Trace Mineral Source Could Negatively Impact Forage Digestion

Utilization of winter forage will be critical in many parts of the country this year due to the extended drought and the quality of forage harvested for winter feeding. Producers should plan ahead to maximize utilization of their forage resources, which should address several areas: reduce wastage, maximize digestibility and extending the supply. 

Wastage of forages by cattle is significantly impacted by the amount of available forage. When grazing stockpiled dormant forages, allowing cattle access to the entire pasture decreases harvest efficiency and utilization. Implementing strip grazing can increase harvest efficiency and reduce wastage thereby extending the forage supply. Allow the animals only enough forage for 1 day and move the electric wire daily. 

When feeding cattle harvested forages, ad libitum access will increase wastage because cattle will select the best parts of the hay and in the process waste much of the remaining hay. Limiting the access to hay will reduce wastage, which can be done in several ways, including:

- Grind hay to feed in fence line bunk. The most effective way is to grind the hay and feed in a fence line bunk, but this may not be cost effective for many operations due to the increase in facility and equipment costs.

- Open round bale feeder. Typical open round bale feeders allow for a large amount of wastage because the bale is sitting directly on the ground drawing moisture, some hay falls through the open sides, and cattle tend to pull out a mouth full of hay then step back allowing some of the hay to fall to the ground outside the bale feeder.

- Closed bale feeder. These keep hay from coming through the sides of feeder where it gets trampled, and cone feeders keep the bale off the ground and restrict access allowing cattle to pull out a mouth full of hay, but hay space to chew with their head inside the bale feeder. Thus, dropped hay still falls inside the bale feeder. 

Maximizing digestion involves providing the nutrients that rumen microbes need to digest the forage. The nutrient with the largest impact is protein, particularly rumen degradable protein. This is protein that microbes in the rumen can digest and use to grow, thus allowing them to digest forage carbohydrates. Feedstuffs with high rumen degradable protein are generally those made from oilseeds include soybean meal, cottonseed meal, sunflower meal, and peanut meal. 

In addition to protein, trace minerals are needed by microbes to digest forage; however, over supply of trace minerals can negatively affect microbial growth and forage digestion. Trace minerals such as zinc and copper have antibacterial properties and can inhibit growth of rumen microbes.

The source of trace minerals affects solubility in different sections of the digestive tract: sulfate-based minerals are highly soluble in the rumen increasing rumen concentrations, whereas, hydroxychloride-based minerals are less soluble in the rumen, but highly soluble in the low pH of the abomasum. The most commonly used sources of copper and zinc in beef cattle trace mineral supplements are sulfate-based. 

In steers fed a medium-quality grass hay with adequate rumen degradable protein, providing a sulfate-based source of copper and zinc decreased digestibility of total forage and forage fiber compared with a hydroxychloride-based source (Figure 1A). Additionally, a recent review of the literature reported that total diet digestibility and fiber digestibility were increased with hydroxychloride compared with sulfate-based sources of copper and zinc (Figure 1B).  

In most cases we have a guestimate of mineral content of the forage, and such we tend to hedge mineral supplementation upward to minimize risk of mineral deficiency. However, this may lead to reduced forage digestibility when using sulfate sources as the greater rumen solubility of these sources may negatively impact rumen microbial growth and forage digestion. Although costly, producers should consider a full mineral evaluation of their forage resources to better deliver appropriate levels of trace minerals for maximum forage digestibility.