Supplementing With Zinc Can Pay Off!

In previous articles I have mentioned zinc as a trace element of importance. Regular supplementation with zinc (other than for facial eczema control) has not been a common practice in New Zealand, but its popularity as a supplement for dairy cattle is growing.

The classic signs of zinc deficiency include a crusty growth and cracking of the skin called parakeratosis. Although mainly recognised in pigs, it has occurred in cattle and other species. To my knowledge, parakeratosis has never been reported in New Zealand cattle. Our soils contain reasonable levels of zinc, although levels tend to be higher in the North Island than in the South.

So if soil levels are seemingly adequate and a gross deficiency of zinc has never been seen in NZ, why would supplementation be of any benefit?

To answer this, we must firstly consider what we mean by trace element levels being “adequate”, or “deficient”, as opposed to “optimal”. We usually base the categorisation of “adequate” or “deficient” levels , on a numerical scale. If soil, pasture, or animal samples are below a certain level, we may say the samples indicate a deficiency of the mineral. If they are above a certain level, they may be categorised as adequate. (If very high levels occur, they may be regarded as “toxic”).

These numerical levels have commonly been set by production response trials. Essentially, if animals respond in controlled supplementation trials by a measurable improvement in production, then their pre-supplementation level may be considered deficient. If no measurable production response occurs, then the level at and above that observed before supplementation may be considered adequate. In such case, supplementation may be considered unnecessary and uneconomic.

This categorisation has served well as a practical way to decide whether supplementation is worthwhile or not. However, the level that is regarded as “adequate”, may not represent that which is “optimal”. It may take measurement of parameters other than growth rate, milk or wool production, or fertility, to determine what may be the “optimal” level of a particular mineral that best maintains the health and well-being of a particular herd on a particular farm.

A decline in immune system competence or physiological performance often occurs well before declining trace mineral levels result in production loss or clinical signs of deficiency. A “sub-optimal” trace element level may display as greater susceptibility to disease, decreased stamina, or inferior quality of a product such as milk. So supplementation of a mineral or maintaining a level higher than is “adequate” may be beneficial to overall health and well-being.

Secondly, although mineral levels measured in soil, feed, and even from animal tissues, may appear “adequate”, the mineral may not be biologically available for the animal to utilise.

This brings us back to zinc. Although soil levels may seem adequate, and no gross symptoms of zinc deficiency have been observed, this does not mean that livestock are getting the amount of usable zinc that they may optimally require on a regular basis.

Zinc is used by the body in many ways and demand for the mineral varies daily. As mentioned, it is necessary for maintaining healthy skin structure. But in particular, it is required by the immune system to maintain natural defence mechanisms. The responsiveness of the immune system depends on having a ready access to zinc. This may be why the response of dairy cattle to small but regular amounts of highly bioavailable zinc, such as zinc amino acid complexes, have included a reduction in SCC’s, lowered incidence of mastitis, and better immune competence of the transition cow.

This effect has been recorded in dairy cows in many countries where there is no gross deficiency of zinc, including New Zealand, which suggests that on many farms the diet may provide “adequate” zinc, but not an “optimal” amount of zinc.

Supplementation with a bioavailable zinc starting about three weeks before calving and continuing during lactation, can be a cost effective option on many farms. The benefits reported include improved milk yield as well as quality, better resistance to disease, and reduced incidence of lameness due to improved hoof hardness.