Final answer:
The common rule for feeding growing horses and lactating mares is to offer a higher protein and mineral rations to meet their increased nutritional needs. This includes a balanced diet with sufficient calories, protein, vitamins, and minerals, with special attention to essential amino acids and trace minerals.
Step-by-step explanation:
The common rule of thumb for feeding growing horses and lactating mares is to provide a diet with higher protein and mineral rations. Growing horses have higher nutritional requirements to support their rapid development and growth. Similarly, lactating mares need additional nutrients to produce milk and maintain their own body condition. It is important to ensure that the feed is of good quality and that the diet is balanced with the right amount of calories, protein, vitamins, and minerals.
For a growing horse, the diet should contain about 14-16% crude protein along with adequate levels of essential amino acids. Trace minerals such as calcium, phosphorus, magnesium, and vitamins like vitamin A, vitamin E, and vitamin D are also important. For lactating mares, an even higher protein content might be necessary, especially in the early stages of lactation.
While providing unlimited hay might be a good practice to ensure horses have enough fiber and are not left without food for extended periods, it does not have the concentrated nutrients that growing and lactating horses require. Therefore, simply providing unlimited hay does not meet the guideline.