This post is a part of the series An Acre of Sunshine.
Every time I pass in or out of our property I have to open and close two cattle gates as our small one lane road passes through a neighbor's pasture. If our little road saw any more traffic than it does, the road would need to be fenced out of the grassy areas beside them, but for now we often have to slow down and honk to get the cattle to shuffle off to the side of the road. You really get to know the cows when you have to shoo them out of the way on a regular basis.
In the area immediately surrounding our property most of the agriculture consists of raising beef cattle, as well as some draft horses. The soil is too rocky and the hills too steep to allow our area to be economically competitive in growing row crops, but the sloping fields grow grass just fine. The fields on our own farm have been used primarily to support cattle for about the last 50 years. Before that there was a wider variety of agriculture, but these others were mostly abandoned as cattle became the mainstay of the local farms.
For these operations, the farmers are in the grass business every bit as much as the cattle business. Grasses only grow from the spring through the fall, but since cows need to eat during the winter also, farmers must harvest enough grasses to provide for the snowy months. During the summer, technically May to November, cattle are brought to the grasses, to feed on pasture. This reduces the work of the farmer tremendously, as the cows harvest their own food. The farmer does have to fence off the paddocks, ensure water supplies, and move cattle between the fields, but this is less intensive than preparing for the winter months. To provide for the winter food needs, the farmer needs to maintain other grass fields for hay, cutting and baling the growth and setting it aside to be doled out as needed to keep the animals well-fed. The same fields can be used for both haying and pasture, but can only primarily provide for one of these in a given year.
It doesn't seem like that much would be required to grow grass, just to cut down any trees, and then let the cattle come through to eat as they would. But in actuality good pasture is a crop like any other, just that it is a perennial crop that only needs to be replanted every 20 or 30 years, rather than each spring. The usual way of establishing pastures is quite similar to planting row crops. One plows up a field to prepare the soil and kill off competing plants, and then the seeds of a variety of grasses and forbs are planted, with names like alfalfa, orchard grass, Timothy, fescue, and clover. These fields often are helped by the addition of trace nutrients as well as fertilizers. Once the fields grow in, they can be maintained for many years. The degradation of pastures and hayfields can be from nutrient depletion or changes in the species composition of the grasses present. When cows are allowed a lot of space, they work through and eat only the choicest morsels, leaving all of the less desirable plants standing. Over time, these undesirable plants can come to dominate the entire fields, to such an extent that the fields must be plowed and replanted.
Once the growing of the grasses is accounted for, one has to look at the business of actually raising beef cattle. Every year there is a seasonal ebb and flow that takes advantage of the natural cycles of the region. During the winter, the herds are almost completely made up of pregnant females, with just a few bulls that are only there to sire the next generation. In the spring all of the cute little calves are born, drinking only milk for their first weeks of life, transitioning over the summer months to the adult diet of grasses. As soon as the fields are showing good signs of growth, the herd is released out to pasture for the summer. The calves put on an amazing amount of growth throughout their first year of life. In the late fall, at around the same time as the pastures go dormant for the winter, the vast majority of the calves are sold off, most often to a feedlot where the calves will continue to put on weight for up to another year before becoming someone's dinner. The calves that are kept by the local farmers are generally the best females, which become the next generation of mothers. These cows, known as heifers, can be bred when they are as young as 15 months.
The cycle actually begins again during the early summer, as cows have a gestation period of about 280 days. This means that in order for a cow to have one calf each spring, it needs to be bred the prior summer, when the prior calf is only 2-3 months old. This also means that all cows should be pregnant in the fall when the calves are sold off, and very often those cows that didn't conceive are sold at the same time. It turns out that roughly 15% of cows don't get pregnant in a given year, which can be due to age, illness, or just random chance in whether the bull did his job. Cows may continue to breed for 10 years or more before age catches up with them.
Putting all of this together, one needs to grow enough pasture and cut enough hay to maintain a mother cow for the entire year to produce an 8 month old calf for sale. Those calves sold and destined to become beef will be fed mostly grain, including a lot of corn, for the rest of their lives. We won't include this feedlot part of cattle production in our calculations here, though I'll try to return to it at a later time in another post. So how much cow does an acre support?
Estimate #1. First principles
As discussed in 'Energy capture, conversion, and storage, a good first rule of thumb is to assume that each time a new level of an ecosystem consumes energy, that only 10% of that energy goes into the next level. We already made an estimate of the total amount of energy captured by photosynthesis, which in this case would be by the grasses. This energy is then used for metabolism, growth, reproduction, etc., of the plants, and only roughly 10% would of that energy would be available to the cows in the form of leaves and stems. Then the cows of course have their own metabolism and growth to deal with, meaning that only about 10% of the energy that the cows consume will end up in the form of cow flesh, which is what the farmer is most interested in.
35788 kWh/acre/year of photosynthesis * .1 for leaves and stems eaten by cows * .1 for efficiency of cows in turning food into weight gain = 358 kWh/acre/year of cow produced
Estimate #2 Going on available data for actual production
We can also look at typical agricultural yields, and see how much food a pasture typically produces, as well as the data on how efficient cows actually are in their growth and reproduction. I wasn't able to easily track down data for western Quebec, but did find what should be roughly comparable data, from the Manitoba Forage Council. This data shows that pasture produces from 2000 to 4000 pounds of forage per year, depending on plant species, fertilization, and water availability. Lets call it 3000 pounds of dry matter, as it is called, for the sake of calculation. Hay contains roughly 800 Calories per pound, so...
