This largely depends on how big this cow is and how much milk she's producing, or has to produce to meet the demands of those two calves. It also depends on what breed-type she is.
It also really depends on what the moisture content of the feed is, and how good of quality it is. Generally, the higher the moisture content of the feed, the more that cow will eat (to a certain point). Also, the better the quality the feed is, the more she'll eat (again, to a point).
On a dry-matter basis, a lactating cow (or a cow that is suckling one or two calves) is expected to eat between 2.5% to 3.0% of her body weight per day.
For example, if a cow weighs 1200 pounds and eats 2.5% of her body weight, she's expected to consume about 30 pounds of dry-matter feed per day. If the feed is really good quality, that same cow could eat 36 pounds of dry-matter ration per day.
A bigger cow will always eat more than a smaller cow. If we were comparing the consumption rate of a 900 lb Dexter cow suckling twins versus an 1800-pound Simmental cow also suckling twins, then, once again on a dry matter basis, we would be seeing a range of 22.5 to 27 pounds per day for the Dexter, versus a 45- to 54-pound-per-day range for the big ol' Simmental. Both of these breeds are great-milking breeds, with the main variable of concern being cow size.
THESE CONSUMPTION NUMBERS ARE NOT AS-FED VALUES. This is very important to know!
Dry-matter basis values are a great starting point, but they don't tell you how much a cow will actually eat when she has the actual hay or silage or pasture in front of her nose. They only tell you, based purely on forage/feed quality and not moisture content, how much a cow will eat on average. Removing the moisture content of a feed is done when feeds are tested for nutrients; this is important because it helps ensure as high an accurate of nutritive analysis as possible. Moisture content has a way of skewing numbers to make them be either way higher or way lower than expected.
When we include the moisture content of the feed in terms of how much a cow will eat, that's where we are looking at "as-fed" values.
Thus, if this 1200-pound cow with twins was being fed hay with an 85% dry matter content (or 15% moisture content), then the amount of hay she'd be actually eating is closer to 35 pounds of hay, if she were eating at 2.5% of her body weight in dry matter. Here's how I calculated that out:
1200 lb x 0.025 = 30 lb / 0.85 = 35.29 lb as-fed per day.
If her intake is 3.0% of her body weight, then:
1200 lb x 0.03 = 36 lb / 0.85 = 42.35 lb as-fed per day.
If this cow was consuming silage at 60% moisture (or 40% dry matter), then she would be consuming:
1200 lb x 0.025 = 30 lb / 0.40 = 75 lb as-fed per day.
And again, if she were eating more (3% of her body weight) of the same feedstuff:
1200 lb x 0.03 = 36 lb / 0.40 = 90 lb as-fed per day.
If this was pasture at 80% moisture (20% dry matter), using the same calculations above, she would be eating between 150 to 180 pounds of pasture forage per day.
All these calculations are easy to do; the main variables are cow size and moisture content of the feed.
If you want to get a whole lot more in-depth with building rations, there are computerized rations built for that exact purpose, such as CowBytes v. 5.32 available with Alberta Agriculture & Forestry (via the Government of Alberta).
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
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