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.
Research took place in Merced County, California on 40 dairy farms. Milk yields varied from less than 55 to more than 86 Lb/cow per day, and concentration of total solids in water varied from less than 200 to more than 1490 mg/L. Accounting for drinking water minerals in the diets increased dietary concentrations by <4% for all minerals except for Na and Cl, which increased by 9.3 and 6.5%, respectively. Concentrations of P and K in milk were essentially the same as the National Research Council (NRC, Nutrient Requirement for Dairy cattle, 2001) value to estimate lactation requirement. However, NRC milk values of Ca, Cl, and Zn were 10 to 20% greater than dairy farm values; and Na, Cu, Fe, and Mn were no less than 36% below NRC values. Estimated excretion of minerals via manure varied substantially across farms. Low milk yield farms had 2 to 3 times less estimated mineral excretions than high milk yield farms (depending on mineral). Although including water minerals increased excretion of most minerals, the actual median effect of Ca, Mg, S, Cu, Fe, and Mn was less than 5%, and about 8% for Na and Cl. Replacing assayed concentrations of minerals in milk with NRC constants resulted in reduced estimated excretion of Ca, Na, Cu, Fe, and Zn (Table 4), but median differences were <5% except for Na, which was 7.5%.
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. 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. Although not measured in this experiment, ignoring disappearance of free choice minerals in the mass balance equation is another potential error and should be included when calculating whole farm mineral balances or when developing Nutrient Management Plans.
(1) Based on: " Castillo, A.R., N. R. St-Pierre, N. Silva del Rio and W. P. Weiss. 2013. Mineral Concentrations in Diets, Water, and Milk and Their Value in Estimating On-Farm Excretion of Manure Minerals in Lactating Dairy Cows. Journal of Dairy Science (accepted for publication). For a copy of this article please contact Alejandro R. Castillo (), Farm Advisor - Dairy Science. UC Cooperative Extension, Merced, CA.