Precision Mineral Nutrition for Dairy Cows

Introduction

Precision nutrition means diets are formulated as close as possible to requirements while avoiding any deficiency issues. Benefits of precision mineral nutrition can include reduced diet costs, reduced environmental impact, and reduced risk of toxicity. The major problem with precision mineral nutrition is increased risk of deficiency. Deficiency can be manifested as a clinical deficiency (e.g., grass tetany caused by inadequate magnesium), or as numerous subclinical responses including impaired health, reproduction, and production. Diets should never be formulated below the average mineral requirements for the pen. The main question is, how much greater than pen average mineral requirement should the formulation goals be? The answer to that question is farm and mineral specific. The nutritionist job is to consider all factors affecting requirements and absorption of minerals for each specific situation and formulate accordingly.

Mineral Requirements

The NRC (now NASEM) published updated requirements for dairy cattle in late 2021 (NASEM, 2021). All known essential minerals were considered by the committee and then based on data, either requirements or adequate intakes (AI) were established for each mineral. A requirement is the amount of mineral needed to meet the needs of an average animal within a defined population (e.g., the average for 650 kg cows producing 35 kg of milk). To establish a requirement adequate data had to be available to have high confidence in the values. An AI is similar to a requirement except that it means the committee did not have the same degree of confidence because of limited data. Often this was because of limited number of studies or studies that did not include multiple levels of the mineral. An AI is not necessarily the optimal amount that should be fed but in the opinion of the committee, meeting the AI will prevent problems and data are available showing that positive effects occurred compared with feeding lower amounts. For diet formulation a requirement and an AI can usually be considered the same thing. The committee evaluated mineral needs for calves, heifers, and dry and lactating cows but this paper will concentrate on lactating cows. An overview of changes in requirements (or AI) compared with NRC (2001) is shown in Figure 1.

Figure 1. Comparison of requirements of minerals calculated using NASEM (2021) to those calculated using NRC (2001). 0 = essentially no change, - = new requirements are less than previous and + = requirements are greater than previous. The number of symbols indicate the degree of change.

Discussing all the changes made to mineral requirements in the NASEM (2021) is beyond the scope of this paper and they will only be discussed briefly. The NASEM set requirements for calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na), chloride (Cl), sulfur (S), copper (Cu), and zinc (Zn), and AI were established for cobalt (Co), iodine (I), iron (Fe), manganese (Mn) and selenium (Se). For most minerals (exceptions are S, Co, I, and Se) requirements or AI were estimated using the factorial approach. The amounts of absorbed mineral needed for maintenance plus the amount of minerals secreted in milk, retained in new body tissue, and retained in conceptus are summed and then divided by an absorption coefficient (AC) to get dietary requirements or dietary AI. Almost all the equations used to estimate requirements or AI were updated from NRC (2001) but in several cases the final dietary values changed very little for ‘average cows’ fed ‘typical diets’. However, the committee thinks the new equations are more biologically correct and therefore should be better when cows are not average and typical diets are not fed. Compared with NRC (2001) the greatest changes in requirements or AI were for Mg, Cu, and Mn (discussed below).

For most of the other mineral requirements (or AI) did not change greatly from NRC (2001). Because of substantial reductions in the AC for Ca supplements, diets with low basal Ca (e.g., high corn silage diets) will probably require more Ca supplementation than NRC (2001) estimated. Requirement for P and the method used to estimate AC were changed slightly usually resulting in slightly lower dietary requirements. The AC calculation should improve accuracy over a wider range of diets. Zinc requirements were modestly increased by about 15%. The AI for Co is about twice the requirement estimate in NRC (2001); however, many diets likely contain adequate basal Co so this probably will not affect diet formulation greatly.

