Tuesday, October 21, 2014

Managing Drought Risk on the Ranch

Clip and Dry to Determine Carrying Capacity

Forage production -The weight of forage that is produced within a designated period in a given area. The weight may be expressed as either green, air-dry, or oven-dry. The term may also be modified as to time of production, such as annual, current year's, or seasonal forage production. (NRCS handbook)

This method will help you determine the carrying capacity of your various pastures.  In addition, as you clip areas with high, medium, and low levels of herbage production, you'll begin to develop mental (and photo) references that will someday help you visually monitor range conditions. 

Take Pre-clipping Photograph

Use digital cameras and software to develop a permanent record of clipped plots for refreshing your memory of the vegetation and corresponding carrying capacity calculated from the air-dry herbage. 

  1. Pre-clipping photographs of quadrats should include the clipping frame and a 3 to 6-foot graduated staff next to the frame. 
  2. The staff should be slightly taller than the tallest grass and forb species. Use 1" to 1-1/2" PVC pipe or wooden dowel. 
  3. Staves should be painted with alternating 6-inch black and white increments for the first 2 feet and 12-inch increments for the remaining height. 
  4. Attach a sharpened metal rod to the bottom to anchor the staff. 
  5. Focal distance and angle should be the same for all photographs. Early and late daylight are best with the sun at your back. Use a long enough focal distance to keep your shadow out of the picture.

Build clipping frame

  1. To estimate standing herbage, build a square frame using 3/4" PVC pipe with inside measurements of 37 3/16" x 37 3/16". 
  2. With schedule-40 PVC, cut four pieces at 37 1/8". This will account for the lack of closure inside the elbows. 
  3. Glue PVC elbows at the ends of 2 sections of pipe. 
  4. Temporarily insert the 2 free sections into the non-glued ends of elbows and use a large flat work surface to properly align elbows when gluing. The frame can then be disassembled for easy transport and storage. 
  5. Snugly assemble the frame and place it flat on the ground. 

Dimensions of this quadrat will allow you to easily convert from grams of air-dry herbage inside the plot to pounds/acre. Simply multiply the grams by 10.

Measure Standing Herbage

  1. Slide herbage of plants not rooted inside to the outside. 
  2. Place a graduated staff at the left rear corner and take 1 or 2 pictures. 
  3. Review the pictures on the camera monitor. 
  4. Clip all herbage at the ground level from plants rooted inside the frame. Discard previous year(s) herbage. 
  5. Place the current-year herbage in a paper bag. Label the bag with a level of production (high, intermediate, or low), sample number, and a date. It is helpful to clip several sample plots for each level of herbage production. 
  6. Place the open bags in an outside vehicle with closed windows for several days.  
  7. Use a 100 or 250-gram spring scale to weigh dried samples. Samples may have to be divided into several bags for drying and weighing. Subtract the weight of the bag from the total weight. 

Calculate Carrying Capacity

Calculate the stocking rate (AUM/ac) and note this number on the digital photograph for each clipped plot.

Moderate stocking rates are based on use of about 25% of peak standing herbage. This accounts for the fact that half of total forage should be left for plant and soil health, and that cattle will only fully utilize about half of available forage (the other half will be trampled, bedded in, and urinated on).

  1. Convert grams of dried forage to lbs/acre by multiply by 10 (as long as you used the size of frame outlined above).
  2. Divide this number by 4 (to account for proper production and utilization factors as described above).
  3. Divide this number by 780 lbs of forage per AUM (feed needs of one AU for one month) to get carrying capacity.

Example:

  1. Convert grams to lb/ac: 164 grams x 10 = 1,640 lb/ac
  2. Determine the amount of herbage to be used: 1,640 lb / 4 = 410 lb
  3. Calculate carrying capacity: 410 lb / 780 lb/AUM = 0.53 AUM/ac

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