Hi and thanks for contacting Ask Extension. If the deer ate the branches back to the main trunk they will not come back even if you spray a soluble fertilizer on them. If there is some green growth left there is hope. Trim off the branches that are bare, brown or beyond repair with a sharp hand pruner. Enhancing plant health by providing adequate irrigation and proper nutrition will also help arborvitae recover over the course of the growing season. Most landscape plants benefit from receiving at least one inch of water each week, either through rainfall or supplemental irrigation. If fertilizing, note that trees and shrubs absorb nutrients readily in the spring before growth starts, or in mid-summer after shoot-growth ceases. A soil test will help you decide what nutrients your soil needs. You can purchase a soil test here https://homesoiltest.msu.edu/ Once you send in your soil sample, Michigan State University will send you a personalized recommendation for your soil.

Most research indicates that a foliar spraying of fertilizer can be used as a "quick fix" for minor nutrient element deficiencies. Foliar feeding is not effective in supplying essential nutrients in quantities necessary for satisfactory growth. Based on your soil test results a foliar spray may or may not help you. The most effective time to spray foliage with micronutrient solutions is just before or during the growth period. Liquid soil injection is the method most often used by professional arborists because it is quick, easy, and also leads to rapid uptake of nutrients. It utilizes high pressure injection of liquid fertilizer into the soil. Injection points should be 2-3 feet apart depending upon pressure and about 8-12 inches deep. Slow-release forms of liquid injection fertilizers are also available. To find an arborist near you go to this website and enter your zip code and it will provide you with a list of certified arborists. https://www.treesaregood.org/findanarborist (ISA Michigan | Directory)

If deer continue to be a problem arborvitae may not be your best choice. The arborvitae variety called Green Giant tends to be more deer resistant.

Here is a link to a good article.

https://extension.unh.edu/blog/2019/04/deer-damaged-number-plants-my-garden-over-winter-what-can-i-do-prevent-happening-again