Nothing in your management efforts will affect production of your pasture more than providing adequate irrigation. Different soil types hold various amounts of water. If all of the water held by the soil was removed, the plants would die. A general rule is that when the plants have removed 50% of the water held by the soil, it is time to irrigate and refill the rooting zone. For established pasture grasses, the rooting zone is 2-3 feet.
Determining when to irrigate. In all but the sandy textured soils, you can do a rough check on soil moisture by using the soil ball method. Dig a small hole and remove a handful of soil from a section 6" to 12" deep. Squeeze the soil into a ball. If, when you open your hand and bounce the ball (in the palm of your hand), the soil remains in a stable shape, your soil has more than 50% of its available water holding capacity. If it crumbles, it needs irrigation.
Am I applying too much water? Using too much water washes away plant nutrients. When this happens grasses appear yellow. Growth of aquatic weeds, such as sedge or rushes, are also indicators of too much water.
What if I'm short on water? If you grass is going dry and dormant between irrigation turns in the hottest past of the summer, it is time to sacrifice some pasture. Let some land dry up in July and August, the grass will green up again in the fall. IN the meantime, keep your most productive areas green and unstressed by focusing your irrigation here. You will get more production from a smaller area of unstressed grass than a larger area of grass going in and out of dormancy.
How long should I irrigate? While this depends on the irrigation supply rate, in general, irrigate sandy soils for short periods (9-12 hours). Before considering rain as a replacement for an irrigation turn, use a shovel to see how deep it has penetrated into the soil. Not often does a rain storm provide enough water to fill the rooting zone.
Sprinkling is one of the best ways to efficiently irrigate your pasture, but it requires a significant monetary investment in equipment. There is little runoff with sprinklers and it is easy to measure how much water is being applied.
Flood Irrigation Methods. With all flood irrigation systems, it is essential that provisions be made to drain excess tail water off the bottom of the field. This water may carry fertilizers and sediment and thus be a source of contamination to streams.
Graded Border Irrigation is when water is flooded down the field between two dikes. It works well, but the land should be leveled for best efficiency. It is difficult to know how much water soaked in and how much ran off the bottom of the field.
Corrugations are closely spaced small furrows that carry water from the top of the field to the bottom. This approach doesn't require quite as level a field as border irrigation, but still doesn't provide for easy monitoring of the water actually retained by the soil. It is also very rough running hay harvesting equipment across the corrugations.
Wild Flood is just running water from the top of the field to the bottom with no mechanism for insuring an even distribution. For this method to work efficiently, the field must be very level with a uniform fall from top to bottom.
Here are more resources regarding irrigation to help you manage your water effectively and efficiently.