Significant Changes to Mineral Requirements

Based on newer data, the maintenance requirement for absorbed Mg more than doubled in NASEM (2021) largely because it is now a function of DMI rather than body weight as was done in NRC (2001). Endogenous fecal losses of mineral should be a function of DMI so the new equations are biologically correct and should be more accurate. The AC for basal and supplemental Mg also changed markedly. The greatest change occurred for Mg supplements. In NRC (2001) the AC for Mg supplements were likely calculated incorrectly resulting in very high values (e.g., 70% for Mg oxide and 90% for Mg sulfate. The new values fit available data very well and are much lower (approximately 23% for Mg oxide and 27% for Mg sulfate. In addition, the new software (NASEM, 2021) uses an equation to reduce AC for Mg as dietary K increases. Overall, because of changes in absorbed requirements and AC, dietary requirements for Mg are about twice as high using NASEM (2021) compared with NRC (2001).

Because of increased concerns about Cu toxicity, the committee conducted a very extensive review of Cu requirements. Adequate data were found to estimate metabolic fecal Cu using DMI rather than body weight which resulted in an approximate doubling of the maintenance requirement. This will result in an approximate 40% increase in dietary Cu requirement for dry cows compared with NRC (2001). However, newer data on concentration of Cu in milk (used for lactation requirement) was 0.04 compared with 0.14 mg/L which greatly reduces the lactation requirement. Overall the dietary Cu requirement for cows producing about 35 kg of milk did not change very much but for cows producing 45 kg of milk, NASEM (2021) dietary requirement for Cu is about 45% lower than NRC (2001).

Manganese was also evaluated carefully because pregnant beef heifers fed to meet the NRC (2001) Mn requirement produced calves that were clinically deficient in Mn (Hansen et al., 2006). Adequate data were not available to establish a requirement but rather Mn has an AI. Based on a single experiment, maintenance AI was increased 30% compared with NRC (2001), and some new information on AC of Mn supplements resulted in a substantial decrease in the AC for Mn. Combining both changes, the dietary AI for Mn about doubled compared with NRC (2001).

Formulation Goals

The equations used to estimate requirements or AI are based on more data and the new data better reflects current cows and diets; therefore, the estimates should be more accurate than values from NRC (2001). However, even if the equations were perfect (which they are not because all statistical equations have prediction error) the estimates would still be for the average which means half the cows with similar characteristics would be overfed and half would be underfed. The potential problems caused by underfeeding are almost always more expensive than the costs associated with overfeeding. Several factors must be considered when determining how much overfeeding should be done including cost of supplemental mineral, source of supplemental mineral, maximum tolerable level of the mineral, accuracy of the estimates, variation in requirements and dietary supply and potential problems if inadequate mineral is fed. Assuming typical supplement costs, based mostly on variation in diet composition but also some on variation in requirements (or AI) most diets should be formulated to provided about 20% more than NASEM (2021) requirements. That degree of overfeeding has essentially no toxicity risk, will not add too much extra costs to diets and will meet the needs of most of the cows in a group. For example, if the average Ca requirement for a pen of cow was 110 g/d, the diet should be formulated to provide 110 x 1.2 = 132 g/d at pen average dry matter intake (DMI). This approach will work for most, but not all, minerals and in most, but not all, situations.

Adjustments to standard safety factors

For most minerals on most farms, including a safety factor of 20% above average NASEM requirements (or AI) is appropriate (Table 1). However, there are exceptions; S, P, Se, Cu, and Mg. For S and P, no safety factor is needed or should be applied. Moderately excess sulfur reduces the absorption of Cu, Mn, and Se so in the long term (months) excess S can cause deficiencies of those minerals. In addition, excess S can reduce dietary cation anion difference (DCAD) which is detrimental to lactating cows. Because of the risk of excess S, it should be fed at about the NASEM requirement (i.e., 2 g/kg of diet DM). Diets with excess S are often fed during the prefresh period to lower DCAD and reduce hypocalcemia. These diets are only fed for a few weeks which is not long enough to cause problems with trace minerals. The requirements for P are very well defined and because of recycling within the cow, P deficiencies almost never will occur when animals are fed at NASEM (2021) requirements. A safety factor often cannot be applied to Se because supplementation may be limited by regulation. In most areas of the world, nutritionists should formulate dairy diets to the maximum legally allowable Se concentration.

