As cows get closer to parturition, their dry matter intake is reduced significantly. In the last week of gestation, a cow’s dry matter intake can be reduced by as much as 90%. A couple of factors cause this reduction. The first is due to physiologic changes in the cow. The developing fetus has occupied more room in the abdomen reducing the amount of space for the rumen. The increased weight of pregnancy also causes the cow to spend more time laying down and less time eating. In addition to these physiologic changes, management practices of feeding anionic salts to prevent milk fever leads to a decrease in palatability of the ration. If the estimated intakes are less than what the ration was balanced for, it is very likely that our mineral needs are not being met simply due to decreased intakes.
Getting cows to eat the ration that contains the trace minerals is only the beginning of the battle. Once inside the cow, these minerals must still be absorbed into the blood stream. Depending on the mineral and the form in which the mineral is found, cows may absorb as little as 1% of the mineral that is consumed. Feeding organic or hydroxy minerals can greatly increase this absorption. These mechanisms help protect the mineral from the rumen environment and allows the minerals to be absorbed in the small intestines along with the nutrient to which they are attached.
Since only a percentage of what is consumed from the ration is able to be absorbed, the remainder of the minerals are going to pass through the cow and be excreted in the feces. In addition to what is excreted in the feces, as minerals are absorbed from the gut into the blood stream, the kidneys will filter some of those trace minerals out excreting them in the urine. Events that raise the serum mineral concentration such as stress, illness, vaccine, or fetal development leads to increases in urinary excretion. Biliary excretion makes up the final major method of mineral excretion from the body.
Minerals don’t like being by themselves; they always come to the party with a partner. Once to the party, they often break apart exposing themselves to other minerals. As minerals bind up to some of these new partners, they can become less absorbable by the cow. Most of these processes take place in the rumen, but some antagonism can even occur in the blood stream. Sulfur, calcium, iron, and molybdenum are some of the major mineral antagonists. During the close-up period, sulfur is often fed at higher levels in an attempt to help in the prevention of milk fever (hypocalcemia). Feeding levels above 0.3% of the diet of sulfur can decrease the absorption of copper by over 30%. Since cows still need to be fed these minerals for normal biological function, eliminating these antagonists from the diet is not feasible, but assuring that proper levels are fed without excess is important to minimize negative mineral interactions in the body.
The transition period of a dairy cow has a high demand for trace minerals, but also magnifies these four hurdles of trace mineral nutrition more than at any other time during her lactation. Recognizing these hurdles helps us to implement strategies to assure that mineral needs are met for the cow. Feeding organic or hydroxy minerals can help to increase absorption and protect the mineral from antagonism in the rumen. During the close-up and fresh period, utilization of injectable trace minerals can help to overcome decreased feed intakes while also avoiding rumen mineral antagonism. Injectable trace minerals have also been shown to rapidly increase the trace mineral status of the cow within hours because of the high rate of absorption. Assuring proper trace mineral status is critical in maximizing the health and reproductive potential of the herd.