18 July 2023

Prevalence and seasonality of trace mineral concentrations in sheep-grazed pastures

Minerals perform many important functions in the body and imbalances (deficiency or toxicity) can have detrimental effects on the performance, fertility, health and profitability of livestock.

An adequate supply of minerals is essential for maintaining optimum growth, health and reproduction of livestock and marginal deficiencies can have a substantial impact on performance and health.

In lowland sheep production systems, herbage, either grazed or conserved, can provide up to 95% of annual feed requirements. Grazed herbage is therefore an important source of minerals for animals in grass-based systems.

Results from the Teagasc National Farm Survey show that 69% of Irish sheep farmers supplement their flock (ewes and/or lambs) with minerals at least once annually. The most used methods are drenching for lambs and mineral buckets for ewes.

Only 35% of farmers who supplement base their decision on veterinary advice or laboratory analysis. Information on monthly variation in herbage trace element concentrations and, hence, the adequacy of supply from a grass only diet, would provide producers with evidence on when supplementation may be necessary and which minerals are likely to be deficient or marginal and thus result in reduced animal performance.

Therefore, producers could better match mineral supplementation strategies with pasture herbage concentrations allowing for more targeted mineral supplementation, thus improving animal performance and reducing production costs in grass-based sheep systems.

Ireland study

Herbage was sampled monthly, pre-grazing, from three paddocks on 56 lowland farms, which were selected, based on geographical location, soil type and farm system, as being representative of lowland sheep producing areas throughout Ireland.

Herbage was cut to the expected post-grazing sward height (4 cm in March and April, 5 cm in May and 6 cm from June to November). The herbage samples were analysed for a suite of 22 minerals.

Herbage mineral concentrations

Cobalt, copper, iodine and selenium are considered the main essential trace minerals for sheep production. The mean concentration of these minerals is presented in Table 1. The concentrations of these minerals varied throughout the grazing season. The concentration of cobalt was lowest in June and July and increased in September and October (Figure 1). The concentration of iodine declined between March and June and was higher in September and October relative to all other months. The concentration of selenium was at a minimum in April/May and was higher in June, July, August, September and October. Herbage copper concentration increased during the grazing season being lower in March than in May, July, August, September and October.

Figure 1: Monthly mean concentrations of cobalt, selenium and iodine

The proportion of farms classified as deficient, marginal or sufficient for cobalt, copper, iodine, selenium and zinc is presented in Table 1. The majority of farms were deficient for cobalt (73%) and iodine (80%), deficient or marginal for selenium (100%) and zine (78%). The herbage on all the farms was classified as sufficient for copper.

Table 1: Dietary mineral requirements of sheep and the incidence of deficient, marginal and sufficient farms

¹Based on NRC (2007) maintenance requirements of an 80 kg ewe and requirements of growing lambs. ²Mean farm herbage concentration of < 0.10, 0.2, >5, ≥ 0.5, >0.3 and >32 for mg/kg DM (for sufficient) for Co, Cu, I, Se and Zn, respectively.

This article by Tim Keady first appeared in the Sheep Open Day Booklet 2022.

Also read: Cobalt supplementation – effect on ewe and lamb performance

Also read: Weaning lamb performance from the INZAC flock