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Adult meat goats must not be fed more than 1% of their body weight in grain. That is, the weight of grain fed must not be more than 1% of the goat's body weight. If the goat weighs 30kg, don't feed more than 0.3kg grain (i.e. 1/100 x 30kg = 0.3kg or 300g). This is
necessary so as to avoid a metabolic disease called acidiosis. For lactating does, grain shud not exceed 1.5% of her bodyweight.

If you feed grains to your goats place a bowl of baking soda. They will occasionally lick it whenever their stomach acidity increase.

Update: When introducing grain in your goats' diet, start with very small amount of grain and increase it very slowly such that in 2 weeks time, the goat will be eating the required amount of grain.
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Also, the grain should not be grinded into flour-like texture to avoid serious digestive problem.