A Pasture Is A Terrible Thing To Waste
By Dr. Hubert karreman, VMD
Added May 19, 2014. Cows grazing pasture can enjoy live, fresh feed loaded with vitamins, minerals, protein, fiber, fats, and carbohydrates as well as immune enhancing essential oils, tannins, flavanoids, terpenoids, polyphenols and carotenoids. They thrive on pasture since it is what they are biologically programmed to eat. They get to exercise in fresh air. It’s also deeply satisfying for us to watch and listen to a herd of cows grazing.
Would you believe that a Holstein herd of 50 cows could get 100% of its daily dry matter intake needs by grazing a total of 1.3 acres of pasture that day? It’s true - if there is 1600 lbs/acre of dry matter available in the pasture for them to eat. How about 42 Jerseys getting 70% of their daily dry matter intake while grazing less than 1/3 acre for 12 hours? They can - if there’s 2000 lbs/ac dry matter available there for them to eat. If you’ve been giving excessively large paddocks in relation to estimated dry matter intake from pasture, correctly sizing paddocks will likely free up ground to make hay or baleage.
Sizing paddocks is a two-step process:
Estimating DMA requires:
With the above in mind, you are well on your way to providing the amount of pasture you desire your cows to be getting, without wasting or skimping.
Steps to estimate dry matter available per acre of pasture (DMA)
Dry Matter Available per acre
1 sq.ft. fresh pasture (ounces) x 456 = Dry Matter Available per acre
1 sq.ft. fresh pasture (grams) x 15.2 = Dry Matter Available per acre
The above conversion numbers take into acount expanding the grams or ounces of 1 sq. ft. to pounds of forage dry matter in 1 acre, as well as taking into account the average dry matter content of 20% for fresh pasture and an 80% utilization rate when using new paddocks every 12 -24 hours. This conversion number is only for nice green carpets of pasture – it does not take into account lots of bare ground, boulders or other unusable areas.
If you can eyeball the yield of a pasture field in dry matter pounds per acre, then you don’t need to do direct clippings. However, very few individuals can do that. Even if you can, it is good to do clippings to reassure yourself of estimates.
Factors that modify DMA
Utilization rate (50-80%) | Bare ground (% observed) |
Unedible, ungrazable areas (% observed)
The utilization rate is based on NRCS work done in Pennsylvania. The utilization rate decreases relative to the time that the animals are in the same paddock, due to trampling, increased number of manure paddies and urinating on plants – all factors in that decrease plant palatability.
For instance, a rocky pasture with patches of bare ground and weeds needs to be taken into account when tryiing to figure out dry matter availability. For example, if a group of cows will be grazing a field for 7 days that has 10% bare ground and 15% boulders, the initial 1 square foot of weighed fresh pasture would still be multiplied by the standard 20% (0.20) to get dry matter content - but then multiply by 50% (0.50) utilization rate and then multiply for bare ground ( x 0.10) and boulders ( x 0.15). Then multiply by (x 95) if grams or (x 2850) if ounces to convert the 1 sq.ft. area to arrive at dry matter available per acre.
Sizing paddocks: animal size, DMA and pasture DMI
Paddock Size = DMI divided by DMA
Recall the two examples from the begining: (1) the 50 Holsteins getting 100% of dry matter intake from 1.3 acres of pasture, with 1600 lbs dry matter available per acre and (2) the 42 Jerseys getting 70% dry matter intake from pasture on less than 1/3 acre for 12 hours, with 2000 lbs dry matter available per acre. How did we arrive at paddock size?
Holsteins in the example herd are estimated to weigh an average of 1200 lbs. From the table, that size animal would eat 42 pounds of total dry matter per day. Multiply dry matter needs for one cow x 50 cows. You will need to provide 2100 pounds of feed for the day for the herd (from any source). The pasture has 1600 lbs DMA.
Paddock size = 2100 needed /1600 available = 1.3 acres/day or 0.65ac/12 hrs.
Jerseys in the example herd are 1100 lbs on average. From the same table, that size animal would eat 38.5 pounds of total dry matter in a day. Multiply dry matter needs for one cow x 42 cows. You will need to provide 1617 pounds of feed for the herd (from any source). 70% of that is desired from pasture. So 1617 x 0.70 = 1132 lbs from pasture needed. The pasture has 2000 lbs DMA.
Paddock size = 1132 needed /2000 available = .57 acre/day or .28 acre/12 hrs.
The “summer slump” of hot dry weather affects farmers everywhere. Most native cool season pastures will go dormant, leaving essentially no grazable ground. The absolute best way to keep your animals grazing throughout the summer is to plant a warm season annual like sorghum sudan grass. This plant grows well all the way into Vermont. It provides lots of dry matter availability per acre. It grows best at precisely the worst time for native pastures – July and August. In southeastern PA it should be planted the last week of May about ½” deep and when the soil is at least 60°F. In many of my measurements, only about 1/8 of an acre of sorghum sudan per 12 hour period will provide the minimum 30% dry matter intake from pasture for 40-cow organic Holstein herds.
Averages from southeastern PA
In the 200 pasture samples for paddock sizing that I’ve done over the last 3 years, it was observed that the best quality and quantity for grazing is the height of pasture when you would make hay. This follows common sense in a big way as well. For example, when clover would be 7-8”, orchard grass 12-18” and alfalfa around 12-14”. This gave an average of about 250lbs/inch/acre. Incidentally, the brix (plant sap sugar) values were also the highest at the time to make hay: Grasses 4-7, Legumes 10-12, Sorghum up to 20. The higher the brix level the better - the sweeter, more palatable and more energy within the plant.
Water and Back Fencing
Keep in mind that cows will drink between 10-30 gallons per day, depending on size, stage of lactation and season. They must have free access to plenty of good water in order to maximize milk production. Also, back fencing with a simple poly wire is needed so recently grazed paddock plants can start their re-growth. Cows should only be in the paddock that they are put into for fresh grazing and there should be water accessible to them in that paddock.
While there is a lot to be said for giving animals lots and lots of space to roam around on pasture, the flip side is that there can be invisible losses of potential yield from your farm acreage. By sizing paddocks based on what is actually standing there and matching that to how much pasture you want your animals to eat, you will be providing more precisely what they need. The benefit to this is that it allows pasture that is still in front of them to keep growing more. In doing this you may find you have excess forage from which to make hay or baleage for winter feeding and become more self-sufficient - certainly a good situation. … and even potentially have more forage to graze later into the grazing season!
Hubert Karreman is a veterianarian for the Rodale Institute in Kutztown, PA, doing applied research, educational activities and raising certified organic livestock. He is also owner of Bovinity Health, LLC.