Does Dry Grass Increase Cattle Drinking Water Needs?

By Scout Nelson

Cattle need clean drinking water every day, but they also get water from the grass they eat. The amount of water in grass can change during the growing season. Young, green grass often contains more moisture than mature, dry grass.

These changes may affect how much water cattle need from livestock tanks. Understanding this relationship can help ranchers manage grazing and plan water supplies.

Hector Menendez, Assistant Professor and SDSU Extension Livestock Grazing Specialist, and Federica La Manna studied the relationship between grass moisture and cattle drinking-water use in South Dakota.

Their research examined how water from forage relates to the amount of water cattle drink from tanks during summer. The findings may help ranchers better understand cattle water needs as pasture conditions change.

Two Sources of Water for Cattle

Cattle get water from two main sources: drinking water and the plants they eat. These sources are often described as blue water and green water.

Blue water comes from stored water sources, such as ponds, streams, rivers, and reservoirs. This water can be supplied to cattle through livestock tanks. Green water refers to rainfall and snowmelt that support plant growth on rangelands. Cattle receive some of this water when they eat grass and other forage.

Grass moisture changes with plant growth and weather conditions. Grass in early growth stages, including tillering, often contains more water than mature forage. Moderate temperatures and enough soil moisture can also support plant growth and affect the amount of water held in the grass.

Both drinking water and water from forage contribute to cattle's daily water needs. However, the amount each source provides changes with pasture conditions and the growing season.

Research Conducted in South Dakota

During the summer of 2025, researchers studied 127 yearling Angus steers grazing six native-grass pastures at South Dakota State University's Cottonwood Field Station in western South Dakota. The study took place from June through August.

The pastures used different grazing methods. Some had continuous grazing, while others used rotational grazing with virtual fencing. They also differed in stocking rates, including low, moderate, and heavy levels. Plant communities varied as well, with cool-season grasses such as western wheatgrass and warm-season grasses such as buffalo grass.

Researchers installed water meters at drinking tanks to measure how much water each herd used daily. About every two weeks, they collected grass samples and measured their moisture levels.

The team then estimated how much water cattle could obtain from the forage they ate. This estimate used an assumed dry matter intake equal to 2.5% of the animals' full body weight. It was not a direct measurement of water consumed from grass by each steer.

Grass Moisture Drops as Summer Progresses

The study found that grass moisture decreased as summer continued. Average moisture levels were about 45% in June, 40% in July, and 38% in August.

These results show that cattle graze on drier forage later in the summer. When grass contains less water, it provides less moisture through feed. As a result, drinking water from tanks may become more important in meeting cattle's daily needs.

The researchers also found a relationship between estimated water intake from forage and measured drinking-water use. When estimated forage-water intake was higher, drinking-water use tended to be lower after the researchers adjusted for pasture area.

However, this result reflects water use across entire herds. It does not show exactly how much water an individual steer saves when eating grass with higher moisture levels.

What the Findings Mean for Ranchers

Grass moisture is one factor that may influence cattle water needs throughout the grazing season. Green grass in early summer can provide more water than dry, mature grass later in the season. Ranchers can consider these changes when planning water supplies and managing pastures.

Other factors also affect how much cattle drink. These include hot weather, animal size, feed intake, and grazing conditions. Therefore, grass moisture alone cannot explain all changes in drinking water use. Cattle must continue to have access to clean drinking water throughout the day.

The research team plans to study how weather, grazing practices, and forage moisture work together to affect water use at both herd and individual animal levels.

Further research may help explain seasonal changes in forage dry matter intake and identify cattle that use water efficiently. It may also improve understanding of how grazing practices affect water needs under different pasture conditions.

Knowing how grass moisture changes during summer can help ranchers make informed decisions about water supplies and grazing management. This research offers a useful starting point for understanding how forage water and drinking water work together to support cattle.

For more information or to enroll a ranch in the SDSU water use program, contact Hector Menendez.

Photo Credit: South Dakota State University