A closer look at inorganic and organic copper and zinc supplementation in nursery pig diets
The use of pharmacological concentrations of zinc (Zn) and (or) copper (Cu) in nursery pig diets to enhance growth has been widely accepted in the swine industry. A routine recommendation is to add 2,000 to 3,000 ppm Zn in the form of zinc oxide and (or) to add 250 ppm Cu as copper sulfate to the growing swine diet. Swine producers may use these high concentrations of inorganic zinc and copper sources for the entire nursery period of up to eight weeks and into the grower period. The effects of these high trace mineral concentrations in swine diets on the environment are of concern. Therefore, the development of more bioavailable organic trace mineral sources may allow lower inclusion rates of copper and zinc and still enhance growth performance. The major problem is that a majority of the research in this area has been inconclusive.
While the cumulative effects of long-term use of pharmacological concentrations of zinc and copper on soil nutrient management are not known, intuitively the suspicion is that the effects will be negative and utilization of manure nutrients on cropland may come under further regulation. This is particularly important in situations where a swine nursery production system is the main source of nutrients for a landmass.
Reasons and responses to high dietary copper
Copper has been acknowledged for many years as an essential nutrient needed for the immune system and normal growth and development of humans and animals. Pigs require copper to maintain basic systems such as synthesis of regulatory peptides, enzyme activity and iron metabolism. A copper deficiency is known to cause microcytic hypochromic anemia due to poor iron mobilization, diarrhea, bone disorders such as bowing of the legs and fractures, neonatal ataxia, hair depigmentation, poor keratinization, decreased synthesis of collagen, elastin and myelin, infertility, cardiovascular disorders, impaired glucose and lipid metabolism and depressed immune function.
In 1955, Barber et al. reported an enhancement in growth performance when growing pigs were fed 250 ppm Cu in the form of copper sulfate. Subsequent copper research (Bunch et al., 1961; Cromwell et al., 1989) has shown a similar response over a wider range of supplementation, 125 to 250 ppm Cu, even though the copper requirement is 5-6 ppm for the weanling pig (NRC, 1998). In general, copper supplementation at 125 to 250 ppm improves average daily gain and feed intake by 8 to 10%, which subsequently improves feed efficiency because the extra nutrients can be used for growth instead of maintenance. The improvement in growth in the weanling pig fed high copper is independent of source.
The mechanism behind the growth promotant effect of feeding 250 ppm Cu as copper sulfate remains unknown. It has long been hypothesized that this inorganic source of copper acts as an antimicrobial agent (enterically) on the intestinal microflora, thus reducing turnover of the intestinal cells and leaving more nutrients available for absorption by the pig (Fuller et al., 1960). Other researchers have reported that copper enhances growth through a systemic effect rather than an antimicrobial effect in the intestinal tract (Zhou et al., 1994ab). The idea that supplemental copper may be acting systemically is supported by the wide variety of biological systemic functions of copper that are related to growth. If a systemic effect is the more likely mode of action, then nutritionists and the feed industry need to develop ways to improve the efficacy of delivering copper into the circulation. This may be accomplished by using organic trace minerals.
Research has shown that pigs fed organic forms of copper have similar growth and feed intake responses to pigs fed 250 ppm Cu as copper sulfate. Recently, a nursery pig study was conducted to evaluate lower concentrations of an organic source of copper (proteinate source, Bioplex Cu, Alltech, Inc.) compared to 250 ppm Cu as copper sulfate, with pig growth performance for 28 days post-weaning as the criteria. The 28 day growth performance is shown in Figure 1. These results demonstrated that feeding 50 ppm Cu as Bioplex Cu improved pig growth performance during the first four weeks post-weaning (Carlson et al., 2000a).
Future research is planned to evaluate trace mineral balance (copper and zinc) and excretion from nursery pigs fed either organic or inorganic sources. The bilary system is the major pathway of copper excretion; therefore, feces contain most of the unabsorbed copper with very little being excreted via the urine. Nursery pigs fed either 150 or 3000 ppm Zn with 125 ppm Cu excrete approximately 50 mg Cu/kg fecal DM (Hoover et al., 1997). Copper concentration in both feces and urine are higher in pigs fed elevated copper from either inorganic or organic sources.
