Persistent organic pollutants may accumulate in the tissues of suckler cows and their calves. Agroscope and Empa studied the processes by which these undesirable substances accumulate as well as the decontamination of previously exposed animals.
Persistent organic pollutants or ‘POPs’ such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) have harmful effects on human health and ecosystems. Between the 1920’s and the 1980’s PCBs were manufactured as industrial chemicals in large quantities and used in a wide range of products. Their production and use were regulated in the International Stockholm Convention and the Aarhus Protocol around twenty years ago. Since POPs are easily dispersed into the environment and are poorly degradable and lipophilic, their accumulation and toxicity along animal food chains is still a matter of concern today.
Airborne organic pollutants are deposited on plants and soils
In addition to accidental point sources of organic pollutants, there are elusive and hence difficult-to-identify diffuse sources. POP emissions into the atmosphere and deposition on plants and topsoils therefore lead to the exposure of livestock – especially those with outdoor access in the case of grassland-based production. To ensure food safety in these production systems, it is crucial to understand the fate of PCBs and PCDD/Fs. In the case of suckler-cow systems, ensuring meat chemical safety requires an understanding of the process by which cows transfer PCBs and PCDD/Fs to their calves.
Absorption and accumulation of POPs during gestation and lactation
We studied the absorption rate and speed of elimination as well as the tissue distribution of PCBs and PCDD/Fs stemming from grass silage or soil in cows and their calves during gestation (109 days prepartum) and over the entire lactation period (288 days). We quantified the absorption, distribution, metabolism and excretion (ADME) processes to explore the interaction with animal physiological traits. The study therefore focused on the dynamics of body lipids.
Eight cows were fed grass silage contaminated with soil. Four of them were decontaminated after 164 days of lactation by switching them to an uncontaminated grass silage-based diet. Each calf was fed with the milk of its respective mother. An input-output mass balance was computed during gestation and lactation from PCB, PCDD/F and lipid inputs (intake of solid feed, soil and milk), the respective outputs (feces and milk) and body storage (initial/final burdens).
POP absorption rate is not the same for cows and calves
In cows, PCB and PCDD/F absorption rates as well as the metabolised fractions decreased with increasing chlorination of the organic pollutants. For calves, however, PCB absorption rate was not a function of chlorination. The absorption rates of highly chlorinated PCBs and PCDD/Fs decreased further when they were ingested through contaminated soil.
Cows eliminate organic pollutants more readily than calves
Cows excreted PCBs and PCDD/Fs via feces (50% relative to total intake) and milk (9%) and accumulated only 5% in their bodies, whilst calves accumulated a much larger share of the total intake in their bodies (44%).
POPs have higher concentrations in liver lipids
Tissue distribution shows a specific accumulation in the lipids of the liver, followed by those of the kidneys, and to a lesser extent of the muscles. POP concentrations in intermuscular adipose tissue were very close to those in the carcass and the whole body. Consequently, they can be used to estimate the body burden provided that a reliable estimate of body lipid mass is available.
Food safety not dependent on POP level in feed only
To ensure meat chemical safety in suckler-beef production, it is important to bear in mind both the physicochemical properties of POPs (e.g. the degree of chlorination) as well as body lipid dynamics and the exposure route (feed, milk, soil). Physiology-based toxicokinetic (PBTK) models are useful tools for understanding the complex interactions of ADME and physiology. These new findings will be useful to the livestock sector for ensuring irreproachable beef quality.
- Cows absorb POPs from grass silage at a higher rate than those from soil.
- The exposure of calves to POPs occurred primarily during lactation (through milk suckling) rather than through placental transfer.
- POPs are more concentrated in the lipids of the liver, kidneys, and – to a lesser extent – muscle than in those of adipose tissue.
- Cows are more efficient than calves at eliminating organic pollutants, which explains why calves show higher concentrations of POPs than their mothers.
- Young (3-month-old) calves had lower estimated metabolic rates than older (10-month-old) calves or cows. Consequently, young calves have the highest pollutant concentrations in their tissues.