When thinking about beef cows on pasture we oftentimes figure that what they drink is all the water they get. There are however three additional moisture sources cows rely on (excluding snow!) which, by increasing order of biological and practical significance are:
- Metabolic water (water generated in the body particularly from fat depots)
- Plant surface water (such as dew)
- Embedded water (water contained in the plant structure itself)
Particularly useful to species in dry environments metabolic water relies on the synthesis within the body mostly from the high ratio of hydrogen present in fat depots and the oxygen liberated in internal reactions. Camels (store fat in the hump) and some sheep species (store fat in the tail) are examples of this adaptation to dry conditions.
Plant Surface Water
Dew is the condensation that occurs during the morning, very common in some regions and times of the year. Favorable weather conditions for its build-up include clear skies, no wind, and decent soil moisture. When there is no rain for more than at least a week, chances for significant dew accumulation are minimal. Even in the best conditions, research has measured values that rarely exceed 0.02 inches. In spite of getting your boots wet, the contribution to the cow metabolism is minimal.
Plant Embedded Water
Aside from the drinking water this other source is the most critical to the cow. Its importance varies with different plant species and their maturity stage. Young plants have high water content, which is reduced as the plant reaches reproductive maturity, and later senescence. Early spring pasture for example can have 85 percent water but as it matures and bleaches during the summer can drop to 50 percent or less. Needless to say if it’s dried for hay the bales will end up with only 12-15 percent water.
Water embedded in feed during the spring
It is estimated that in general a lactating beef cow needs two gallons of water per 100 pounds of body weight during the summer. A 1,200 pound cow then may drink roughly 24 gallons of water, maybe up to 10 more to produce milk for the calf. If a cow eats roughly 2 percent of their body weight as dry matter during the spring, she could very well be “eating” 24 pounds of dry matter of pasture. If this “spring pasture” has 85 percent moisture this cow will be “eating” 17 gallons of water so she will reduce the amount she needs to drink.
Water embedded in feed during a dry summer
During a dry summer or when a drought has developed the situation differs. For once, the total pasture to the cow is limited. Depending on the forage available she may now be eating half of what was offered during the spring. In addition, as the plants mature and bleach under the summer heat, their water content is reduced oftentimes to 50 percent or even less. So the same cow that still nurses the calf will graze 1 percent of her body weight, or 12 pounds of dry matter, bleached pasture. This is where the rancher steps in and supplements with round bales reserved for the occasion. However, the amount of water the cow is now “eating” with the feed is less. If the pasture has now a maximum of 50 percent water the cow will be “eating” 3 gallons of embedded water. It does not really matter much to the “water budget” that the rancher supplied 12 pounds of dry straw because in essence they will only supply roughly 1 quart of water in total.
Balancing the cow’s water budget
So here is where things get dicey, the cow is now forced to drink more water to make-up for the one she didn’t “eat” with the forage. If the quality of the water in the ranch was borderline bad during the spring it is not as dangerous. The cow was ingesting 17 gallons with the spring pasture, so she would have to drink much less water from the source to make-up for the 24 plus gallons required.
The situation becomes far worse in the heat of the summer. Her water “eaten” with the feedstuffs is 3, maybe 4 gallons. The cow has to make-up for the difference with the water she has available to drink. To make matters worse, if is water from a pond, and no significant rains have happened, water evaporation would have concentrated chemicals, algae, etc. This water that may have been borderline acceptable in the spring, can become a health threat for the cows in the summer. The point to drive home for every rancher during this situation is first and foremost to analyze water. Second, baled forages to be used need to be of fair to high quality, if by any chance there is nitrate accumulation in the forage (1 percent and above), its combination with increased intake of low-quality water can have dire consequences for the cattle enterprise.
Source: Alvaro Garcia, South Dakota State University