October 27, 2011
by Allan Nation
Irrigated pasture is a high management activity and for best results requires a variable stocking rate. It is most economical when used to “pop” growing animals that have developed frame but little flesh, to “finish” grass fed animals for slaughter or dairying.
It is less economic when used with brood animals or to grow and frame lightweight cattle and lambs.
The right combination of stocking rate, water and fertility can produce per acre gains in excess of one thousand pounds per acre in the central USA. Profits from growing cattle and irrigated pasture can be far in excess of irrigated commodity crops and can rival specialty crops in return per acre.
Because properly irrigated and fertilized grasses are not under stress they are more palatable and digestible than natural pastures. As a result, per head average daily can be considerably higher than those normally seen on non-irrigated pastures.
Before investing in any irrigation equipment make sure you have an adequate reliable water source.
In general, you will need five to 20 gallons per minute from a well or stream or two to four acre-feet of water in a farm pond per acre irrigated. Low volume irrigators can help you conserve water and may use less total water as well.
You also need to test the quality of your irrigation water. Water high in salt or excess minerals can build up in your soil and tie up your soil fertility. Water high in sand or silt can reduce the life of your irrigation equipment.
If you do not have guaranteed full season access to irrigation, you should not plant perennial grasses.
Also, during excessively dry, warm, snowless winters, perennial grasses will need some occasional winter irrigation to survive.
Irrigated pastures require precision fertilization to prevent trace mineral deficiencies - particularly on sandy soils. Bawling, restless cattle are always a good indication something is wrong with the taste of the grass and their distress should not be ignored. Poorly balanced soils produce inferior animal performance and can eventually result in severe animal health problems as well.
A common problem with irrigated pastures is the fast-growing, low dry matter grass does not contain enough “scratch factor” and can cause bloat and animal distress. This can be alleviated by keeping free-choice hay available or allowing access to slower growing, non-irrigated grass on a free-choice basis.
PASTURE PUGGING IS A BIG PROBLEM
An irrigated pasture should not be grazed during or immediately after having been irrigated as this will cause severe soil compaction. For this reason, all irrigated pastures require some system of rotational grazing.
Similarly, an adjacent area must be provided so it is possible to remove the cattle from irrigated pastures during periods of heavy natural rainfall. Many irrigated pastures have been ruined by leaving cattle on irrigated pastures during natural rainfall events.
In humid climates (over 30 inches of rain), having more than 50 or 60 percent of your pastures irrigated is not recommended due to the high potential for destructive pasture pugging.
Wheel move (sideroll) irrigators are very labor intensive particularly during the grass germination period when light, frequent applications are necessary. Often the irrigators must be moved every two to four hours and becomes an onerous full-time job.
Flood irrigation is poorly suited to pasture irrigation due to the lack of precise control over the amount of water put out at any one time. Unfortunate by-products of flood irrigation are sun scalded plants, the development of soil hardpan (caliche) and soil salt buildup.
Typically flood irrigation alternates very dry conditions with overly wet. This is hard on both the pasture and the soil and is highly conducive to pasture pugging. The whole idea of pasture irrigation is to keep the soil moist at all times but not excessively wet.
There are now innovative, European designed portable irrigation units available which can work well in irregularly shaped or tree studded pastures. Other graziers are having good results using New Zealand effluent sprayers as low-cost irrigators.
Low labor, solid set irrigators work well on hillside pastures.
Excessive paddock subdivision is not necessary with irrigated pasture as the pasture is seldom under stress and needs less rest than “natural” pastures. In a center pivot system, four subdivisions are normally enough with rapidly growing pasture.
The irrigation water can be turned off and the pivot moved forward through the animals dry or it can be reversed.
Early morning is the best time to irrigate as wind speeds are lowest. Some areas have more favorable night-time electricity rates.
Pivot wheels cutting deep trenches in the soil is a sign of over-irrigation. These ruts tend to be a particularly bad when the irrigator has to climb a hill.