Reasons and responses to high dietary zinc
Zinc is an essential trace mineral required for numerous biological and physiological processes in all livestock species throughout the life cycle.
Figure 1. Effect of supplementing Bioplex Copper to nursery pigs on average daily gain.
In general, zinc plays a major role in protein synthesis, carbohydrate metabolism, and basic functions in growth and development, reproduction and healing of wounds. Zinc is involved in boosting the immune system in response to disease outbreaks. In 1955, Tucker and Salmon first reported zinc to be an essential nutrient and a zinc deficiency in swine resulted in parakeratosis (a skin disorder) and growth retardation. Due to the detrimental effects of zinc deficiency, the inclusion of this trace mineral in the diets of livestock is a common practice and has been for decades.
In recent years the inclusion of supplemental zinc above the requirement of 100 mg/kg for nursery pigs (NRC, 1998) has been studied. The investigation of feeding pharmacological concentrations of zinc to nursery pigs began following reports from Europe (Poulsen, 1995) that suggested supplemental zinc oxide decreased the incidence of non-specific post-weaning scours, and more importantly Escherichia coli proliferation. In addition, several reports from the United States have shown that the addition of pharmacological concentrations of zinc as zinc oxide can promote growth performance of the newly weaned pig (Hahn and Baker, 1993; Hill et al., 1996; Smith et al., 1997). The improvement in growth performance is observed in either early or traditionally weaned pigs fed pharmacological concentrations of zinc provided as zinc oxide for a minimum of two weeks immediately post-weaning (Carlson et al., 1999). An average from these reports shows that nursery pigs fed 3,000 ppm Zn as zinc oxide have approximately 14% greater daily gains and 7% greater daily feed intakes. A North Central Region Swine Nutrition study (Hill et al., 1999) has shown that 2,000 ppm Zn as zinc oxide is just as efficacious as 3,000 ppm Zn as zinc oxide in increasing daily gain, feed intake, and feed efficiency for four weeks post-weaning.
These results demonstrate the potential for nursery pigs fed high concentrations of zinc for the first two weeks post-weaning to be approximately 2 lb. heavier going into the grower-finisher period. This provides a huge economical advantage since each additional pound leaving the nursery results in three days less needed to reach market weight in most cases. It has become routine in the swine industry to add high concentrations (2,000 to 3,000 ppm) of inorganic zinc to nursery diets for improved growth performance. However, the biological mechanism behind the enhanced growth performance of nursery pigs fed 3,000 ppm Zn as zinc oxide is unknown.
The mode of action behind feeding 3,000 ppm Zn as zinc oxide in stimulating growth would appear to be similar to that of feeding 125-250 ppm Cu as copper sulfate. Carlson et al. (1998) reported that feeding pharmacological concentrations of zinc (3,000 ppm Zn as zinc oxide) altered duodenal morphology (deeper crypts and greater total thickness) and increased intestinal metallothionein concentrations, which indicates that high amounts of zinc have an enteric effect on the nursery pig. The interest in using lower concentrations of organic trace minerals in place of pharmacological concentrations of inorganic trace minerals has increased. Organic trace minerals have higher bioavailability than inorganic trace mineral sources and possibly exhibit greater metabolic activity, which could result in better responses in performance and less nutrient excretion (Wedekind et al., 1994).
It has been reported that zinc retention is affected by zinc status of the pig as well as dietary zinc concentration. Fecal zinc excretion (major route) increases while percent retention decreases with increasing dietary zinc content (Poulsen and Larsen, 1995). Balance studies have reported that by the third week post-weaning, pigs are excreting 110-170 mg Zn/kg fecal DM when fed the traditional concentration of 150 ppm Zn; and excreting almost ten times more (1,022 mg Zn/kg fecal DM) when fed 3,000 ppm Zn from zinc oxide. However, when nursery pigs are fed a combination of 250 ppm Zn from sulfate and 250 ppm Zn from zinc methionine (organic form) for a total of 500 ppm Zn, fecal Zn excretion is approximately 250 mg Zn/kg DM (Hoover et al., 1997). Urinary zinc excretion follows the pattern of dietary zinc concentration and fecal zinc excretion, but accounts for less than 1% of total zinc excreted (Poulsen and Larsen, 1995). Pigs fed 3,000 ppm Zn as zinc oxide excrete approximately 3 mg/l Zn via urine while pigs fed either 150 ppm Zn as zinc oxide or 500 ppm Zn from organic and inorganic sources excreted 1 mg/l Zn in urine.
