A review of the new NASEM 2021 Nutrient Requirements of Dairy Cattle

The NASEM 2021 Nutrient Requirements of Dairy Cattle were presented during a meeting from August 30 to September 2. The meeting was hosted by the American Dairy Science Association. Davi Brito de Araujo, DVM, Global Trace Mineral Program Manager in Trouw Nutrition reviews the important changes that have been made and provides some guidance on how to implement them. The content of this article is based on presentations by the NASEM Committee members and other speakers during the conference. Additionally, information from the previous NRC (2001) Nutrient Requirement for Dairy Cattle and NRC (1989) Nutrient Requirement for Beef Cattle were used.

It is well understood that a large number of nutrients are required for dairy cattle to support health, growth, reproduction, and performance. Feeding diets that provide adequate, but not excessive amounts of nutrients can improve profitability of dairy farms and reduce their environmental impact.

Two important parameters for trace minerals are included in the new NASEM 2021 guidelines.

Trace mineral requirement (Req): the daily requirement is defined as the average trace mineral nutrient intake required to meet the needs of half of the healthy individual animals in a specific gender group at a certain stage of the production cycle.

Adequate trace mineral intake (AI): the adequate trace mineral intake is defined as the average daily nutrient intake that a group of experts has defined in case only a limited amount of experiment data is available. In other terms, AI is used when requirements cannot be identified.

The new NASEM 2021 guidelines include nine trace minerals that are considered essential for dairy cattle:

• Chromium (Cr)
• Iodine (I)
• Molybdenum (Mo)
• Cobalt (Co)
• Iron (Fe)
• Selenium (Se)
• Copper (Cu)
• Manganese (Mn)
• Zinc (Zn)

Compared to the 2001 NRC Guidelines, the recommendations for Cu, Mn and Zn changed considerably. Recommendations for Cr, I and Co changed, but not dramatically, whereas the recommendations for Fe and Se stayed the same.

In ruminants, Co is a precursor of vitamin B12 or cobalamin in the rumen. Part of dietary Co can be absorbed as cation form. Once absorbed, it can’t re-enter the rumen, making it unavailable for microbes. Most of this Co is excreted in the urine, and a smaller amount is found with the bile.

The NRC 2001 requirement for Co was set at 0.11 ppm of dietary DM, resulting in an intake of approximately 1.2 mg/day for a dry cow and 2.4 mg/day for a lactating cow. This requirement was based on the amount of dietary Co required to maintain plasma levels of vitamin B-12 above 0.3 µg/L.

Since the NRC 2001 guidelines were published, new research in beef cattle suggests that total dietary intake of Co should range from 0.13 to 0.25 mg Co/kg of DM. In addition, a few production response studies in dairy have been published. The lowest concentration evaluated in those studies was 0.20 mg Co/kg of diet DM. Based on these data, the adequate intake of total Co was set at 0.2 mg of Co/kg of DM.

New NASEM 2021 equation for Co: Cobalt AI (mg/d total Co) = 0.2 X DM (kg)

Typical diets without supplemental of Co will contain about 0.1 ppm of Co, therefore the addition of 0.1 to 0.2 ppm of supplemental Co should be adequate. Diets that provided more than 0.4 ppm Co did not influence vitamin B-12 or Co status in dairy cows.

Cu is a component of several proteins, including cytochrome C oxidase, cytosolic superoxide dismutase, lysyl oxidase and tyrosinase. Additionally, Cu is required for hemoglobin synthesis and iron metabolism.

Intestinal absorption of Cu in mammals is upregulated when diets with low amounts of Cu are fed and downregulated in case of high intakes of Cu. Age of the animal, chemical form of dietary Cu and the presence of antagonist affect intestinal absorption of Cu.

In the NRC 2001 guidelines, a nutrient requirement model was used to establish Cu requirements for dairy cattle. The maintenance requirement for absorbed Cu was set at 0.007 mg/kg of body weight (approximately 4.3 mg/day for an average cow), the growth requirement was set at 1.15 mg/kg of growth, the lactation requirement was set at 0.15 mg/kg of milk, and the pregnancy requirement was set at between 0.5 mg/day (less than 100 days in gestation) to 2.0 mg/day (more than 225 days in gestation). To meet NRC 2001 Cu requirements, diets for dry and lactating cows needed to contain about 13 to 15 ppm of Cu.

In the current NASEM 2021 Guidelines, the effect of the copper source and its bioavailability has been discussed. Cu levels in most of the common feedstuffs range from 4 to 15 mg Cu/kg DM, and true absorption of Cu in those feeds was set at 0.05. The absorption coefficient (AC) for Cu from CuSO4 was set at 0.05 in the previous edition and the same value was retained in the NASEM 2021 guidelines.

The AC for other Cu supplements was evaluated on relative bioavailability (RBV) studies using liver Cu concentration as a parameter when available. Inconsistent results were found, so in the new NASEM 2021 Guidelines, the AC of generic commercial Cu supplements is also set at 0.05. In this case users of supplements other than CuSO4 will have to modify this value based on available data for the specific product and situation.

The Cu requirements for maintenance in the NASEM 2021 guidelines increased to almost double (0.0145 mg Cu/kg BW vs. 0.007 from previous version) whereas the requirements for lactation reduced dramatically from 0.15 to 0.04 mg/kg of milk.

