Abstract

Calculations performed in consideration of a continuously increasing world population have revealed that animal production needs to be enhanced worldwide by at least 2 % each year so as to provide sufficient feed. Yet, effective growth promoting agents, in terms of in-feed antibiotics, have been completely banned throughout Europe due to the possible development and spread of multiresistance in bacteria. New efficient, safe and inexpensive feed additives are therefore needed in order to maintain or even further improve performance levels in animal husbandry. Based upon this information, rare earth elements have been considered as promising natural feed additive. Thus, this study was designed to bring together the current research on rare earths in order to analyze the data obtained and to facilitate the discussion of its relevance to agricultural utilization. The term rare earth elements comprises the elements scandium (21), yttrium (39), lanthanum (57) and the 14 chemical elements following lanthanum (58 -71) called lanthanoids. Favoring the tripositive oxidation state, rare earths present a high affinity for ionic bonding, thus a large number of both organic and inorganic rare earth salts may be formed. Nevertheless, rare earths may also form complexes especially with chelating oxygen ligands. In nature, rare earths occur in multiple minerals, such as bastnaesite and monazite which are mainly used for industrial production. Today, rare earths are part of several daily used devices such as lighters, television sets and computers. Additionally they are found in medical technology, nuclear engineering, automobile industry, military devices and even in spacecraft. Furthermore, rare earth-containing drugs are used for the treatment of hyperphosphatemia in chronic renal failure patients and for burn treatment. Based upon their paramagnetic properties, rare earths, especially gadolinium, have also been arranged as contrast agents in magnetic resonance imaging and computer tomography. In the future, among other uses, rare earths might be involved in cancer therapy, treatment and prevention of osteoporosis and atherosclerosis as well as organ transplantation. In China, rare earths have been successfully used at low concentrations as feed additives and fertilizers for decades. Yet, careful interpretation of Chinese data is recommended due to the fact that Chinese papers are often only available in native language and furthermore not up to standard with Western scientific research reports, hence lacking statistical treatment of data and details of experimental methods. However, in China, both yield increases and quality improvements were achieved in multiple plant species including cereals, fruits and vegetables after rare earth application. Recommended application rates vary with the crop species, the application technique (soil, foliar or seed dressing) as well as the timing. As feed additives, rare earths were shown to improve body weight gain and feed conversion in nearly all categories of farming animals (chickens, pigs, ducks, cattle). Additionally, improvements in milk production in dairy cows, in egg production in laying hens and in output and survival rate of fish and egg hatching of shrimps were noticed. Feed additives used thereby predominantly contain light rare earths (La, Ce, Pr, Nd) but even though both organic (nitrates, chlorides etc.) and inorganic (ascorbates, citrates etc.) rare earth feed additives are commercially available, organic ones are claimed to provide better results. Based on the effects reported in Chinese studies, experiments were initiated under Western conditions in order to investigate the action of rare earths on both plant and animal growth. Several Western feeding trials conducted on animals have been able to demonstrate significant performance enhancing effects after dietary rare earth application, while results obtained from experiments on the effects of rare earths on plant growth have been controversial. In pigs, improvements in body weight gain of up to 19 % and in feed conversion rate of 10 % were observed after their diets were supplemented with low-dosed rare earth chlorides. Even better effects were however noticed after rare earth citrates were added to the feed of pigs. Furthermore, under field conditions, rare earths were shown to increase body weight gain by up to 10 % and improve feed conversion by up to 9 % in pigs. Following these results, rare earth containing feed additives in terms of Lancer® have entered the market in Switzerland, where a temporary permission has been granted for their use in pig production. In addition, in broilers, rare earths were also shown to increase final weights by 7 % and improve feed conversion by up to 3 %. Very recent studies also confirmed performance enhancing effects in broilers with increased body weight gain and feed intake of up to 6.6 % and 6.9 %, respectively. In rats, which were used as a small animal model, improvements in body weight gain and feed conversion of 4 -7 % and 3 -11 %, respectively, followed the application of rare earths. Thus, clear performance enhancing effects were achieved in Western studies on rats, pigs and poultry due to dietary rare earth supplementation. However, there are also studies in which positive effects of rare earths on animal performance were not as obvious or not observed at all. A comparison between the results of these feeding experiments as to the mixture of rare earths, the concentration as well as the compound applied showed that these parameters are involved in the magnitude of performance enhancing effects of rare earths. At present, no definitive statement on optimum composition can be made. However, a dose-dependency was observed in several trials and better effects have been achieved when the mixture of rare earths was applied instead of single lanthanum. Additionally, it seems that organic rare earth compounds have a higher impact on animal performance than inorganic ones. This is probable ascribable to different chemical characteristics, which lead to variations in both absorption and bioavailability. Generally, absorption of orally applied rare earths is very low, with more than 95 % being recovered in the feces of animals. According to minute gastrointestinal absorption of rare earths, oral toxicity is very low and comparable to usual table salt. LD50 values determined in various animal experiments rang from 830 mg/kg to 10 g/kg body weight. None of the feeding trials performed reported any effects on the state of health of the animals, which coincides with low oral toxicity and additionally supports the safe application of rare earth feed additives to animals. In addition, no effects on either meat or carcass quality were observed. Likewise, rare earth concentrations determined in organ samples were very low and similar or even lower than in control animals. This is attributed to the ubiquitous occurrence of rare earths, thus also in plants and soils. As a result they also appear in commercial diets and subsequently in animal and human tissue. It has also been shown that rare earth contents in usual vegetable foodstuff are still higher than those in meat obtained from animals additionally fed with rare earths. Therefore, the application of rare earths as feed additive is also considered to be safe for humans. Furthermore, as to current knowledge, no damage is to be expected on the environment as a consequence of rare earth application to agriculture. In fact, as rare earths can improve feed conversion, they may support the efficient use of natural resources, while additionally reducing environmental loads in terms of animal excrements. Hence, with respect to animal, human and environmental safety, rare earths meet legal recommendations of the European Union for their registration as feed additive. Although the mechanism underlying performance enhancing effects of rare earths is not completely understood, several proposals have been made. According to current research, rare earths might exert their action locally within the gastrointestinal tract, including effects on the bacterial micro-flora as well as on nutrient uptake, digestibility and utilization. Likewise, anti-inflammatory and anti-oxidative effects may also contribute to positive effects. Additionally, actions on the intermediate metabolism in terms of effects on cellular functions, growth-and digestibility-related hormones and enzymes or the immune system have also been considered. It might also be possible that rare earths are not yet identified essential elements. Based on the information gained in this study, it has been concluded that rare earths are of high interest as possibly new, safe, inexpensive feed additive in Europe, especially in pig and poultry production.