Reduced dry matter intake caused by limited forage during drought can have an adverse effect on reproductive efficiency, especially if cows are thin at calving.

Cows that enter the winter in poor body condition can have lower survivability, but they also have a greater chance to have a weak calf, produce an inadequate quality and quantity of colostrum for their calves, have a calf with a reduced weaning weight and fail to become pregnant during the next breeding season.

Adult cows need to calve in a body condition score (BCS) of 5.5-6.0 while bred heifers need to be in even better condition than adult cows. Average milking heifers should calve at a BCS 6.0-6.5. Work from Oklahoma State University reported that pregnancy rates after 60 days of the breeding season were 56% for cows with a body condition of 4 vs 96% for cows with a body condition of 6. Heifers raised on a restricted diet weighed less, reached puberty later and had lower pregnancy rates than those fed an adequate amount of nutrients.

When facing drought conditions during the breeding season it appears the rancher has two choices to consider: supplement the deficient nutrients and/ or reduce the size of the cowherd to match reduced forage available. When dealing with drought, remember to provide the big nutrients first by asking two questions: Is there adequate protein and/ or energy (TDN) in the diet? What symptoms are the cows showing during or after a drought? Do you observe an increase in respiratory sickness (immune system), is there decreased reproductive efficiency (lowered conception), a change in hair color (red hair on an Angus) and finally did cows not “clean” after calving (retained placenta)? If you answered yes to these  observations, then there may also be a mineral deficiency.

Both macro minerals (required in larger quantities, such as calcium, phosphorus, magnesium) and micro minerals (required in smaller quantities such as copper, zinc, selenium, and manganese) can cause decreased conception, delayed estrus, abnormal estrus, depressed immunity, and reduced forage intake. Subclinical trace mineral deficiencies probably occur more frequently than recognized by most producers and may be a larger problem than an acute mineral deficiency. This is because the rancher does not see specific symptoms that are characteristic of a trace mineral deficiency. With a subclinical deficiency, the animal grows or reproduces at a reduced rate, uses feed less efficiently and operates with a depressed immune system.

The macro mineral phosphorus is usually provided in a loose, free choice mineral supplement and is important in most parts of the United States, but it becomes more important in areas of the country where cattle are maintained on grazed forage year around. In areas of the country such as Arizona, New Mexico, Texas, and Oklahoma. A phosphorus deficiency is more likely to be observed, especially during periods of drought.

However, one major disadvantage to free-choice minerals is lack of uniform daily consumption by animals. Factors which influence consumption of loose minerals include: (1) soil fertility and forage type, (2) season of year, (3) available energy and protein, (4) individual requirements, (5) salt and mineral content of water and supplement, (6) palatability of mineral mixture, and (7) physical form of the minerals. An injection of trace minerals containing copper, zinc, selenium, and manganese has been shown to quickly overcome some of these problems by complementing loose mineral containing these trace minerals.

Regarding the utilization of trace minerals, antagonists in water or forage can reduce the digestibility of copper and selenium. In the late 1930’s, scientists in England described a severe scouring disease of cattle called “teart” that was caused by ingestion of forage containing high levels of molybdenum. Later, scientists discovered that feeding large doses of copper sulfate could prevent this condition. In addition, it was shown that molybdenum limited the retention of copper in the body, especially in the presence of inorganic sulfate in the diet.

Molybdenum in the presence of sulfate reduces the deposition of copper in organs and increases the excretion of copper in the urine. When the copper to molybdenum ratio of the diet in the presence of adequate sulfate was less than 2.8 to 1, a copper deficiency was evident. A copper to molybdenum ratio of no less than 4:1 has been proposed to ensure that the copper requirement will be met. High levels of dietary zinc and iron also depress copper absorption and tend to increase the requirements. High dietary levels of zinc (100 ppm) reduce liver copper storage, and therefore it is recommended that the zinc to copper ratio be maintained at least a 3:1 ratio. A higher ratio is better.

Much of the water available to cattle in is not sufficient in quality to sustain desired performance and health. Water sources that are high in sulfate caused by drought can induce a copper deficiency in cattle despite adequate copper intake. This is because sulfur and molybdenum separately or together can form insoluble complexes in the rumen with copper and markedly reduce its availability to the animal. High dietary molybdenum in combination with moderate to high dietary sulfur results in formation of thiomolybdates in the rumen which greatly reduce copper absorption. Independent of molybdenum, high dietary sulfur reduces copper absorption probably by the formation of copper sulfide.

Work from SD showed that high levels of water sulfate (3900 ppm) resulted in slower daily gains, lowered water consumption, an increase in mortality, morbidity and polio in steers compared to steers consuming water with 400 ppm sulfate.

Selenium bioavailability is also reduced by high dietary sulfur, and high dietary iron can also reduce copper bioavailability. This is one reason that an injection with trace minerals will overcome ruminal effects on absorption caused by antagonism. Research showed that plasma copper, selenium and zinc concentrations increase rapidly after injection compared to an inorganic form fed as an oral supplement.

Work from Iowa State University found a significant boost in trace mineral status 14 days after calves received an injection of trace minerals compared to calves fed an inorganic and chelated organic mineral combination which reached the same level after 28 days. The group supplemented with only inorganic minerals took about 45 days to reach the same trace mineral status as calves previously given an injectable trace mineral.

Research from Brazil showed that an injectable trace mineral injection increased pregnancy rate of cows with body condition scores less than 5, but a nonsignificant response for cows with a body condition score greater than 5. Cows in a thin body condition due to drought appear to be good candidates for  a trace mineral injection as part of a strategic mineral supplementation program.

Summary:
1. Drought causes a decrease in forage intake which results in decreases in protein and energy consumption.
2. Cows showing poor body condition (<5) have a greater chance of having a weak calf, producing an inadequate quality and quantity of colostrum for their calves, having a calf with a reduced weaning weight and failure to rebreed during the next breeding season.
3. During a drought, water quality from ponds may be lowered due to an increase in sulfate and iron. These two minerals plus forage molybdenum cause an antagonism in the rumen which reduces bioavailability of copper and selenium. A trace mineral injection should overcome these antagonisms and has a faster uptake into the blood stream that feeding an inorganic source of minerals.
4. Thin cows with a body condition score of less than 5 caused by drought appear to be a good candidate for an injectable trace mineral injection.
5. The use of an injection using a product like Multimin90 in conjunction with feeding a loose mineral makes practical sense to prevent a decline in reproduction.