Supplementing minerals on pasture — a necessary evil!

Nutrition with John McKinnon

Supplementing minerals on pasture — a necessary evil!

Hopefully, as you read this article, “spring has sprung” and pastures across the country are showing signs of early growth. For cow-calf operators, the arrival of spring signifies the transition from winter feeding to a new grazing season. Spring pasture growth is considered an excellent forage source for meeting the nutritional requirements of lactating cows, particularly with energy and protein content. Seldom is there a need for supplementation, particularly if cattle numbers are matched to existing forage supply.

However, there continue to be questions, even skepticism, on the part of some producers regarding the need to supplement mineral on pasture. Questions range from the need to supply minerals to the type of mineral required for a given grazing situation. With this column, I would like to address both of these questions.

First let’s address the need to supply minerals on pasture. The specific macro (i.e. calcium, phosphorus, magnesium) and trace (i.e. copper, zinc, manganese) mineral content of any given pasture will depend on numerous factors including forage species, soil type, plant maturity (i.e. spring versus fall) and environmental factors that influence pasture growth from one year to the next. To illustrate this point, let’s look at the results from a survey conducted by the Saskatchewan Forage Council (fact sheets available at The survey looked at macro and trace mineral content of a variety of grass and legume pasture species sampled in the spring and fall of two growing seasons. The results showed that spring-sampled alfalfa pastures averaged 1.8 per cent calcium, while the calcium content of various grass species ranged from 0.3 to 0.4 per cent or almost a five-fold difference. Spring alfalfa samples also tended to be higher in magnesium (i.e. 0.43 versus 0.14 per cent) than comparable grass samples.

More importantly, when meeting the requirements of a lactating beef cow shortly after calving, only 70 per cent of spring-sampled pastures met calcium requirements, regardless of plant species. Only 65 per cent and 25 per cent of samples met phosphorus and magnesium requirements. The situation worsens when looking at trace minerals. For example, only 15 per cent of pastures met copper and zinc requirements. To further confuse the issue, some minerals — such as calcium, magnesium and manganese — tended to increase in concentration from spring to fall. Meanwhile others — such as phosphorus, zinc and copper — tended to decrease.

While these results are a snapshot of pasture mineral content in one province, they illustrate several important points. First, there is a need for macro-mineral supplementation (i.e. calcium, phosphorus and magnesium) of lactating beef cows on pasture, particularly those grazing early spring pastures with a high proportion of tame or native grasses. As the season progresses, the need for calcium and magnesium supplementation may decline in part due to increasing grass concentration of these minerals and in part due to reduced animal requirements, particularly as milk output declines. In contrast, meeting phosphorus requirements will continue to be a challenge, particularly as plants mature over the summer and fall.

These results also indicate that regardless of pasture species or stage of maturity, trace minerals need to be supplemented. It is also important to realize that regardless of forage trace mineral concentration, digestion and absorption of trace minerals such as copper and zinc can be seriously impeded by factors such as age of animal (i.e. copper is poorly absorbed by mature ruminants) and high dietary levels of antagonistic minerals such as molybdenum, sulfate or iron.

What do these results indicate about the type of mineral required on pasture? First, they highlight the importance of understanding the forage makeup of your pastures and the need for a yearly forage mineral analysis. This information can guide you in choosing the appropriate calcium-to-phosphorus ratio (i.e. 2:1 vs. 1:1) as well as in pinpointing where you need to focus on meeting trace mineral requirements. A forage mineral analysis is also useful to evaluate if projected free choice mineral consumption, as indicated on the mineral tag, will offset specific mineral deficiencies such as that of magnesium or copper.

The results also underscore the importance of magnesium supplementation, particularly for spring-calving cows, as well as the potential for widespread trace mineral deficiencies. To offset these deficiencies, many feed manufacturers offer a mineral specifically designed for the breeding season. These minerals are formulated to have elevated levels of macro minerals such as calcium, phosphorus and magnesium as well as specific trace minerals such as copper, zinc and manganese. Another common characteristic is to have a proportion of the trace mineral package as chelated minerals. Chelated minerals are better absorbed than conventional sources and can lead to a more rapid restoration of the animals’ trace mineral reserves.

Finally, as the pasture season progresses and/or as animal requirements change due to cessation of breeding or a decline in milk production, switching to a “range” mineral designed to supplement the macro and trace mineral profile of your pasture species is a logical step not only to meet requirements but also to reduce cost.

Returning to our initial questions, I hope it is clear that supplementing minerals on pasture is a necessity! To avoid unnecessary pitfalls when selecting which mineral is right for your situation, it is advisable to seek professional advice from your local feed company representative and/or from a consulting nutritionist/veterinarian.

About the author


John McKinnon

John McKinnon is a Professor Emeritus at the University of Saskatchewan and a consulting nutritionist who can be reached at [email protected].



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