Elaborate gates are generally not needed to allow the pivots wheels to pass over the fence. Usually the irrigator can walk over a one-wire fence equipped with flexible plastic fence posts with no problem as long as the wheel unit is provided with a special guide rail to prevent the wire from getting caught up in the gearing or under-carriage of the wheel unit.
The pivot should be perimeter fenced as a square with interior square paddocks and not in a circle. We never want to confine the animals to only the irrigated area. Usually the cattle will prefer to sleep and loaf off the irrigated grass and this practice should be encouraged. There have been reports of sleeping calves being crushed by the wheel unit.
The cattle should be watered in the non-irrigated corners rather than at the center. The stock tanks should be built so that the animals’ front feet are higher than his back feet. This will help prevent pasture bloat. Also, building small hillocks in the corners will also allow the animals to relieve the gasses that cause bloat. Perfectly flat irrigated pastures are prone to bloat.
Some grazing operations are large enough that they have multiple pivots and a whole pivot can be used as a single paddock. The fence surrounding the pivot should be fenced square. The corners not being used for stock watering can be irrigated with fixed point (Rainbird) irrigators.
Using multiple pivot points as a system generally gives you more flexibility than running each pivot as an individual and separate unit.
In most of the central and southern USA, center pivots are generally not able to match daily evaporation in mid-summer. This moisture deficit must be made up from soil resources. Therefore it is important to build up your soil moisture reserves prior to the onset of hot weather.
Soils that are high in organic matter provide the most even pasture growth curve and allow the use of high quality perennial grasses such as perennial ryegrass.
On low organic matter soils, only use drought hardy perennials such as fescue and smooth brome or annuals such as oats, turnips, corn and annual ryegrass. In general, annuals will require one to one and a half inches of water per week while cool-season perennials need two to two and a half inches depending upon the air temperature.
In the southern half of the USA, a double crop of fall-planted cool-season annuals followed by crabgrass or grazing maize makes an excellent, high quality, long season pasture.
There have been some spectacular per acre gain results with irrigated improved bermudagrass in the South when both a variable stocking rate and a high level of nitrogen fertilization were used.
Nitrogen should be applied in multiple small doses so as to not exceed 40 lbs of actual N per application. Liquid nitrogen can be applied through the irrigators.
It’s time to water your pasture when your soils have lost 50 percent of the water available to plants in the root zone. The root zone for pasture is two feet. For grazing maize it is three feet and for alfalfa it is four feet.
To determine the soil root zone moisture level use a shovel or soil auger to take a soil sample from the root zone. With 50 percent available water, sandy soils will roll into a ball under pressure. Clay soils will form a ribbon and loamy soils will form a ball and show water at their surface.
VARIABLE STOCKING RATE
A difficult lesson to learn is that your stocking rate is always increasing with growing cattle as time progresses even when per head numbers remain the same. Trying to run a fixed number of growing cattle through the whole season invariably means you will run out of grass.
Bermudagrass and other warm-season grasses which are heat and drought tolerant are highly sensitive to day-length. The stocking rate must build to a peak around the fourth of July and then decline as the days become progressively shorter.
All perennial cool-season grasses will require a decrease in stocking rate during the mid-summer period even with irrigation. Cool-season grasses virtually go dormant when the temperature is over 86 degrees F (30 C) and warm-season plants will go dormant at 95 degrees F (37 C). Temperatures in interior North America frequently exceed both of these extremes.
In very hot summer areas, sensitive cool-season perennials such as perennial ryegrass must be completely deferred from grazing during heat waves to survive. This is particularly true when night time temperatures are above 56 degrees F (14 C).
Both cool-season and warm season pastures in the fall will only support approximately half the stocking rate of their peak growing seasons. Regardless of the class of animal you graze, some sort of “put and take” system must be in place to match the stocking rate to the seasonal variation in grass growth and to keep the grass in a young and vegetative state.
With dairy cattle, this “put and take” is usually a haying or pasture silage operation during the spring peak growth season. With a more flexible beef stocker operation, this is best accomplished by selling and buying animals.
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