Pasture Irrigation

Irrigated Pastures

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

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

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– Limiting factors (i.e... hardpans)

Soil Types

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

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

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

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

How does a plant extract water?

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.

What determines how fast the plant uses water?

Growth stage Location Climate

– wind, temperature, humidity, Solar radiation, ground temperature

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

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

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

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

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

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

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

Formula No. 1

Dry - out period + Grazing period +1= Irrigation frequency

or

Irrigation frequency - Dry out period -1= Grazing period

Formula No. 2

Number of pastures =Regrowth period(days)

Grazing period(days)1

Formula No. 3

Grazing cycle= grazing period number of pastures

Formula No. 4

Irrigation Interval (days) numberof irrigations = grazing cycle

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

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.

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.

Example

DetailsGiven: 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

Step 1. Find irrigation frequency

Soil water x root depth= total water2in/ft x 3ft =6in

Use 50% of total water 6in x .5 = 3in3in available water

available waterwater use /day

3020

15inin d

days. /

Irrigation frequency = 15days

Solution:

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

Number of pastures =regrowth period

grazing period 1

22

111 3

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

irrigation interval 2grazing cycle

15 230

1

Good !!

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P2

P3

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Irri

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10 days

20 days

14 days

20 days 20 days

20 days20 days

14 days

14 days14 days

14 days14 days

10 days

10 days

10 days

Step 5. Calculate set times

set time=inches applied

intake rate in/hr3inches.3in/hr

hrs 10

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

Now let’s check efficiencies

40 acre field3 pastures/field

ac / pasture

100 miner inches = 2 cfs

13.33 ac 3 in = 40 ac-in = 3.33ac-ft needed

1333.

2 cfs = 4 ac-ft/day so 3.33ac-ft needed4ac-ft applied

083.

or 83% water each pasture in two sets 12hrs each

Summary Irrigation frequency 15 days Dry- out period 4 days Grazing period 10 days Regrowth period 20 days Grazing cycle 30 days 3 pastures 13 acres 2 set per pasture 12 hours per set 3 inches of water replaced at 83% efficiency

Questions