A common recommendation is to feed supplemental zinc to nursery pigs only during the first two weeks post-weaning. This is because by the third week, pigs fed diets at or above 500 ppm from inorganic and (or) organic sources of zinc are in negative zinc balance. Negative zinc balance can be defined as more zinc being excreted than consumed on a daily basis. One can conclude that accumulation of zinc in plasma and tissues reached a threshold after two weeks and the ability to absorb zinc decreased, resulting in a negative balance. This may occur because fecal zinc contains not only unabsorbed zinc, but also endogenous zinc, i.e. absorbed zinc that is re-excreted into the gut via biological secretions.
Additional research has shown that pigs fed lower concentrations of organic zinc forms have similar growth response as pigs fed 3,000 ppm Zn as zinc oxide (Ward et al., 1996). Recently, a nursery pig study was conducted to evaluate lower concentrations of an organic source of zinc (proteinate source, Bioplex Zn, Alltech, Inc.) compared to 2,000 ppm Zn as Zn oxide, with pig growth performance for 28 days post-weaning as the criteria. Over the entire 28 day nursery study, dietary zinc treatment had no effect on growth performance, feed intake or feed efficiency (Table 1). During week 1 and Phase 1 (weeks 1–2), nursery pigs fed either 50 or 100 ppm Zn as Bioplex Zn had the greatest average daily gain (P < 0.1) compared to the other dietary zinc treatments (Figure 2). It was concluded that feeding 50 to 100 ppm Zn as Bioplex Zn may replace 2,000 ppm Zn as zinc oxide based on 28 day post-weaning growth performance (Carlson et al., 2000b). However, more research is needed to further evaluate the efficacy of organic trace minerals in swine nutrition.
Table 1. Effect of supplementing nursery pig diets with Bioplex Zn on growth performance during the first 4 weeks post-weaning.
Figure 2. Effect of supplemental Bioplex Zn on nursery pig growth performance during the first two weeks post-weaning.
The bottom line is that as zinc concentrations increase in the diet regardless of form, urinary and fecal zinc excretion increases. Pigs fed nursery diets containing 3,000 ppm Zn as oxide had increased urinary and fecal zinc concentrations compared to pigs fed diets containing 150 ppm or 250 ppm Zn from an organic zinc source.
Why move away from high concentrations of inorganic trace minerals?
It has been generally accepted that nutrients in swine manure should be used to fertilize cropland at agronomic rates. In other words the application of manure should be restricted to the amount of nutrients taken up by crops. Most attention has been paid to macronutrients such as nitrogen, phosphorus and potassium, but it is useful to be aware of the agronomic rates of the micronutrients including zinc and copper as well. A corn crop of 150 bushels/acre has been reported to take up about 200 g Zn and about 50 g Cu per acre while a soybean crop of 40 bushels/acre takes up the same 50 g Cu, but only about 100 g Zn per acre. It is not clear what the long-term effects of micronutrient accumulation might be. We have no direct evidence of detrimental effects. Some countries (e.g. Canada, Japan, Denmark and the European Union) have taken the cautious approach and restricted the concentration of these minerals in swine diets (Table 2). Other countries may follow this path in the future.
Table 2. Recommended and allowed dietary levels of copper and zinc in Denmark.*
Conclusions
The feeding of pharmacological concentrations of inorganic copper and (or) zinc post-weaning results in improved growth performance and subsequently heavier pigs entering the grower-finisher period. However, there is a very realistic concern about potential mineral buildup in manure and subsequently in the soil when feeding high concentrations of copper and zinc to nursery pigs. Cropland acres needed per nursery pig space per year calculated from figures presented herein would be very rough estimates, but provide a reasonable impression of the magnitude of land required.
Farm practices will continue to apply swine excreta to soils, but when these manures are enriched in both copper and zinc, they will require an enormous increase in land mass in order to avoid mineral accumulation in the soil. Today, there is no documentation of any detrimental environmental impact. In the future, regulations on feed copper and zinc concentrations may be applied, similar to those concerning selenium.
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