On average it takes 1.25 to 1.44 kg of feed to produce 1 L of milk or 0.63 to 0.72 kg of dry matter, which is the nutrient content of the feed with all the water removed.
Dairy farmers feed their cows to aim to produce 1.4 to 1.6 L of milk per kg of dry matter intake. In early lactation ( less than 100 days in milk), cows will produce an average of 40 L of milk per day and eat 50 kg of feed per day, which averages 50% water and 50% dry matter. So, early lactation cows need 1.25 kg of feed per L of milk, which results in 1.6 L of milk per kg of dry matter intake. In late lactation (over 200 days milking), cows will only produce an average of 25 L of milk and eat 36 kg of feed per day. So, late lactation cows need 1.44 kg of feed per L of milk, which results in 1.4 L of milk per kg of dry matter intake.
In general, dairy cows are fed a total mixed ration, which should contain all the nutrients a cow needs in every bite. A total mixed ration will contain roughly 50 to 60% forage (hay and silage) and 40 to 50% concentrates (grain, protein meal, fat, vitamin/mineral) and each kg of dry matter should break down to 70% carbohydrate, 18% protein, 6% fat and 6% vitamins and minerals.