Mineral Nutrition for Grazing Beef Cows: Intake and Quality
When beef cows are grazing over the next few months, nutrient requirements for energy and protein are generally met by the forage, and thus vitamin and mineral supplementation is the primary nutritional concern. Most producers provide free choice mineral to cows during this time and assume that it is providing adequate amounts of the necessary minerals. However, there are some important factors to consider to ensure mineral requirements are met.
First, and most important is mineral intake. Cows do not consume free choice mineral consistently throughout the summer grazing season. Some believe that cows will consume what they need, but that is not the case. Salt intake is the driving factor affecting mineral intake. If cattle are consuming significant amounts of salt in the forage, then they will decrease intake of high salt mineral and vice versa if they are consuming small amounts of salt in the forage. Adjusting the amount of salt in the mineral is the best way to regulate intake, and if plain white salt is provided mix it with the mineral otherwise cattle will consume the salt instead of the mineral.
Availability and location of clean water can also impact mineral intake. Mineral supplements generally have a high salt content, and cattle will want a drink after consuming mineral. And may stop consuming mineral sooner if water is not readily available. Placing mineral feeders relatively close to water sources can help if mineral intake is less than desired, but having mineral feeders too close to loafing areas can result in overconsumption of mineral.
The form mineral is provided can also affect intake. Cows will generally consume less mineral per day when in block form than in loose form just for the simple fact that the amount of mineral consumed per minute is lower with blocks. It is important to provide several blocks at once so multiple animals can consume mineral at the same time. Also, lower intake of mineral blocks means that the mineral concentration in the block should be increased accordingly. Blocks may be a good option during times of the year when cattle want to overconsume loose mineral. Mineral tubs on the other hand are consumed in greater amounts than loose mineral by cows, generally by design as tubs usually provided added protein and sugars to aid in digestion of more mature forages. However, the high palatability of tubs can easily result in overconsumption.
Individual cows also do not consume the same amount of mineral with some cows consuming less and some cows consuming more than desired. In a recent study at the USDA-ARS For Keogh Livestock & Range Research Laboratory, there was a 3-fold difference in mineral intake among cows in the herd. Currently, very little is known about the variability in mineral intake among cows and the reasons this variability exists. In this study there was no statistical difference in calf weaning weight or postpartum interval between cows consuming the most versus the least amount of mineral, but there appeared to be a linear trend that as cows consumed more mineral, calf weaning weight decreased. There could be several reasons for this that need to be explored before management strategies can be developed.
The most important aspect of mineral intake is to monitor intake. Adjustments to salt content or mineral form cannot be made accurately if the current level of mineral intake is not known. The date mineral is delivered, amount of mineral delivered, and the number of cows in the herd should recorded to compute the average daily mineral consumption. Mineral intake can then be managed accordingly.
The second factor of mineral supplementation is the quality of the mineral, which includes concentration of individual minerals in the supplement, bioavailability of the mineral sources and mineral deficiencies of the forage, all of which affect the ability of the mineral supplement to meet the mineral requirements of the cow herd. Soil mineral content and plant availability, forage species, and forage maturity affect the mineral content of the forage consumed by the cow. Different regions of the US differ in mineral content of soils resulting in different mineral content of forages grown in that region. Other characteristics of the soil, such as clay content and pH, also affect the availability of soil minerals for uptake by the forage plant. Thus, mineral supplements should be formulated for the region of the country where they will be used.
Forage species also differ in mineral content even when grown in the same soil. In broad terms, legumes are different than grasses, and cool-season grasses are different than warm-season grasses. For all forage species, mineral content is generally greater in lush growing plants than more mature plants. Additionally, more mineral is associated with the plant cell wall, which is the less digestible fraction, as the forage plant matures. The proportion of individual minerals also changes as the forage plant matures. Thus, the concentration of individual minerals in the supplement should change in different times of the year.
There are several sources of minerals that can be used in manufacturing a mineral supplement: oxide, sulfate, chloride, and organic. These sources differ in bioavailability, which is the ability of the mineral to be absorbed and function in the body. Generally, organic sources are the most bioavailable, but the increase in bioavailability compared to the other sources is not equal among mineral elements. For example, organic selenium is 40% more bioavailable than selenite, but organic manganese is only 25% more bioavailable than manganese sulfate. Sulfate and chloride sources have similar bioavailability, and oxide sources have very low bioavailability with the exception of magnesium oxide, which is similar to magnesium sulfate or magnesium chloride. Knowledge of the source of the mineral element in the mineral supplement can be important, especially in areas where mineral antagonists are prevalent in forage plants.
There are several factors to consider and manage when it comes to mineral supplementation of grazing beef cows. Make sure that the mineral supplement is formulated for the soils and forages in the appropriate region of the US. Monitoring mineral intake is the most important aspect because even the best formulated mineral cannot meet cow requirements if the mineral is not consumed in the proper amount.
