Last time i looked at macrobiotics was about 20 or 30 years ago and it seems things have moved on and occasional white fish is allowed. However, it still is based mainly on whole grains and certain veggies and cooking processes and even materials are discouraged. See below. In the UK, GM foods are not yet accepted for growing and the EU has mechanisms for testing imported grain feeds for GM and other unwanted features so I rather feel the horses will just be eating hay and ordinary cereals.
A macrobiotic diet combines elements of Buddhism with dietary principles based on simplicity and avoidance of "toxins" that come from eating dairy products, meats, and oily foods. Older versions of the macrobiotic diet were quite restrictive. One variation allowed only the consumption of whole grains. Current proponents of the diet advocate flexibility but still discourage dairy products, meats, and refined sugars,
The standard macrobiotic diet of today consists of 50 to 60 percent organically grown whole grains, 20% to 25% locally and organically grown fruits and vegetables, and 5% to 10% soups made with vegetables, seaweed, grains, beans, and miso (a fermented soy product). Other elements may include occasional helpings of fresh white fish, nuts, seeds, pickles, Asian condiments, and non-stimulating and non-aromatic teas. Early versions of the diet excluded all animal products. Proponents still discourage dairy products, eggs, coffee, sugar, stimulant and aromatic herbs, red meat, poultry, and processed foods. Some vegetables, such as potatoes, tomatoes, eggplant, peppers, asparagus, spinach, beets, zucchini, and avocados, are discouraged. The diet also advises against eating fruit that does not grow locally (for example, in most of the United States and Europe, bananas, pineapples, and other tropical fruits).