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Sizing Up Pastures

By Hubert Karreman, VMD


Foldable yardstick creating a square foot quadrat
for forage sampling.






How can we best and most easily verify that
what the cows are being given access to graze
is actually providing what is being documented?
Also, how can we make sure we are not wasting
pasture – especially in areas where there is a
high animal density on farms of relatively
small land base?


Added January 17, 2011.
With the new pasture rule released in February 2010, it is now reality that pastures need to be managed as a crop and that they are to provide a meaningful source of feed for ruminants. And of course the 30/120 requirement, which the organized groups fought so hard for, is now in effect.

The National Organic Program (NOP) has given public training sessions since the release of the new pasture rule. Additionally, certifiers and organic outreach organizations have also given training sessions during the grazing season this past year. Most of the trainings have focused on what inspectors have traditionally looked for during inspections: the feed inventory in the barn and how much feed was grown or purchased. The feed ration has traditionally been looked at as well. Looking at the ration is a residue from the old "80/20" rule, which had farmers accounting the use of 20% non-organic feed in the first 9 months of their final transition year. Since most inspectors are fairly well versed in some of these basic calculations, the NOP has actively promoted these barn components for figuring out the dry matter intake from pasture. Their main formula is to subtract what the animals are being fed in the barn (winter time) from what their dry matter demand is and do this again during the grazing season, thus arriving at what the animals are eating on pasture.

But it doesn't take a rocket scientist to realize that the NOP and certifier focus is on barn feeding – not actual pasture feeding! It is plain to see that the most important factor in the whole equation is not even there – pasture availability! All the focus is on barn feeding and then an "oh, by the way the rest is coming from pasture". But what does the pasture actually look like? How much is out there when the cows arrive? Is enough pasture being supplied to match what the paper barn ration says they're getting from pasture? How do we know? I can imagine there are situations where the size of the pasture is not changing, even though the recorded ration says they are getting a different percent dry matter from pasture than previously stated.

How can we best and most easily verify that what the cows are being given access to graze is actually providing what is being documented? Also, how can we make sure we are not wasting pasture – especially in areas where there is a high animal density on farms of relatively small land base?

These two questions were central to the part-time work I did for Midwestern Bio-Ag in Lancaster, County, PA during the summer of 2010. Having practiced management intensive grazing as a herdsman in the late 1980's, I am very into the proper pasturing of cows. Also, some farmers were worried if they would "make it" with the new rule. I worked with about twenty farmers in total but worked closely with four on a weekly basis. I would walk a field prior to grazing, scout it out with a sense of how the cows would see it, choose an area to clip, place the clippings in a 1 gallon size ziplock bag, bring the sample home, weigh it fresh (in grams), dry it down in a food dehydrator for about 8 hours or overnight (has 4 layers for 4 samples), weigh it dry, calculate how much pasture dry matter is available, then size the paddocks for the cows. I did this about 115 times from late April to late September.
While scouting pastures, I thought like a cow – where and what would she go eat? Then I'd consider how much of the field is like that and then I'd choose a spot to take one representative sample. If there were very different components to a field, then I would take an extra sample or two for averaging. But most fields, whether a monoculture of alfalfa or sorghum sudan all the way to a field with lots of different species (forage species and "weeds"), were actually fairly uniform when looking at "the big picture" from the perspective of cows being set into the pasture.

I got a lot of really great exercise, was able to observe cows eating lots of different types of plants, helped farmers understand their pastures better, and also felt good helping farmers comply with the new pasture rule. The main outcome was the farmers gaining a true awareness of pasture as a significant source of feed – something the back calculating from the paper barn ration doesn't do at all.

Between late April and early July I did a lot of trial and error work but saw some trends that were occurring. I then developed a formula which would subdivide a pasture field into appropriate paddock sizes for the upcoming week. The formula allows the farmer to plan ahead rather than "blindly" moving the wire and adjusting things only after seeing how the cows milked or ate in the barn. How much pasture are they actually being exposed to – enough to pass the 30% dry matter requirement from pasture? And in areas with high animal density on small acreage, blindly moving fence ahead might be wasteful if too large an area is given relative to what is growing whereas careful measurement will conserve the most available pasture dry matter for the longest time.

So how do I go about utilizing the pasture sizing formula? I choose a random area to sample by walking around a bit in the paddock, seeing the general growth and then stopping at an area which just "seems" right. I put down my foldable yard stick onto the ground to give 3 one foot sides to the area. The sample size is thus one square foot. I then measure the height of the canopy of vegetation by lightly letting my hand push down upon the tops to where the vegetation seems to give resistance. I jot down this height as "H". Then I clip the sample to a height of 4" since this is usual height cows leave behind after 12 hours and it's also a nice height to start re-growth. If the canopy height is 8 inches or less, I'll clip down to 3" since the cows will graze it lower. When cows are put out into sorghum sudan at about 20" or higher, they tend to leave about 10-12" of residual. Watch the cows, they'll show you themselves.
The most important numbers needed are the "H" (the height, in inches, of the sample) and the dry weight of the sample (in grams), labeled as "DW". From this everything else flows. By dividing the Dry Weight (DW) by the Height (H) of the sample, you get the amount of pasture in grams per inch per square foot. Multiplying by 95.2875 will convert grams /inch /square foot to pounds /inch /acre (P).

