1. Pasture Irrigation

2. Irrigated Pastures
Maximum production from irrigated pastures requires timely irrigation and the exclusion of livestock when the soils are wet.

3. What effects irrigation?
Type of soil
How much and how long to apply
Plant water use - (ET or Consumptive use)
how much and how often to apply

4. Soil properties Plant growth depends on the soil
depth
texture
Limiting factors (i.e... hardpans)
Irrigation also depends on the soil
Infiltration
Water holding capacity

5. Soil Types

6. Infiltration
Infiltration is define as how long the water takes to move through the soil
this effects the length of an irrigation
also runoff and deep percolation

8. What is water holding capacity?
The soil is made up of solids and voids or pore space
Pore space is made of a water and an air component
The soil is limited to how much water it can hold, thus water holding capacity
Water holding capacity very dependent on soil texture

10. What happens if too much water is applied?
Fills all the pore space - saturation
wetlands
Drainage downward - Deep percolation
water unavailable for plants
loss of nutrients

11. Is all the water in the soil available for the plants?NO
Available water is between field capacity and wilting point. These are measured as pressure required to extract the water
To reduce plant stress keep water extraction to about 50% of the available water

13. How does a plant extract water?

15. Most of the roots, soil micro-organisms and available plant foods are concentrated in the upper one or two feet of the soil
From the preceding slide we can see that Seventy percent or more of the water used by grasses and legumes is taken from that area also.

16. What determines how fast the plant uses water?
Growth stage
Location
Climate
wind, temperature, humidity, Solar radiation, ground temperature

17. Consumptive use
Various methods have been determined to predict crop water use
Most are not direct measurements but use empirical methods
Where can this information be obtained?
Local NRCS offices
State department of Water Resources
Internet

19. Irrigation Frequency We know how much water the soil holds
We know how much and how fast the plant uses the water
We can estimate how often and how much water to apply

20. What is the Goal? Maximum production
If the interval between irrigation is too long the grasses go dormant and production and quality are lost
Current research shows that alfalfa-grass on deep medium-textured soils decline occurs after 15 days. Clover is approximately 7 days

22. The goal of Irrigation
Replace used water with as little waste as possible (runoff and deep percolation)
Runoff carries pollutants in to other water bodies, has a bad public perception
Deep percolation - not visible, but can create temporary water tables, drive out soil air and can leach plant foods
Too much water can also reduce yields as shown in the following graph

24. Irrigation Frequency and Grazing Schedules
Optimum irrigation frequencies as well as the amount of water to be applied each irrigation has been determined for most crops and soils
It must be correlated with the grazing schedules to be used effectively

25. Dry out time
Each irrigation must be scheduled to allow adequate dry-out time before stock are admitted.
Dry-out periods of 3 to 4 days are adequate for most soils
Without dry-out time compaction and damage to the plants

27. The dry-out period plus the number of days each pasture is to be grazed (grazing period) within the rotation cycle plus one day to irrigate should be equal or less than the planned irrigation frequency

28. Formula No. 1
Formula No. 2

29. Formula No. 3
Formula No. 4

30. Once Irrigation cycle equals grazing cycle
Don’t adjust irrigation interval to meet seasonal water needs.
In cooler months that require less water - just apply the water for shorter time periods instead of adjusting the irrigation interval

31. Studies show...
1. Shortening the irrigation interval requires an increase in the number of pastures and a reduction in the grazing interval
2. That 2 or more pastures can be watered at a time when there are an even number of pastures or when the grazing period is a multiple of the irrigation interval. Works good for when water is delivered on a rotational basis.

32. 3. That a different pasture is watered each time for odd number of pastures unless the grazing period is a multiple of the irrigation interval. Works best with sprinklers or water delivered on a demand basis.
4. That flexibility in irrigation intervals and grazing periods increases as the number of pastures are increased.

33. Example

34. Details Given: 40 acres, deep medium textured soil
Grass clover pasture, Water use 0.20in/d
Soil water 2 in/ft, root depth 3ft flow rate 100 inches, Soil intake .3in/hr
Find: Irrigation frequency, Grazing period,
number of pastures, grazing cycle, and
irrigation cycle, irrigation set times

35. Solution:
Step 1. Find irrigation frequency
Soil water x root depth= total water
2in/ft x 3ft =6in
Use 50% of total water 6in x .5 = 3in
3in available water
Irrigation frequency = 15days

36. Step 2. From Tech note select a dry out period of 3 days grazing period = 15days - 1 day - 3days = 11 days
Step 3. From tech note select regrowth period
Step 4. Calculate the grazing cycle
grazing period x number of pastures = 3x11 = 33 days
Doesn’t quite match

37. adjust drying time to 4 days and regrowth time to
20 days. this gives a grazing period of 10 days
and a grazing cycle of 30 days
Good !!

38. P1 P2 P3 14 days 14 days Irrigation Irrigation Irrigation 10 days

39. Step 5. Calculate set times
If you are sprinkling that’s fairly good. Use a
12 hour set.
If you are flood irrigating then you need to add the
advance time for the water to get from the head of
the field to the end. 2 hours just might do it, so 12
hours set would probalby work for both.

40. Now let’s check efficiencies
2 cfs = 4 ac-ft/day so
or 83% water each pasture in two sets 12hrs each

41. Summary Irrigation frequency 15 days Dry- out period 4 days
Grazing period days
Regrowth period days
Grazing cycle days
3 pastures acres
2 set per pasture
12 hours per set
3 inches of water replaced at 83% efficiency

42. Questions