Formulation goals for Cu and Mg need to be considered carefully. Modest overfeeding of Cu increases liver Cu concentrations which increases the risk of Cu toxicity which is most cases is fatal. This risk should encourage nutritionists to feed very close to the requirement. On the other hand, diets and water often contain antagonists that reduce Cu absorption which should encourage nutritionists to overfeed Cu to a greater extent. The most common antagonists to Cu absorption are excess S (includes both diet and water), excess molybdenum (Mo), and grazing on high clay soils. Dietary S (including S from water) at 3 and 4 g/kg of diet DM may reduce expected Cu absorption by 15 and 30% when dietary Mo is around 1 mg/kg diet DM. If diets contain 5 mg Mo/kg diet DM, Cu absorption may decrease by 18 and 37% for diets with 3 and 4 g/kg S). This means that if diets have 4 g/kg of S and 5 mg/kg Mo, an appropriate safety factor may be close to 1.5 times NASEM requirement. Diets with 4 g/kg S cause other problems so the first goal should be to try to reduce S concentrations in diet and water. If that is not possible you may need to formulate diets to provide 1.5 times as much as the average Cu requirement as calculated by NASEM (2021). That recommendation is for Holsteins. Jersey cattle accumulate more Cu in their liver than do Holsteins fed the same diet. Therefore, Cu should be fed closer to requirement for Jerseys. Although data are limited, based on hepatic accumulation differences safety factors probably should be about half as much for Jerseys as for Holsteins.

Magnesium has a very wide safety margin, so overfeeding is usually not a concern, and several factors can inhibit Mg absorption encouraging overfeeding. In addition, the AC for Mg from Mg oxide (most common Mg supplement) can vary greatly. Magnesium absorption decreases as dietary K concentration increases. This antagonism starts occurring even before the K requirement is met. The NASEM (2021) model includes an equation to adjust the AC for Mg based on diet K so if using that model, the safety factor (i.e., 1.2X) does not need to be increased for higher K diets. Monensin inhibits absorption of Mg from Mg sulfate by about 25% but it increases Mg absorption when Mg oxide is fed (Tebbe et al., 2018). The AC of Mg from Mg oxide averages about 25% but can be close to 0 for some low-quality sources (large particle size, improper calcination procedures). Increased concentrations of dietary fat can inhibit Mg absorption. Considering all the potential antagonists, the wide safety margin, and the problems associated with inadequate Mg, diets should probably include a safety factor of about 1.4X (assuming the AC is already adjusted for high K).

“Non-requirement” effects

Feeding some minerals above requirements can elicit responses such as reduced risk for some diseases or increased milk production or milk components. These effects are considered responses, rather than requirements (i.e., feeding these levels are not necessary to obtain high milk yield or healthy cows). NASEM (2021) did not include a response model for minerals (it does include a response model for amino acids). No requirement or AI was established for chromium (Cr) because data are not available to determine it. However, several studies have shown a positive milk response when transition and early lactation are supplemented with Cr at a rate of about 0.5 mg/kg of diet DM. Feeding Mg at about twice the requirement to dry and prefresh cows reduces the risk of hypocalcemia. To meet the requirements for Na, K, S and Cl of a lactating dairy cow, DCAD (calculated as concentration of Na + K minus concentrations of (Cl +S) where concentrations are in mEq/kg) needs to be about 175 mEq/kg of diet DM. However increasing DCAD above that value is expected to increase milk yield, milk fat yield and DMI ((Iwaniuk and Erdman, 2015). Replacing sulfate forms of trace minerals with hydroxy or certain types of organic trace minerals can increase fiber digestibility (Faulkner and Weiss, 2017) or alter gut microbiome in ways that may improve hoof health (Faulkner et al., 2017). In a meta-analysis, organic trace mineral from a single manufacturer increased milk yield (Rabiee et al., 2010). Dependent on economics and some other factors, feeding extra mineral to elicit a production response can be warranted.

Table 1. Dietary concentrations (DM basis) of minerals that approximately meet the average requirements (NASEM, 2021) for a Holstein cow producing 35 kg of milk, and recommended formulation goals based on expected variation in mineral supply and requirements.