New NASEM 2021 equations for Cu: Copper requirements, as mg absorbed Cu/d =

• For maintenance = 0.0145 X BW (kg)
• For growth = 2.0 X ADG (kg/d)
• For gestation (90 to 190 d pregnant) = 0.0003 X BW (kg)
• For gestation (> 190 d pregnant) = 0.0023 X BW (kg)
• For lactation = 0.04 X Milk (kg/d)

For example, the total requirement for absorbed Cu for an average lactating Holstein cow, producing 35 kg of milk, 650 kg BW and 150 days pregnant is 11.0 mg/d (vs. 11.4 mg in the 2001 NRC). Assuming a DMI of 23 kg and a dietary AC of 0.045, a dietary Cu concentration of about 11 mg/kg DM would meet this requirement. On the other hand, the Cu requirement for a dry pregnant Holstein cow (700 kg BW; 260d of gestation) is 11.7 mg/d, or approximately 20 mg dietary Cu/kg DM, assuming a DMI of13.5 kg.

Mn is a cofactor in several enzymes and proteins, and is required for normal metabolism of lipids, carbohydrates and amino acids. Out of all trace minerals, the requirements for Mn are the least well defined. Concentrations in body tissues and fluids, as well as the absorption from the gut are extremely low, making accurate measurements difficult. Data regarding absorption of Mn from basal feeds or from inorganic sources of Mn other than MnCl2 are not available.

Relative differences in AC of supplements are the same in the new NASEM 2021 Guidelines compared to the NRC 2001, but actual values were reduced by half. The AC of Mn from MnCL2 and Mn2SO4 were changed to 0.005 (vs. 0,01 from last version) and basal ingredients were assigned an AC of 0.004 (vs. 0.0075 from last version). Additionally, the ACs for Mn carbonate and Mn monoxide were set at 0.0015 and 0.003, respectively, based on RBV studies.

In the previous NRC 2001 guideline, the maintenance requirement for absorbed Mn was set at 0.002 mg/kg BW (around 1.2 mg/day for an average Holstein cow), the growth requirement was set at 0.7 mg/kg of growth, the pregnancy requirement was set at 0.3 mg/d and the lactation requirement was set at 0.03 mg/kg of milk. As result, for a typical Holstein cow producing 32.5 liters of milk, the total requirement for absorbed Mn was about 2.2 mg/day and for a dry cow it was about 1.7 mg/day. Assuming typical dry matter intakes for that production and based on established AC, dairy cattle diets containing 7 to 18 ppm of Mn would meet the NRC 2001 requirement.

New NASEM 2021 equations for Mn: Manganese requirements, as mg absorbed Mn/d =

• For maintenance = 0.0026 X BW (kg)
• For growth = 2.0 X ADG (kg/d)
• For gestation (> 190 d pregnant) = 0.00042 X BW (kg)
• For lactation = 0.03 X Milk (kg/day)

In the current NASEM 2021 update, the requirements for lactating and dry pregnant cows are substantially greater than the previous NRC 2001. A late-gestation 700-kg Holstein dry cow requires 1.8 mg/d of absorbed Mn for maintenance, plus 0.3 mg/d for gestation, resulting in a daily requirement of 2.1 mg absorbed Mn. A 650-kg Holstein 45 kg/d milk yield cow requires 1.7 mg/d for maintenance and 1.35 mg/d for lactation, resulting in a total requirement of 3.1 mg/d absorbed Mn. Assuming an AC of 0.004 and a daily total DMI of 12.5 kg and 26 kg/d, a concentration of 40 mg and 27 mg total Mn/kg of diet DM would meet the requirement of a dry and a lactating dairy cow, respectively.

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)

NASEM 2021 recommendation for Chromium (Cr) Cr is a component of a small peptide called chromodulin which is responsible for enhancing the action of insulin. Requirements or AI have not been quantified for cattle. Reliable data on Cr concentrations in feeds are difficult to obtain because Cr concentration are very low. Presently, he only approved Cr supplement that can be fed to cattle in the U.S.A. is chromium propionate, and the maximum legal rate is 0.5 mg Cr/ kg of DM. Although, adequate intake for Cr in lactating dairy cows cannot be established due to lack of titration studies, supplementation of 0.01 mg Cr/kg BW often increases milk yield in early lactation, and can improve immune function in beef and dairy cattle.

NASEM 2021 recommendation for iron (Fe) Fe deficiency is very rare in adult cattle, therefore, and accurate indicator of Fe status is not available. The NASEM 2021 recommendations for iron are identical to the NRC guidelines from 2001. A 650-kg cow producing 25 kg of milk/d at 205 days of gestation and consuming about 20 kg/d DM needs to absorb only 41 mg Fe/d. With an AC of 0.1, it needs to or be fed a diet containing only 20 mg Fe/kg DM. Milk fed calves are the only group of cattle that require Fe supplementation. About 50 mg Fe/kg of diet is adequate to maintain physiological function in growing veal calves.

NASEM 2021 recommendation for Molybdenum (Mo) Mo is a cofactor of xanthine oxidase, aldehyde oxidase and sulfite oxidase. Deficiencies have not been reported and supplementation of Mo is not suggested. Because of antagonistic effects of Mo on Cu absorption, clinical signs of Mo toxicity are similar to those of a Cu deficiency. Increasing dietary Cu concentration will usually fix clinical signs of Mo toxicity.

NASEM 2021 recommendation for Selenium (Se) The previous 2001 NRC guidelines defined Se requirement as 0.3 mg/kg of dietary DM for all classes of dairy cattle. No changes were made for the NASEM 2021 Guidelines. Current FDA regulations limit Se supplementation to 0.3 mg/kg of diet, which is sufficient to maintain Se status in most dairy cattle.

The new NASEM 2021 Guidelines are providing key updates on trace mineral requirements, based on adequate intake. Knowledge on trace mineral levels in the total diet but also of individual diet ingredients as well as knowledge about trace mineral is essential to formulate a correct diet. Comparing to the NRC (2001) Nutrient Requirement for Dairy Cattle, changes in the recommendations related to Co, Cu, Mn and Zn in particular are extremely relevant, to ensure high quality intake of essential trace minerals.