3000 lbs/acre/year * 800 Calories/pound * 1 kWh/860 Calories = 2790 kWh/acre/year of grasses
Further, a cow (pregnant and/or milk producing) requires roughly 30 pounds of food a day, or 10950 pounds through a year. In that same year, the calf will grow from an embryo up to roughly 500 pounds by the time of sale in late fall. The calf primarily drinks milk for the first couple of months, transitioning to the adult diet of grazing over the summer. All told, that calf will consume perhaps 1500 pounds of forage on top of the mother's intake over the summer and fall. Put together, it then takes...
10950 lbs forage (for cow) + 1500 lbs forage (for calf) * 800 Calories/lb * 1 kWh/860 Calories = 11580 kWh to maintain a cow for a year and to produce a 500 pound calf.
Finally, how much energy is harvested out of this system in a year? It is of course all of the calf, but it also ends up being the mother cow, around 15% of the time. As mentioned above, the cows generally aren't kept another year if they do not get pregnant over the summer. These cows average about 1200 pounds. When butchered, about 50% of a cow is meat, distributed over lean and fatty cuts. Some rough estimates suggest that this meat averages around 800 Calories per pound. It was difficult to find data on the embodied energy in the rest of the cow, including entrails, bones, skin, etc., so I will assume that these other parts have the same energy density as the meat. Put together, this means that...
(500 lbs (calf) + 1200 lbs (cow)*.15 (harvest rate of female cows)) * 800 Calories/pound * 1 kWh/860 Calories = 633 kWh of energy per year from raising a cow/calf pair.
The last step is to level out this amount of energy from a cow/calf pair back to a single acre:
2790 kWh/acre/year * 1 cow calf pair/ 11950 kWh * 633 kWh/ 1 cow calf pair = 148 kWh of energy in the form of cow harvested from one acre in a year.
Estimate #3 Actual production from our farm
Finally, we can make an estimate of the productivity of our farm from the actual production that we have observed over the last few years. We have 18 acres of pasture, and have used these fields both as pasture and hayfields over the last five years. In the first couple of years after the purchase of our property, we had one of the farmer neighbors put cow/calf pairs out to pasture on our farm. Since then, another local farmer has cut hay off of the same fields.
When we had cattle on our property, this was a herd of 20 cow/calf pairs which were rotated between our property and another nearby, such that the herd was on our property half of the time. This effectively makes 10 cow/calf pairs for the six month growing season. The calculations from estimate #2 can be adapted, as we know that each cow/calf pair needs 11580 kWh per year:
11580 kWh of forage /cow calf pair * 20 pairs/18 acres * 1/2 of the year * 1/2 of the time = 3217 kWh/acre of forage produced in a year.
In the years since we switched to haying, there has been quite a bit of variability in the weather, including both one very wet as well as one very dry summer. The summer of 2016 was a more average year, and in this year the property produced 50 large round bales of hay from a cut in mid-summer. In many climates farmers can get multiple cuttings of hay from a single field in a year, but with the relatively poor soils and shorter growing season, most of the local fields are cut only once. This does mean that the late summer and early fall growth aren't available for cattle unless the cattle are allowed to graze through later in the season. Below is the calculation for the amount of energy found in those bales, with weight and Calorie estimates for the hay drawn from here and here.
50 bales/18 acres * 1000 lbs/bale * 800 Calories/pound * 1 kWh/860 Calories = 2580 kWh/acre/year of grasses
These two estimates, that our fields produce between 3217 and 2580 kWh, are very much in line with Estimate # 2 so going forward we will go with that the final figure estimated there, of 148 kWh/acre/year of cow being harvested per year from grassy fields.
Visualizing this growth
As discussed above, before getting to cattle one has to have grasses. Below is a picture of a big round haybale, weighing around 1000 pounds. Each acre of our fields can produce about 3 of these per year. Averaged out over the entire year, each acre is growing 7 pounds of grass per day, a big handful.
Round haybale with author and son for scale
In the picture below, the calf is approaching that 500 pound size, typical for when they are sold off in the fall. The mother is still over twice that weight, around 1200 pounds, and stands around 5' tall. To support that mother and calf for the year, it requires about 4 acres of hayfield and pasture.
Growing other animal species, or for other products
The above discussion was all about cow and calf cattle farming, the mostly small scale operations feeding their animals on pastured grass. I didn't address the 'finishing' process for beef cattle, where the calves live in a more constrained environment eating more grains as they put on additional weight and size. Nor did I discuss growing other animals for food, or such products as the milk or eggs that can be obtained from those animals. In terms of the amount of energy that can be converted from sunlight to the end agricultural product, growing beef cattle is one of the least efficient. The adult cows must be maintained for many years, and they usually have only a single calf per year. Further, cows are somewhat less efficient than some other types of animals at converting feed into weight gain. And just look at other examples like chickens or pigs. Each of these produces many more young in any given year, as well as those animals reaching market size much more quickly. The takeaway is that these other types of animal husbandry can produce significantly higher yields per acre. The upside of beef cattle is that they take much less effort on the part of the farmer, they can be raised on land of marginal quality, and there is high demand and therefore a good price for beef. Needless to say, I may try to make a more quantitative comparison in a future post.
Estimate for total cow production: 148 kWh/acre/year
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