Yes, you can use too much horse manure. Horse manure is a great fertilizer for your garden or lawn; however, it's important to be careful when applying it. Too much of anything isn't good, and the same goes for horse manure – if you apply too much of it in one go then you will run the risk of over-fertilizing your soil and causing an imbalance which could damage or even kill off plants. As with any fertilizer, carefully follow instructions on how to safely use horse manure and make sure not to exceed the recommended amount. Additionally, consider mixing other organic matter into the soil as well such as composted leaves or grass clippings in order to create a more balanced environment that won't be damaged by an overabundance of nutrients from just one source like horse manure.