We know that cows will not eat everything that is out there – there will always be some refusal due to trampling, fresh manure and urine. The NRCS has data that shows cows moved every 12-24 hours utilize 80% of a given paddock. (Utilization decreases with longer stays in the same paddock). This utilization (U) needs to be taken into account. Additionally, the original height (H) of the sample now has to be taken into account again. By multiplying the pounds/inch/acre (P) by the utilization (U) and the actual height (H) to be grazed, we arrive at the Dry Matter Available (DMA) – the most critical component in pasture management, something which the NOP has not promoted nearly enough, in my opinion. Hopefully you can understand that the subtraction/back-calculation from a paper barn ration may be simple for inspectors but it does absolutely nothing for farmers to actually manage pasture better than prior to the new rule. With DMA we truly have a sound basis to know how much the herd will actually encounter and potentially consume when arriving in a new paddock.

We need to consider one more critical component: the daily dry matter from pasture required (R) for a group of animals. This (R) is the product of average animal weight x DM Demand daily x Desired % DM intake from pasture. For example, an average 1300 lb milking Holstein herd need 3.7% of body weight dry matter intake daily (from nutritionist) x 30% of dry matter intake from pasture = (R) = 14.5 pounds dry matter from pasture daily. For 900 lb Holstein heifers, the Dry Matter Demand of 19.1 lbs (pg 8 NOP DMD table) x 30% from pasture = (R) = 5.73 pounds dry matter from pasture daily.

Dividing (R) by the Dry Matter Available (DMA) will give pasture acreage (A) needed for one animal. Then multiply by the number of animals in the group and you will then know what size paddock is needed for the group.

This is repeated weekly as pasture conditions can change significantly. By repeating this process over a hundred times, I have thought a lot about the formula and also my observations. When scouting a pasture, I quickly realized if a field needs a longer resting period. I would alert the farmer and he could make the appropriate field change. As far as the formula goes, any factor is completely replaceable depending on what one wants to focus on (percent pasture intake, number of animals of a given size, specified amount of days on a field, etc). For instance, if the nutritionist declares on the paper ration that the cows shall get a certain percent of their ration from pasture (like 68%), then we plug in .68 into the (R) factor and not the minimum .30 (30%). By the way, it is always better to use a minimum of .35 (35%) in the (R) factor than to sit right on the 30% line. If there is a mixed group of weights of animal (like Jerseys and Holsteins in a group), take the average weight and size the pasture for that. Being reality and not hard science allows for common sense to guide pasture measuring and general weights of the groups. At least there is awareness of actual pasture availability and the animals that will be eating it.

Though the formula is based on only one representative sample, I was purposely conservative. Consider the following: the 12"x12"x12" foldable yard stick allows only the inside area to be clipped. And when encountering two or more obvious canopy heights, use the taller canopy to give the height (H), since this (H) will be a larger denominator and give a smaller initial grams/ inch/ acre from which the dry matter available is calculated. Also keep in mind bare soil areas. Estimate such areas of pasture (10%, 20%, etc) and then add that to the calculated acreage needed (A) for the group.

By sizing each week, the weather forecast can be taken into account: if it is to be a dry week, slow down and use a smaller size whereas for a wet week speed up by increasing the paddock size.

I had a lot of fun while helping my local farmers. Cows would often seem to know that I was checking out how good the next paddocks were by following me closely with their eyes. I really enjoyed watching cows eat many different species of plants – especially reaching under the fence line for "weeds". Cows will eat essentially anything that is young and lush, even if technically a weed. They absolutely love lambs quarters and smooth pigweed. They will even eat the tops off of spiny red root pig weed. Young headed out foxtail is devoured if cows are in a tight area with it. And these weed eating cows were not skinny and in search of desperately needed groceries – they looked sleek and athletic. They do avoid toxic plants like horse nettle.
A completely new experience for me was coming upon a large bovine family group living pretty much independent of people. The "extended family" of about 40 or more animals consisted of two bulls, nurse cows and very healthy calves, some yearlings and dry cows. Upon arrival a large group of the youngest calves were under tall sumac trees being watched over by the two bulls. As the two bulls "babysat" the calves, the cows and yearlings were stretching their necks to devour 6 to 8 foot high smooth pigweed from the top on down. Truly a small bovine society totally at peace and contented out on natural pasture.

Dr. Hubert Karreman has been involved in organic dairy and grazing since being a herdsman at Seven Stars Biodynamic Farm in the late 1980's. He was on the National Organic Standards Board between 2005 and 2010 and actively involved with the discussions and formulation of the new grazing rule. His newest book "The Barn Guide to Treating Dairy Cows Naturally" (Acres USA, 2011) is especially for farmers, containing only the most practical tips for 90+ real cases and nearly 200 colored pictures.

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