- Email: [email protected]
Measuring Forage Dry Matter Yield Using Clipped Forage Samples
Appendix 2
Measuring Forage Dry Matter Yield Using Clipped Forage Samples Select or make a sampling quadrat, a square, or rectangular frame of convenient size to throw onto a population of plants for sampling. An inside measurement of 1 ft 1 ft provides 1 sq. ft of sampling space. These can be made of stiff wire, wood with metal-reinforced corners, or plastic water pipe. Measure the inside dimensions, and multiply length width to determine the area. Record this number and its units (either square inches or square feet). Walk across the pasture to be sampled in a zigzag pattern. Gently and randomly throw the quadrat. To just learn the procedure, two or three throws would be enough. Accuracy of the estimated forage yield of the whole pasture increases with an increasing number of throws and samplings. The first time that a field is sampled, about 10e30 throws and samplings would be reasonable. At each place where the quadrat lands, gently pull up on the forage plants to straighten them. Use a ruler, a pasture stick, or similar device to measure the plant height in at least four and preferably 10 places within the quadrat. Record these height measurements and average them. Use grass clippers or electric hedge clippers to cut the forage plants as near to the ground as is practical (about 1/4e1/2 inch). Collect all the clipped forage from each quadrat in a separate paper or plastic bag. Fine-mesh laundry bags have good air flow and do not take up water. Ensure that the bags can handle the heat of your drying system without burning. Label the bags with the pasture and paddock name and/or number and its type of forage. What you do next depends on the size and type of oven you have for drying samples. Commercial and research laboratories use large convection ovens to dry forage samples at temperatures around 100e176 F for 8e72 h. Microwave ovens, Koster testers, and food dehydrators are used. Table A2.1 indicates features of alternative devices for drying forages.
TABLE A2.1 Features of Various Forage Drying Devices Devices
Accuracy
Relative Cost
Number of Samples
Disadvantage
Convection oven
High
High
Overnight to 3 days
Depends on size
High cost
Microwave oven
Medium high
Low
5e30 min
1
Needs constant monitoring
Heater/fan (Koster)
Low
Low
25e50 min
1
Highest dry matter loss
Food dehydrator
Medium high
Low
2e10 h @ 165 F
<10
Electronic probe
Low
Moderate
A few seconds
1
Time Required
Lowest accuracy
371
372 Appendix 2
No matter which drying oven you use, an accurate, reliable balance such as a postal or dietetic scale is recommended. It should have a capacity of at least 100 g and precision to 1 g or less. Weigh one of the sample bags. If it is porous, dry it for a short time before weighing. Weigh the whole wet, freshly cut forage sample. After drying, weigh the dry sample in its container. Record all these weights. Alternatively, use an electronic balance with enough capacity to weigh your biggest samples and the container used for weighing and drying samples. Set an empty sample container (paper bag, pie plate, etc.) on the balance and press the “tare” button. The reading should go to zero. If all of your containers have the same mass, you do not need to subtract the mass of the bag or plate from your sample, and your calculations will be a little simpler. If your balance loses the tare setting, just put another empty container on it and press tare again. If your sample is larger than the capacity of your drying device, then you need to use a subsample for dry matter analysis and multiply the dry matter percent by the wet weight of the sample. If your entire sample fits easily into your oven (such as a convection oven) or other dryer, the dry weight of the sample minus the weight of the container is the amount of dry matter from your sample area. If you plan to subsample a large air-dried sample and oven-dry the subsample, remember to compensate for this in your calculations and that determining your actual dry matter yield will be delayed. Heater/fan (Koster) moisture testers formerly came equipped with a spring scale, which is possibly not accurate enough or durable enough for long-term use. Choose an electronic scale (balance). Turn the scale on. Place 100 g of wet forage on the specimen container and spread it out. Place the specimen container on the evaporation unit, plug in the unit, and turn it on. Dry the sample for 30 min, then weigh it again. Continue drying for decreasing amounts of time and weighing the sample until there is