February 10, 2023
Low protein, amino acid supplemented diets have been shown to be beneficial in the swine industry as they lower diet costs and reduce nitrogen excretion, while maintaining growth performance of the animal. But there are some issues with low protein diets, says Miranda Buchinski. For example, animals require both non-essential amino acids and essential amino acids, and these diets assume that pigs can produce sufficient non-essential amino acids when only the essentials are provided.
"This may reduce essential amino acid utilization efficiency for lean gain," Buchinski says. "Another issue with these diets is that dietary nitrogen supply may be limiting, and this results in the metabolism of amino acids to provide nitrogen for the endogenous synthesis of those non-essential amino acids, and this reduces overall essential amino acid utilization for lean gain as well as limits animal productivity."
For efficient amino acid utilization, both essential and non-essential amino acids, or a source of nitrogen for the synthesis of the non-essentials, should be given in the diet, says Buchinski, who is currently pursuing her master's in swine nutrition at the University of Saskatchewan. While the ideal protein concept takes into consideration the essential amino acids required for an animal, another number to examine is the essential amino acid-nitrogen to total nitrogen ratio of a diet. Buchinski says this is a practical way to optimize both essential amino acids and non-essential amino acid sources in a diet.
"So as the ratio decreases, total dietary nitrogen content increases and vice versa," she says. In swine, the ideal ratio has been determined to be 0.48 (Heger et al. 1998)
In a University of Saskatchewan study conducted two years ago, researchers examined the lysine requirement for nitrogen retention in pigs fed a diet with a low essential amino acid-nitrogen to total nitrogen ratio and a high ratio diet. Each set of diets included five graded levels of lysine. The results showed the high ratio diet had a breakpoint of 1.21% SID lysine, while the low ratio diet didn't have a breakpoint at all, indicating the nitrogen was limiting in the high ratio diet.
Buchinski says it's been suggested that non-protein nitrogen sources could be used to provide the dietary nitrogen that low protein diets typically lack, and positive responses have been seen when NPN is included in diets that are deficient in non-essential amino acid-nitrogen. For example, Mansilla et al. 2017 demonstrated that feeding ammonium nitrogen is as effective as feeding synthetic amino acids or intact protein in terms of maintaining growth, improving feed efficiency and nitrogen retention. Buchinski has also done work in this area, feeding pigs diets without ammonium phosphate and with ammonium phosphate at 1.7% as well as graded dietary lysine content. The results of her study also showed NPN inclusion improved nitrogen retention.
The objective of her latest research was to determine the impact of dietary NPN inclusion and lysine content on growth performance, nitrogen digestibility and carcass characteristics in growing pigs. The team hypothesized that performance, nitrogen output and carcass quality of pigs fed a diet containing NPN and a greater lysine content would be improved compared to the pigs fed a diet deficient in the non-essential amino acid-nitrogen.
A total of 240 growing pigs with an initial body weight of 20.2 kilos were grouped into five pigs per pen, either two barrows and three gilts or two gilts and three barrows. The pens were randomly assigned to one of six treatments over three blocks in a two by three factorial design. The non-ammonium phosphate diets had the higher ratio of 0.54 with the three graded levels of SID lysine (1.03, 1.15 and 1.27) and the diets that included ammonium phosphate had the low ratio of 0.48 with the same three graded levels of lysine.
Buchinski points out the non-ammonium phosphate diets had a slightly higher ratio than the ammonium phosphate diets, which was also reflected in the crude protein content of those diets, with the diets that included the NPN having a slightly higher crude protein content than the diets without.
On day 0, 7, 14, 21 and 28, feed intake and body weight were measured. On day 15, fresh fecal samples were obtained via rectal stimulation from two to three pigs per pen. Finally on day 28, backfat and lean depth were measured from two pigs in each pen, one barrow and one gilt. Diet and fecal samples were analyzed for nitrogen and acid insoluble ash to determine apparent total tract digestibility of nitrogen.
Buchinski says the results showed the inclusion of the NPN didn't have much of an impact on average daily gain, however lysine content had a significant impact on the overall average daily gain, with the pigs fed the higher lysine content having a greater overall average daily gain. As for average daily feed intake, the inclusion of the non-protein nitrogen had a significant impact on daily feed intake throughout the entire trial, with the pigs that were fed NPN consuming significantly less feed than the pigs that were not fed NPN.
This was reflected in the gain to feed data from the trial. The pigs that were fed NPN had a greater gain to feed in the first week as well as overall when compared to the pigs that were not fed NPN.
As for body weight data, Buchinski says "the pigs grew pretty much on par with each other the first two weeks and then into the last couple weeks the impact of the lysine started to show, with the pigs that were fed the higher lysine content having a slightly higher final body weight than the pigs that were fed a reduced lysine content."
Back fat was not impacted by NPN inclusion, but lean depth was, with the pigs that were fed the ammonium phosphate depositing more lean depth than the pigs that were not fed the ammonium phosphate.
NPN didn't impact the apparent total tract digestibility of nitrogen, but it did impact the fecal nitrogen output with the pigs that were fed the ammonium phosphate excreting more fecal nitrogen than the pigs not fed ammonium phosphate. The fecal nitrogen output was also impacted by the interaction between nitrogen and lysine content.
"The inclusion of the NPN reduced feed intake and increased gain to feed compared to the diets with no ammonium phosphate, and overall average daily gain and day 28 body weight increased with increasing lysine, but were not impacted by the inclusion of the ammonium phosphate. Pigs fed the diets containing NPN had increased lean depth, and while apparent total tract digestibility wasn't impacted by ammonium phosphate inclusion, the increased fecal nitrogen output was likely a result of the higher dietary nitrogen content of those diets," Buchinski says.
"These results suggest that the inclusion of an NPN can improve feed efficiency and increase lean depth while maintaining a similar average daily gain and final body weight. Ammonium phosphate is also an appropriate source of nitrogen in diets that may be limiting in total nitrogen, such as a lot of low protein diets we see today."
Buchinski was a finalist in the R.O. Ball Young Scientist Grad Student Competition at the 2023 Banff Pork Seminar.
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