Saturday, April 19, 2014

Managing Drought Risk on the Ranch

Sandhills Defoliation Response Index

The Sandhills Defoliation Response Index System (SanDRIS) is an example of response indices that can be used to optimize year-to-year changes in pasture-use sequences that minimize the effects of grazing and drought stress (Table 1). This simple decision-support tool is based on the three variables that have the greatest collective effect on warm-season tallgrass response to grazing on Sandhills prairies: (1) season of defoliation, (2) timing and quantity of precipitation, and (3) quantity and distribution of residual herbage at the end of the growing season.

 Sandhills Defoliation Response Index System (SanDRIS)

Season of Defoliation (Grazing, Hail, Fire & Grasshoppers)

Index

March-April: sedges green-up and grow

+2

May: cool-season grasses green-up +1
June: early growth of warm-season tallgrasses -1
July: rapid growth of warm-season tallgrasses -2

Multiple-events during June-August

-3

August: warm-season grass seed heads emerge 0

September: nearly full growing-season deferment

+2

October-February: dormant season

+4

 

Precipitation Regime (October through September)

Index

Well above average: 5-6 feet of moist soil by April 30 and abundant precipitation during June-July

+2

Near average: 3-4 feet of moist soil by April 30 and abundant precipitation during June-July

0

Drought: Little or no dormant-season precipitation during October-April and/or well below average precipitation during June-July

-2

 

End of the Season Residual Herbage (Hydrologic Condition)

Index

Excellent: standing herbage of palatable species and littler abundant throughout (>1500 lb/ac) +2

Adequate: litter and standing herbage uniformly distributed (700-900 lb/ac)

0

Poor: standing herbage absent, litter uncommon

-2

 

In the Sandhills of Nebraska, prairie sandreed is the earliest developing warm-season tallgrass followed by bluestems and switchgrass. Numerical values for season of grazing in this guide are based on probable response of warm-season tallgrasses which can produce 70% to 90% of the herbage on upland Sandhills sites in good to excellent range condition. SanDRIS is based on numerous ranch observations and published research conducted at the University of Nebraska-Lincoln, Gudmundsen Sandhills Laboratory.


Using SanDRIS

Each pasture is scored in each of the 3 categories based on information in current-year grazing and precipitation records and on-site evaluations of residual herbage soon after killing frost in October. The sum of a numerical value from each of the 3 categories provides a SanDRIS score for individual pastures ranging from -7 to +8.

All summer-grazed pastures should be evaluated during 1- to 2-day time periods after killing frost to determine the end-of-season residual herbage index value. Dates of grazing and percentages of seasonal stocking rates during multiple grazing periods, entered in the grazing records, will be used to determine season of defoliation index values. On-ranch precipitation records or web site data from a nearby weather station should be used to determine the precipitation regime index value. Depth of moist soil is readily determined during spring fencing repairs.

The primary objective is to minimize the occurrence of negative SanDRIS scores in the same pastures in consecutive years. Negative scores in the preceding year should be offset with management practices that are likely to result in positive scores at the end of the subsequent growing season. Running 2-year totals for each pasture should be >0.


Multiple Defoliation Seasons

Pastures should be scored for growing-season use when any combination of summer defoliation processes removes more than 40% of the herbage that would be accounted for by traditional moderate summer stocking rates. For example, if last year's use included 9 AUD/ac during July and 12 AUD/ac during November with a traditional summer stocking rate of 15 AUD/ac, the percentage of summer use would be 60%, (9 AUD/ac) ÷ (15 AUD/ac). Score the pasture strictly for July use (Season Index = -2).

When growing-season stocking rates are less than 40% of seasonal rates, multiply the dormant-season index value, +4, by the percentage of the total stocking rate that occurred during the dormant season. For example, if 5 AUD/ac occurred during June and 16 AUD/ac occurred during December, the adjusted season of grazing index value would be +3, [4 x (16 AUD/ac ÷ 21AUD/ac)].


Pasture-use Sequences

Sustainability of rotation grazing systems in the Nebraska Sandhills can be quickly assessed by using the SanDRIS "Season of Defoliation" index values. It is unlikely that preferred warm-season tallgrass populations can be sustained by Plan A. Two-year running total scores will be negative in six of the seven pastures each year. Use of all pastures from June through September is likely to reduce subsequent-year forage production potential in five of the seven pastures each year when the subsequent year is dry. Plan B increases the number of pastures with positive index values by resting about 25% of the rangeland each year. If the accumulation of residual herbage is not needed for hydrologic condition, wildlife habitat, or other objectives, rested pastures could be dormant-season grazed with cows after the "rested" pastures are deferred from spring green-up to killing fall frost. Resting pastures may be justifiable because grass costs for yearlings ($/a yearling month) are often about 25% higher than grazing leases for cows and calves ($/pair month) when adjusted to a $/animal unit month rate. Another alternative would be to switch from yearlings to pairs and use a six-month grazing system (Plan C). In Plan C, there are an equal number of negative and positive "Season of Defoliation" index values and the absolute value of the positive index values exceeds the absolute value of the negative index values. Consequently, year-to-year changes in the pasture-use sequence can be made to ensure that two-year running total index values are >0 in all pastures in each year.

 

  

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