no change in measurement. The final weight represents the dry matter of the sample. For example: beginning (wet) weight ¼ 100g. End (dry) weight ¼ 85 g. This is the amount of dry matter. The dry matter content is dry weight/wet weight ¼ 0.85 in this case. If you have a large convection oven (oven with a fan), poke about 20 holes in the paper bags with a pen or pencil. No sample chopping is required. Place the bags in the oven overnight at about 100e120 F. A microwave oven should have at least 500-W capacity for drying forage samples. Containers for the forage should be microwave-safe. Glass pie plates work, but they become hot, and because of their weight, your balance must have sufficient capacity for them. Be cautious with paper plates or bags. If the container is porous, preheat it for about 10e20 s to drive out residual water, then weigh it, and record the empty oven-dry container weight (A). Place a chopped forage sample of about 100 g in an uncovered container. Record gross weight of wet sample þ container (B). To prevent the sample from charring and protect the electronics, place a cup with about 6 oz of water in a back corner of the oven. Use full power to heat the sample for 3 min then weigh it. Stir the sample, dry for another 1e3 min, and reweigh. Repeat until the weight loss from each drying is less than 1 g. If charring occurs, use the previous weight. Record final gross weight of the oven-dry sample þ container (C). Calculate the sample dry matter concentration as (C-A)/(B-A) 100%. Example: Empty oven-dry container (A) ¼ 20 g. Wet sample þ container (B) ¼ 90 g. OD sample þ container (C) ¼ 40 g. DM concentration ¼ (40 20)/(90 20) ¼ 0.286 100% ¼ 28.6%. If you use a food dehydrator, weigh the dry sample trays, and then weigh the trays with wet forage samples on them. Dry (dehydrate) the samples according to instructions (8e12 h). Weigh the dry samples on their sample trays and calculate the dry matter concentration as for the microwave oven above. The forage dry matter (FDM) weight from 1 sq. ft of sampled area is multiplied by 43,560 sq. ft/acre to give the weight of FDM per acre. If a dry sample from 1 sq. ft weighed 30 g, then it would represent. 30 g/sq. ft 43,560 sq. ft/acre ¼ 1,306,800 g/acre. This also equals 1306.8 kg/acre, and this is converted to lbs/acre by multiplying. 1306.8 kg/acre 2.2046 lbs/kg ¼ 2881 lbs of FDM/acre. To determine the FDM yield in each inch of forage height per acre, divide the yield/acre by the number of inches of average forage height. For example, if the average forage height was 10 in.: (2881 lbs FDM/acre)/10 in ¼ 288.1 lbs FDM/acre/in.
Measuring Forage Dry Matter Yield Using Clipped Forage Samples Select or make a sampling quadrat, a square, or rectangular frame of convenient size to throw onto a population of plants for sampling. An inside measurement of 1 ft 1 ft provides 1 sq. ft of sampling space. These can be made of stiff wire, wood with metal-reinforced corners, or plastic water pipe. Measure the inside dimensions, and multiply length width to determine the area. Record this number and its units (either square inches or square feet). Walk across the pasture to be sampled in a zigzag pattern. Gently and randomly throw the quadrat. To just learn the procedure, two or three throws would be enough. Accuracy of the estimated forage yield of the whole pasture increases with an increasing number of throws and samplings. The first time that a field is sampled, about 10e30 throws and samplings would be reasonable. At each place where the quadrat lands, gently pull up on the forage plants to straighten them. Use a ruler, a pasture stick, or similar device to measure the plant height in at least four and preferably 10 places within the quadrat. Record these height measurements and average them. Use grass clippers or electric hedge clippers to cut the forage plants as near to the ground as is practical (about 1/4e1/2 inch). Collect all the clipped forage from each quadrat in a separate paper or plastic bag. Fine-mesh laundry bags have good air flow and do not take up water. Ensure that the bags can handle the heat of your drying system without burning. Label the bags with the pasture and paddock name and/or number and its type of forage. What you do next depends on the size and type of oven you have for drying samples. Commercial and research laboratories use large convection ovens to dry forage samples at temperatures around 100e176 F for 8e72 h. Microwave ovens, Koster testers, and food dehydrators are used. Table A2.1 indicates features of alternative devices for drying forages.
