(affirming in part, dissenting in part).
I agree with the majority opinion that, under state law, Reichmann Land and Cattle, LLP (Reichmann) must obtain a State Disposal System permit and that Reich-mann received due process of law in the proceedings before the administrative-law judge. I disagree with the majority concerning the federal permit, however, because I believe that the federal regulation governing a Concentrated Animal Feeding Operation is ambiguous and that the agency’s interpretation must therefore be given substantial deference. Under the correct deferential approach, Reichmann’s winter cattle-feeding operation is a Concentrated Animal Feeding Operation that is required to obtain a National Pollutant Discharge Elimination System permit before it may discharge manure into Minnesota waters. I respectfully dissent from the portion of the court’s opinion that concludes that federal oversight is not required.
In reviewing the relevant federal regulation, we give no deference to the agency’s interpretation when “the language of the regulation is clear and capable of understanding.” In re Cities of Annandale & Maple Lake NPDES/SDS Permit Issuance, 731 N.W.2d 502, 514-15 (Minn.2007) (quotation omitted). But when the relevant language is “unclear or susceptible to different reasonable interpretations, i.e., ambiguous, we will give deference to the agency’s interpretation and will generally uphold that interpretation if it is reasonable.” Id. at 515.
The determination of whether words or phrases are unclear or reasonably susceptible to more than one reasonable interpretation, however, “does not depend on a *51reading of those words or phrases in isolation, but relies on the meaning assigned to the words or phrases in accordance with the apparent purpose of the regulation as a whole.” Id. at 517. Thus, our supreme court has said that “the meaning of statutory language, plain or not, depends on context.” Id. at 518 (quotation omitted). Accordingly, this regulation must be interpreted “within the context of the [Clean Water Act],” a law that “vests in the [Environmental Protection Agency] and the States broad authority to develop long-range, area-wide programs to alleviate and eliminate existing pollution.” Id. at 518, 520.
Here, the parties agree that Reichmann exceeds the size criterion of 1,000 head of cattle that is one of the key components of a Concentrated Animal Feeding Operation. See 40 C.F.R. § 122.28 (2013). And no dispute exists as to whether the animals spend at least 45 days on the land in winter. See id., at b(l). The critical dispute is whether this phrase of the regulation governing an Animal Feeding Operation is met: “[c]rops, vegetation, forage growth, or post-harvest residues are not sustained in the normal growing season over any portion of the lot or facility.” Id., at b(1)(ii).
Interpreting this phrase in the context of the Clean Water Act, I believe that subdivision (ii) of 40 C.F.R. § 122.23 b(l) is unclear and susceptible to different reasonable interpretations about whether it applies to winter feeding operations. Reiehmann’s proposed reading asserts that “sustained” means “maintained” and because the record shows that he maintains “crops ... in the normal growing season” on the lot in question, the winter feeding operation is not an Animal Feeding Operation. While this interpretation is not unreasonable under a plain reading of those precise words, it does not consider other terms in the regulation and it eviscerates federal protection of the environment during the winter months. Under this proposed reading, a farm operation could feed thousands of cattle for many months in the winter without any federal oversight of discharges as long as the farmer planted a crop in the spring. Given that the purpose of the regulation is to define when a certain number of animals confined for a certain number of days creates environmental problems significant enough to require regulation, total exclusion of the winter months from the Clean Water Act defies explanation.
While we may not disregard the letter of the law “under the pretext of pursuing the spirit,” Annandale, 731 N.W.2d at 516, the plain language of the regulation is unclear when one analyzes all the regulation’s terms. The commissioner’s interpretation of “sustained” to mean “to keep up, to carry or to withstand (a weight or pressure),” is also a plausible reading of the regulation, and is one that preserves the protections of federal law whenever the requisite numbers of cattle are feeding on a confined lot for the required numbers of months. By including the word “sustained,” the Environmental Protection Agency intended that the vegetation be examined when there is a force acting on it — a thousand head or more of cattle — to see whether a vegetative cover exists that will protect the waters of the state.
In addition, despite the language “in the normal growing season,” the inclusion of other terms further makes the timeframe of the regulation unclear. For example, “forage growth” supports the commissioner’s assertion that the relevant examination of the vegetative cover on the land takes place when the animals are present. The term “post-harvest residue,” in conjunction with “normal growing season,” *52adds to the uncertainty about the pertinent timeframe.
Because the regulation is susceptible to different reasonable interpretations, and, at a minimum, the definition proposed by the Minnesota Pollution Control Agency is not clearly erroneous, we appropriately defer to the Environmental Protection Agency’s promulgated preamble to the rule at issue. See Auer v. Robbins, 519 U.S. 452, 461, 117 S.Ct. 905, 911, 137 L.Ed.2d 79 (1997) (holding that deference to an agency’s interpretation of its own regulation is controlling unless “plainly erroneous or inconsistent with the regulation” (quotation omitted)); Univ. Med. Ctr., Inc. v. Sebelius, 856 F.Supp.2d 66, 83 (D.D.C.2012) (holding that when preamble confirms interpretation of regulation, “substantial deference” is required). The contemporaneously published preamble states that the regulation specifically applies to wintering operations.
The agency’s interpretation provides that, “in the ease of a winter feedlot, the ‘no vegetation’ criterion in the [Animal Feeding Operation] definition is meant to be evaluated during the winter, when the animals are confined.” 68 Fed.Reg. 7176, 7189 (Feb. 12, 2003). The Environmental Protection Agency further noted that it “assumes that [Animal Feeding Operations] and permitting authorities will use common sense and sound judgment in applying this definition.” Id.
This interpretation is consistent with the language of the rule and it furthers the purposes and principles of the Clean Water Act. Accordingly, given the deference owed to an agency interpreting its own regulation,4 I conclude that Reichmann’s winter feeding practice is an Animal Feeding Operation. See 40 C.F.R. § 122.23(b)(1). Because the winter feeding fields meet that definition, Reichmann is running a Concentrated Animal Feeding Operation that requires a federal NPDES permit before it may discharge waste into Minnesota waters. See 40 C.F.R. § 122.23(b)(2), (d)(1). For these reasons, I would affirm the decision of the commissioner in all respects.
. The Minnesota Pollution Control Agency is charged by state and federal law with the day-to-day responsibility for enforcing and administering this portion of the Clean Water Act in Minnesota. The regulation at issue is properly characterized as the pollution control agency’s own regulation. See Annandale, 731 N.W.2d at 516.