discuss nutrient balance studies for evaluating mineral excretion, methods for estimating excretion on farms, and procedures for managing nutrient excretion on farms.

Several practices are available to assist in managing macromineral excretion on the dairy.  Accuracy in ration formulation, accuracy in feed preparation, and feeding management will play critical roles in controlling macromineral excretion on dairies. It will involve close teamwork between dairy owner, dairy nutritionist, and feeding crew on the farm.

Key practices for managing excretion are discussed below.

  • Identify “major suppliers” for feed analysis. There are two key factors for determining when a feedstuff should be analyzed: 1) How variable is the nutrient content of the feed? 2) How much of the dietary nutrients are provided by the feed? Forages are highly variable in mineral content and provide 50 to 75% of the dietary intake of Ca, K, and Mg. Clearly all forages should be analyzed for macromineral content. Most other feedstuffs are likely to be small contributors for each specific nutrient and thus do not need as frequent chemical analysis. Commercial feed additives and mineral supplements will have a guaranteed feed analysis and not require additional testing.
  • Analyze feeds using wet chemistry methods. Mineral concentrations are best measured in the lab by wet chemistry methods.
  • Create a historical library of feed analysis and manure. Tracking feed analysis over time will provide sufficient data to identify average composition for a dairy’s common feed suppliers and to identify lots of feed with unusually high or low nutrient content.
  • Analyze water supplies. Livestock water supplies typically have a very low concentration of macrominerals and provide only a few grams of each mineral per day under normal water consumption rates. However, “worst case” water supplies can provide 50% or more of the animal’s daily requirements for that nutrient. Ration adjustments may be required if the livestock water supply has high mineral content.
  • Formulate diets to meet nutrient requirement plus some overhead. Dairy cows that consume inadequate amounts of essential nutrients can suffer from a host of health problems and often have reduced milk production and reduced reproductive efficiency. Because of numerous uncertainties associated with mineral nutrition, including variation in mineral concentrations of TMR feedstuffs, the lack of information regarding mineral absorption, and potential antagonism with other minerals, diets are often formulated so that mineral intake by cows exceeds mineral requirements. Overfeeding minerals can inflate feed costs, reduce absorption of other minerals because of increased antagonism, and have adverse effects on rumen microbes and the cow (i.e., toxicity). Even if overfeeding minerals has no negative effects on the cow or feed costs, it will certainly result in greater manure excretion of minerals.
  • Avoid free choice feeding of mineral. Supplemental salt and phosphorus minerals are commonly fed free choice in a section of the feed manger due to their relatively low cost and as insurance that the cow’s needs are met. However, intake of free choice mineral varies widely between cows and is best managed by proper supplementation through the TMR rather than free choice feeding. Utilizing salt blocks will decrease daily free choice consumption over loose granular salt (NRC 2001) if some free choice feeding “must” be practiced.
  • Evaluate DCAD in dairy rations. Dietary cation-anion difference, or DCAD, is a composite measure of mineral intake that is used in formulating diets for dry and lactating cows. In close-up dry cows, a negative DCAD can help prevent metabolic problems and in lactating cows, a positive DCAD can help increase milk production and milk components. The most common equation to determine DCAD is based on the dietary concentration of the cation minerals Na and K.
  • Manure handling impacts mineral distribution in manure storages. Dairies that handle manure by flushing with recycled flushwater have more K and Na in the primary storage lagoon and less in solid manure than dairies that handle manure by scraping. Thus, there is a greater potential for salt accumulation in the crop ground used to land apply liquids from the primary storage lagoons on flush dairies than on scrape dairies.

Norell, R. and Chahine M. (2013). Impact of Macro Mineral Nutrition (Na, K, Mg, Ca) of Dairy Cows on Nutrient Excretion. Idaho Nutrient Management Conference Proceedings in Twin Falls, ID. Volume 7: 23-32. March 6, 2014.

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