Editor's Note: The following information is from an article written by Alejandro Castillo with the University of California Cooperative Extension, Merced County. 

"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 ruminal 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, which could have negative environmental effects.

Accurate estimates of mineral excretion by dairy cows are needed for nutrient management plans. Numerous equations have been derived to estimate mineral excretion by dairy cows, but the underlying function for most equations is: Manure excretion of mineral X = Intake of mineral X – milk secretion of mineral X. In most equations, minerals provided by drinking water are ignored, the concentrations of minerals in milk are assumed constant, and book values are used rather than assayed values. The importance of including information on mineral composition of drinking water and assayed mineral concentrations in milk on estimated mineral balances is not known."

In the April 2013 California Dairy Newsletter, Castillo discusses research that took place in Merced County, Calif., on 40 dairy farms.

"The main conclusion of this study indicates that for some farms, ignoring minerals consumed via water and using NRC constants for estimating milk secretion of minerals rather than assayed concentrations introduced significant errors when estimating manure excretion of minerals via the mass balance technique," Castillo says. "Mineral excretion data from our study are not necessarily applicable to other farms; at this time, it is not possible to identify farms that should include measured mineral data for water and milk. Therefore, sampling and analyzing water and milk for mineral concentrations should be considered for all farms that are estimating mineral excretion via mass balance."

Read more in Mineral Excretion in Lactating Dairy Cows.

Source: California Dairy Newsletter, April 2013