Forage & Grains
10 Steps to Calculate Dry Matter Intake
from Pasture---It’s Not So Difficult!
By Harriet Behar
Organic farmers must provide all ruminants with an average of not less than 30% of their dry matter intake from pasture during the grazing season. You will need to figure out the Dry Matter Demand (DMD) and Dry Matter Intake (DMI) for all ages of ruminants in your herd. In addition, you need to know how long the grazing season is on your farm so you can calculate the average pasture DMI over the grazing season. With this information, you can determine the dry matter intake from pasture and if you meet the new regulation.
The method to determine the percent of DMI from pasture described in this article uses the “subtraction method” and is done through the following basic steps. For each group of animals:
Below are examples of how to work through these steps.
Step 1. Categorize groups of animals by feeding group and figure dry matter intake
Groups are formed of animals that share a similar stage of life or production. Each group will need their DMD and DMI calculated separately. Some examples of groups are:
This exercise will be simplest if you make a chart similar to Table A near the bottom of the page, or use the table provided by your certifier.
Once you have determined the various feeding groups on your farm, determine how many animals you have in each group, and the total amount of each type of feed that you give each group. Some simple math can then be done to figure out the average amount of feed you are giving each animal. For instance, if you know you are feeding 100 pounds of oats to 50 animals per day, then you are feeding 2# of oats per day per animal.
Then use Tables B and C to figure out the Dry Matter Demand per animal per day. Tables B and C are used by MOSA (Midwest Organic Services Association) to predict DMD of dairy cows based on size and level of milk production. These tables predict DMD based on the current weight of the animal and/or how much milk they produce.
To get the pounds of Dry Matter Demand for dry cows, for instance, take the weight of the animal times the % from Table C and multiply (1200 pounds x 1.8% or 0.18= 21.6 pounds DMD).
Step 2. Determine length of the grazing season
The length of the grazing season will vary from one part of the country to another, and from farm to farm. The grazing season as defined in the new pasture rule is not the same as the growing season. The Grazing Season, as defined by the NOP (205.2), is: “The period of time when pasture is available for grazing, due to natural precipitation or irrigation. Grazing season dates may vary because of mid-summer heat/humidity, significant precipitation events, floods, hurricanes, droughts or winter weather events.“
The grazing season will vary from 365 days to 120 days, depending on the location of the farm. The grazing season may be interrupted or non continuous in some parts of the country. The actual length of the grazing season for a given farm can be determined based upon historical documentation of grazing days on the farm.
See the illustration on this page for an example of the grazing season. Source: Training Presentation Slides Part 1 (PDF), National Organic Program Pasture Rulemaking.
Step 3. Determine dry matter intake (DMI) from the types of feed fed per day
In this next step, you need to determine how much DMI is being fed from NON-pasture feeds. This is referred to as “dry matter fed” in the rule. In Table A you calculated the lbs of each feed being fed to each animal each day, now you need to convert this to a Dry Matter basis. To do this, you will need to know the DM content of each of the types of feeds being fed. Use Table D to help you determine the dry matter Demand of the animals and the amount of Dry Matter available per type of feed. Dry matter refers to feed without the water. Therefore if you are feeding haylage that is 65% moisture, it only has 35% dry matter. This means 1000 lbs of haylage sitting in the bunk is equal to 650 lbs of water and 350 lbs of dry matter (the nutrient source).
Step 4. Determine DMI from pasture
This calculation “estimates” your dry matter intake from pasture based on the amount of dry matter fed from other feed sources.
To figure Dry Matter intake from pasture: Total Dry Matter Demand (DMD) (figured from Tables B and C): MINUS total pounds of Dry Matter fed to each class of animal that is NOT from pasture (supplemental feed) EQUALS pounds of DM from pasture.
For example, if dry matter demand is 35 lbs/cow; and dry matter fed from other feed sources = 14.29 lbs; then to calculate the DMI from pasture, you subtract the amount of supplemental feeds from the total DMD for the animal. What you have left is your pasture DMI: 36 lbs (DMD) – 14.29 lbs (DMI other sources) = 21.7 lbs DMI from pasture.
Alternatively, producers may determine the actual amount of intake from pasture through paddock/field measurements.
Step 5. Calculate the percent DMI from pasture
To calculate the percent of dry matter from pasture, you divide the pounds of DM from pasture by the total dry matter demand, then multiply by 100. This formula establishes how much of the total dry matter required by the cow is coming from pasture.
To calculate, take the % DMI from pasture and divide by the estimated DMD for the cow, then multiply by 100 to convert the value to a percent.
Using the example from the ration in Table A for the milk cows. See Table E.
Step 6. Average the DMI from pasture over entire the grazing season
If you feed only pasture during the entire grazing season you only need to do one DMI calculation. If you adjust the amounts and types of feed fed from sources other than forage grazed from pasture several times during the grazing season then you will want to make several calculations of % DMI during the grazing season and average them.
Thanks to MOSA for use of their charts and tables and to the e-organic dairy team for the information provided in this article.
Harriet Behar is an Organic Specialist at the Midwest Organic and Sustainable Education Service, MOSES. www.mosesorganic.org 715-778-5775