TABLE A2.1 Features of Various Forage Drying Devices Devices
Accuracy
Relative Cost
Number of Samples
Disadvantage
Convection oven
High
High
Overnight to 3 days
Depends on size
High cost
Microwave oven
Medium high
Low
5e30 min
1
Needs constant monitoring
Heater/fan (Koster)
Low
Low
25e50 min
1
Highest dry matter loss
Food dehydrator
Medium high
Low
2e10 h @ 165 F
<10
Electronic probe
Low
Moderate
A few seconds
1
Time Required
Lowest accuracy
371
372 Appendix 2
No matter which drying oven you use, an accurate, reliable balance such as a postal or dietetic scale is recommended. It should have a capacity of at least 100 g and precision to 1 g or less. Weigh one of the sample bags. If it is porous, dry it for a short time before weighing. Weigh the whole wet, freshly cut forage sample. After drying, weigh the dry sample in its container. Record all these weights. Alternatively, use an electronic balance with enough capacity to weigh your biggest samples and the container used for weighing and drying samples. Set an empty sample container (paper bag, pie plate, etc.) on the balance and press the “tare” button. The reading should go to zero. If all of your containers have the same mass, you do not need to subtract the mass of the bag or plate from your sample, and your calculations will be a little simpler. If your balance loses the tare setting, just put another empty container on it and press tare again. If your sample is larger than the capacity of your drying device, then you need to use a subsample for dry matter analysis and multiply the dry matter percent by the wet weight of the sample. If your entire sample fits easily into your oven (such as a convection oven) or other dryer, the dry weight of the sample minus the weight of the container is the amount of dry matter from your sample area. If you plan to subsample a large air-dried sample and oven-dry the subsample, remember to compensate for this in your calculations and that determining your actual dry matter yield will be delayed. Heater/fan (Koster) moisture testers formerly came equipped with a spring scale, which is possibly not accurate enough or durable enough for long-term use. Choose an electronic scale (balance). Turn the scale on. Place 100 g of wet forage on the specimen container and spread it out. Place the specimen container on the evaporation unit, plug in the unit, and turn it on. Dry the sample for 30 min, then weigh it again. Continue drying for decreasing amounts of time and weighing the sample until there is no change in measurement. The final weight represents the dry matter of the sample. For example: beginning (wet) weight ¼ 100g. End (dry) weight ¼ 85 g. This is the amount of dry matter. The dry matter content is dry weight/wet weight ¼ 0.85 in this case. If you have a large convection oven (oven with a fan), poke about 20 holes in the paper bags with a pen or pencil. No sample chopping is required. Place the bags in the oven overnight at about 100e120 F. A microwave oven should have at least 500-W capacity for drying forage samples. Containers for the forage should be microwave-safe. Glass pie plates work, but they become hot, and because of their weight, your balance must have sufficient capacity for them. Be cautious with paper plates or bags. If the container is porous, preheat it for about 10e20 s to drive out residual water, then weigh it, and record the empty oven-dry container weight (A). Place a chopped forage sample of about 100 g in an uncovered container. Record gross weight of wet sample þ container (B). To prevent the sample from charring and protect the electronics, place a cup with about 6 oz of water in a back corner of the oven. Use full power to heat the sample for 3 min then weigh it. Stir the sample, dry for another 1e3 min, and reweigh. Repeat until the weight loss from each drying is less than 1 g. If charring occurs, use the previous weight. Record final gross weight of the oven-dry sample þ container (C). Calculate the sample dry matter concentration as (C-A)/(B-A) 100%. Example: Empty oven-dry container (A) ¼ 20 g. Wet sample þ container (B) ¼ 90 g. OD sample þ container (C) ¼ 40 g. DM concentration ¼ (40 20)/(90 20) ¼ 0.286 100% ¼ 28.6%. If you use a food dehydrator, weigh the dry sample trays, and then weigh the trays with wet forage samples on them. Dry (dehydrate) the samples according to instructions (8e12 h). Weigh the dry samples on their sample trays and calculate the dry matter concentration as for the microwave oven above. The forage dry matter (FDM) weight from 1 sq. ft of sampled area is multiplied by 43,560 sq. ft/acre to give the weight of FDM per acre. If a dry sample from 1 sq. ft weighed 30 g, then it would represent. 30 g/sq. ft 43,560 sq. ft/acre ¼ 1,306,800 g/acre. This also equals 1306.8 kg/acre, and this is converted to lbs/acre by multiplying. 1306.8 kg/acre 2.2046 lbs/kg ¼ 2881 lbs of FDM/acre. To determine the FDM yield in each inch of forage height per acre, divide the yield/acre by the number of inches of average forage height. For example, if the average forage height was 10 in.: (2881 lbs FDM/acre)/10 in ¼ 288.1 lbs FDM/acre/in.