var recommendations = new Dictionary<String, UseRecommendationModel>();

foreach (var wg in weed.WeedGroups)
{
    var collection = ndb.UseRecommendations.Where(ur => ur.WeedGroups.Contains(wg));

    foreach (var recommendation in collection)
    {
        string preparationName = recommendation.Preparation.Name;

        recommendations.Add(preparationName, new UseRecommendationModel { PreparationName = preparationName, ... });
    }
}

var recommendations = new SortedDictionary<String, UseRecommendationModel>();
var collection = ndb.UseRecommendations.Where(ur => ur.WeedGroups.Any(wg => weed.WeedGroups.Contains(wg)));

foreach (var recommendation in collection)
{
    string preparationName = recommendation.Preparation.Name;

    recommendations.Add(preparationName, new UseRecommendationModel { PreparationName = preparationName, ... });
}

ndb.UseRecommendations
   .Where(ur => ur.WeedGroups.Any(wg => weed.WeedGroups.Contains(wg)))
   .OrderBy(ur => ur.Preparation.Name)
   .ToDictionary(ur => ur.Preparation.Name, ur => new UseRecommendationModel { PreparationName = ur.Preparation.Name, ... });

using System;
using System.Collections.Generic;

namespace SortedDictionaryApplication
{
    class Program
    {
        static void Main()
        {
            // Create a new sorted dictionary of strings, with string 
            // keys.
            var openWith = new SortedDictionary<string, string>();

            // Add some elements to the dictionary. There are no  
            // duplicate keys, but some of the values are duplicates.
            openWith.Add("txt", "notepad.exe");
            openWith.Add("bmp", "paint.exe");
            openWith.Add("dib", "paint.exe");
            openWith.Add("rtf", "wordpad.exe");

            foreach (KeyValuePair<string, string> pair in openWith)
            {
                Console.WriteLine("Key: {0}, Value: {1}", pair.Key, pair.Value);
            }

            Console.ReadKey();
        }
    }
}

public class ViewModel
{
    public IEnumerable<UseRecommendationModel> Suppressions { get; private set; }
}

// Код, что форичем читает данные, причём и ключ, и значение
foreach (UseRecommendationModel value in model.Suppressions)
{
    string key = value.PreparationName;
}

public class ViewModel
{
    public IEnumerable<UseRecommendationModel> Suppressions { get; private set; }
}

// Код, что форичем читает данные, причём и ключ, и значение
foreach (UseRecommendationModel value in model.Suppressions)
{
    string key = value.PreparationName;
}