By Lee Johnston and Julia Holen, University of Minnesota West Central Research and Outreach Center, Morris; Jae Cheol Jang, Pedro Urriola and Jerry Shurson, University of Minnesota, St. Paul; and Mark Schwartz, Schwartz Farms, Sleepy Eye, Minn.
Genetic improvement in the swine industry has resulted in a steady increase in litter size at birth over the last 10 to 15 years. Often, an undesirable consequence of such large litter sizes is an increase in variability of piglet birth weight and increased incidence of low birth weight pigs.
Low birth weight pigs are generally defined as any pig that weighs less than 1 kilogram (2.2 pounds) at birth (see picture). Preweaning mortality of low birth weight pigs often exceeds 40% and may be as high as 80%. Obviously, this high mortality represents a significant economic loss to producers and decreases efficiency of pork production systems. Small pigs at birth are thought to be the result of intrauterine growth retardation that occurs during pregnancy. These small pigs occur as a result of receiving inadequate nutrition in the uterus during development and are born with low birth weight. Not only is the pig’s chance for survival to weaning reduced, these small pigs often have lower weaning weight, slower growth from weaning to market weight and fatter carcasses at harvest compared with littermates with average birth weights (3 to 4 pounds).
So, the obvious question is: Can we manipulate the sow’s diet during gestation to improve nutrition of the small pigs in the uterus, which would increase birth weight, reduce the proportion of these small pigs at birth, and improve their performance after birth?
Researchers at USDA’s U.S. Meat Animal Research Center report results of a study in which they fed a high level of dietary zinc (0.07% zinc sulfate) to gilts from Day 80 of gestation until farrowing. They found no effect of zinc on piglet birth weight, weight of the smallest pig in the litter or weaning weight of pigs. However, the stillbirth rate of pigs declined as the birth interval of pigs increased. More importantly, pre-weaning mortality of pigs weighing less than 1 kilogram at birth was reduced substantially for gilts fed the high zinc diet (Figure 1).
This was an important finding indicating that feeding a high amount of supplemental zinc during gestation could improve survival of pigs after birth. However, this was only one report that focused on sows in their first reproductive cycle in a research herd. These gilts had 10.6 total pigs born and pre-weaning mortality of 8.3% while commercial herds in the United States during 2017 had 13.9 total pigs born and pre-weaning mortality of 17.8%.
We wondered if this response could be repeated with a greater number of multi-parity sows and in a commercial production setting with total pigs born and pre-weaning mortality more representative of the commercial swine industry. So we designed an experiment to test the effect of dietary zinc in late-gestation on pre-weaning survival of pigs.
The experiment was conducted on a commercial sow farm in southern Minnesota during the summer and fall of 2018. Sows in three production weeks of the farm (339 mixed parity sows; 112 to 115 per treatment) were assigned to one of three dietary treatments from Day 75 of gestation until farrowing.
Dietary treatments included a standard corn-soybean meal gestation diet that contained 125 parts per million of zinc (Control); an intermediate zinc diet which was the Control diet with 365 ppm additional zinc from zinc sulfate (INT); and a high zinc diet which was the Control diet with 595 ppm additional zinc from zinc sulfate (HI). Zinc in the Control diet was supplied by zinc sulfate (75 ppm) and a zinc-amino acid complex (50 ppm). Sows assigned to INT and HI received additional zinc daily in their feed drop boxes as a top-dress composed of ground corn (95.8%), choice white grease (1%) and zinc sulfate (3.2%).
At farrowing, individual pigs were identified with ear tags and weighed within 24 hours of birth. Incidence of mortality were recorded throughout the lactation period and pigs were weighed again individually at weaning. Across all treatments, 4,514 piglets were observed in this study.
Dietary treatments had no effect on litter size at birth (average = 14.2 total pigs born) or weaning (average = 10.6 pigs weaned) and did not influence the number of stillborn or mummified pigs. Dietary zinc concentration had no meaningful effects on average birth weight of pigs, but the INT zinc treatment did reduce the incidence of low birth weight pigs. However, increasing dietary zinc concentration during late gestation significantly reduced overall mortality of piglets from 15.0 to 12.2% (Figure 2).
Most of this reduction was attributable to a significant 10 percentage point reduction in mortality of low birth weight pigs. However, dietary zinc also significantly reduced mortality of heavy birth weight pigs from 7.6% to 3.3% which was an unexpected finding.
In one week of production, we selected 15 litters per treatment to measure growth performance from weaning through the nursery and growing-finishing phases until they were marketed, and carcass data were collected at harvest. These litters included about 50 low birth weight pigs and 100 pigs with average or heavy birth weight. Postweaning mortality of low birth weight pigs was not different than that of pigs born with average birth weights. Postweaning mortality was not affected by dietary zinc treatment of the sows.
However, low birth weight pigs required about 3.5 days longer to produce a carcass that weighed about 5 pounds less at harvest than average birth weight pigs regardless of sow zinc treatment. Fat-free lean percentage of carcasses (53.6%) was not different between low birth weight pigs compared with contemporary littermates born with average or heavy birth weight. Therefore, dietary zinc supplementation of sow diets in late-gestation did not influence postweaning growth performance or carcass characteristics of their litters.
In summary, two experiments conducted by different research groups under very different conditions indicate that elevated dietary zinc in late-gestation improves survival of low birth weight pigs. Presently, it is not clear what mechanism(s) is responsible for this improved performance. There are many unanswered questions regarding dose of zinc, timing of supplementation, source of dietary zinc and many others that need to be answered before we fully understand the effects of zinc on pre-weaning piglet survival and its place in commercial production systems.
We appreciate the financial support of the Pork Checkoff through the Minnesota Pork Board and the cooperation of Schwartz Farms, Sleepy Eye, Minn.