A cow eats 2.5-3% of its body weight a day. So, in theory, if a cow weighs 1,000 pounds, this means that it will eat 25-30 pounds of grass a day (11-13.5kg), 750-900 pounds a month (350-400 kg), and 9,000-11,000 pounds a year (4000-5000kg).
Typically, bigger, more mature cows will eat more grass than younger cows, and cows at their peak milk production will also consume more grass.
Beef vs. Dairy Cows Grass Consumption
On average, a dairy cow lives 5-6 years before slaughter, and a beef cow lives 1-2 years before slaughter. So naturally, you would think that dairy cows consume significantly more grass than beef cows in their lifetime – but it isn’t quite as straightforward as this.
Beef cows consume mostly grass and hay whereas dairy cows require more variety in their diet to maximize milk production, which also includes the use of various mineral supplementation.
Typically, dairy cows consume about 50% forage and 50% grains whereas beef cows can be 100% grass-fed and grass-finished, which means that they only consume grasses and forages, never anything else.
Do All Cows Eat Grass?
All cows eat grass for the first few months of their existence because all cows graze on grass and spend time in pasture. This means that technically all cows are “grass-fed”.
After the first few months of cows’ lives in which they have eaten nothing but grass, some will go to the feedlot where they are fed a high grain diet while others will remain in pasture.
Cows that remain in pasture and are also “grass-finished”, which means that they have access to pasture their entire lives, and also live longer.
Do Cows Graze Outside All Year Long?
Just because most cows’ diets primarily consist of grass, this doesn’t mean that they are grazing outside on grass all year long. An enormous amount of land and time is required for cows to constantly graze outside.
Even if a farmer has the resources to allow this, not all geographical locations have the climate to support year-round grazing.
How Much Grass Do Cows Eat After Giving Birth?
The amount of grass that a cow eats after giving birth depends on its maturity.
A fully mature cow will consume approximately 22 pounds (10kg) of dry matter grass in its first week after calving. This rises by 2.2. pounds (1kg) per week for eight weeks into lactation.
Heifers that have their first calf will consume less than mature cows. They will eat 18 pounds (8kg) of dry matter grass.
Does Grass Have All the Nutrients a Cow Needs?
Pasture grasses undergo two stages of growth – vegetative growth that occurs in the spring and mature growth that occurs mid-to-late summer.
Generally, it is in spring where there is new growth, and the temperatures and rainfall are ideal that the nutrient content of grass is highest. Later in the year during late fall and winter, mature or dormant grass will have lower energy and protein content, as well as lower digestibility.
In the event of a very long winter, or extremely dry spring or summer, the grass might not have all the nutrients that cows require from their diet. When that happens, it’s common to supplement a cow’s diet with the various nutrients that they are not getting enough of.
The Importance of Rotational Grazing
Considering the immense amount of grass that a cow consumes a year, and also taking into account that even hobby farmers are likely to have more than one cow, surely some sort of grass management is required? You’re absolutely right – and it’s called rotational grazing.
Rotational grazing is when cows are moved from one field to the next in order to always get the best grass that is so important to their diet. Once the cows are moved, the grass and field are given a chance to regrow. The frequency of rotational grazing can differ, though it’s common to rotate every one to two weeks.
If rotational grazing wasn’t carried out then what is known as spot grazing would occur. This is when cows will only graze and eat the grass that they find most palatable. So on a pasture that contains several grass species, a patchy grazing pattern may occur.