Zinc is important for macronutrient metabolism, immune function, gene and hormonal regulation and cell signaling. Also, Zn is key component of more than 200 enzymes. In ruminants, molecular and cellular mechanism of absorption of dietary Zn have not been studied yet. In other species, Zn absorption occurs throughout the small intestine by two different mechanisms: a saturable, transport-mediated absorption system (ZIP transporters) and non-saturable diffusion. When Zn is supplemented at low levels, the ZIP transport systems is important, whereas if Zn is fed at high concentrations, diffusion is likely to predominate because of transporter saturation and downregulation.
According to NASEM 2021, measuring true absorption or relative availability of Zn is difficult. Firstly, because fecal excretion of Zn is used to maintain Zn homeostasis, and good markers for Zn are not available. There are several publications on RBV of available Zn sources, but in most of these studies, more Zn than the amount required were used, making conclusions difficult. The AC for ZnCl,2 ZnSO4 and ZnCO 3in the previous NRC 2001 guideline was 0.20 and was not changed. For ZnO, the AC was changed to 0.16 based on RBV studies.
In the NRC 2001 guideline, maintenance requirement for absorbed Zn was set at 0.045 mg/kg of body weight (approximately 27 mg/day for a Holstein cow), the pregnancy requirement (during the last 90 days of gestation) was set at 12 mg/day, the growth requirement was set at 24 mg/kg of growth and the lactation requirement was set at 4 mg/kg of milk. As result, an average Holstein cow producing 32.5 kg/d of milk and assuming dietary AC, diets with 40 to 45 ppm will meet the requirement. For a dry cow, diets with about 23 ppm will meet the requirement.
Now, in the new NASEM 2021 version, a factorial approach was chosen to determine the dietary Zn requirement. No new data were available on Zn uptake of the conceptus, but new data on urinary and endogenous fecal losses were available.
New NASEM 2021 equations for Zn: Zinc AI, as mg absorbed Zn/d =
For maintenance = 5.0 X DMI (kg)
For growth = 24 X ADG (kg/d)
For gestation (> 190 d pregnant) = 0.017 X BW (kg)
For lactation = 4 X Milk (kg/d)
Maintenance requirement for total absorbed Zn was increased but the AC for basal Zn was also increased. Now, a 650-kg 50 kg milk yield Holstein cow, DMI about 29-30 kg, will require 350 mg/d of total absorbed Zn (vs. 230 previously) and a dry cow (270 days of gestation), about 13 kg of DMI will require 65 mg/d of absorbed Zn. In this case, a total diet containing 60 and 25 mg/kg of Zn will fulfill the requirements for lactating and dry cow, respectively.
NASEM 2021 Adequate Intake recommendation for Iodine (I)
Most sources of I are readily absorbed. Iodides of Na, K and Ca are commonly used. Concentration of I in soil is variable. In the 2001 NRC Guideline, the primary determining factor for thyroxine secretion rate (TSR) was BW. Therefore, the AI for maintenance for all dairy groups (dry cows, lactating cows etc.) were based on BW. I is also secreted in milk, therefore, an AI for lactating dairy cow was added and set 0.1 mg/L of milk.
New NASEM 2021 equation for I is: Iodine AI (mg/d total I) = 0.216 X BW (kg)0.528 + 0.1 X Milk (kg/d)