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Pasture Irrigation

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

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**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

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**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

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Soil Types

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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

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**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

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**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

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**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

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**How does a plant extract water?**

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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.

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**What determines how fast the plant uses water?**

Growth stage Location Climate wind, temperature, humidity, Solar radiation, ground temperature

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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

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**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

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**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

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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

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**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

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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

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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

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Formula No. 1 Formula No. 2

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Formula No. 3 Formula No. 4

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**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

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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.

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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.

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Example

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**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

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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

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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

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**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 !!

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**P1 P2 P3 14 days 14 days Irrigation Irrigation Irrigation 10 days**

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**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.

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**Now let’s check efficiencies**

2 cfs = 4 ac-ft/day so or 83% water each pasture in two sets 12hrs each

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**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

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Questions

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Understanding Soil Chemistry

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