A note on absorption of crystalline threonine in pigs *

Two groups of 6 castrated pigs, in which each animal weighed 44 kg and was equipped with a post-valvular T-caecum cannula, were given threonine-deficient and threonine-supplemented (0.1 %) diets. As similar daily flows of N and threonine were found in the ileal digesta after feeding both diets, it is concluded that crystalline threonine added to cereal-based diets is entirely absorbed in the small intestine of pigs.


INTRODUCTION
Increasing quantities of crystalline amino acids are being used to supplement diets for pigs.Yen et al. (1991) found that pigs absorb free lysine and threonine more rapidly than protein-bound lysine and threonine.
Our previous study of amino acid absorption in the small intestine (Buraczewska, 1981) demonstrated that the rate of absorption of free threonine, similarly to histidine, was lower as compared with other free essential amino acids, and depended on the concentration of amino acids in the intestine.
The objective of the present study was to determine whether crystalline threonine is absorbed completely in the small intestine after feeding pigs with a threonine-supplemented diet.
Supported by the Committee for Scientific Research, Grant No 5 P06 E 001 13

Animals
Twelve castrated male pigs (line 990), surgically fitted with post-valvular T-caecum cannulae according to van Leeuwen et al. (1991) were divided into two groups and fed with the respective diets for 12 days.The pigs were kept in metabolic cages and given two equal portions of the diets at 08:00 and 20:00 h.The diets were mixed with water (1:1) just before feeding and offered to the pigs in amounts 3.0 times the maintenance requirements for energy (1.5 MJ ME/kg BW 0 75 ).

Experimental procedure
At the end of feeding the experimental diets, when the animals were at about 53 kg body weight, ileal digesta was collected for 3 days, 12 h a day (between 08:00 and 20:00).The collection was carried out by attaching plastic bags to the cannulae.The bags were changed every hour and their contents immediately frozen at -20° C.After completion of the collection the samples were thawed, pooled per animal within the experimental period, freeze-dried and ground before chemical analysis.

Chemical analysis
Dry matter and nitrogen were analysed using standard methods (AOAC, 1990).Threonine was determined in HCl-hydrolysed digesta using a Beckman 6300 high pressure amino acid analyser.Chromium oxide was determined according to the Kimura and Miller (1957) procedure.

RESULTS AND DISCUSSION
Both nitrogen and threonine concentrations in the digesta were similar after feeding the threonine-deficient and threonine-supplemented diets (Table 1).This means that the added free threonine disappeared completely from the lumen of the small intestine.Consequently, ileal digestibility of protein-bound threonine was similar after feeding both diets.Also, no differences were observed in the digestibility of dry matter or crude protein of the diets.
The results point to 100% ileal digestibility of crystalline threonine in pig diets.However, more rapid absorption of free-than protein-bound threonine (Yen et al., 1991) may affect the utilisation efficiency of the free amino acid, which was shown in experiments with different frequencies of feeding diets supplemented with free lysine (Buraczewski and Buraczewska, 1980;Batterham and Murison, 1981;Cook et al., 1985).It is suggested that feeding more meals (also ad libitum feeding) is more efficient since it may prevent an imbalance of amino acids at the site of protein synthesis resulting from more rapid absorption of free amino acid(s) such as lysine or threonine.

TABLE 1
Nitrogen (N) and threonine (Thr) content in the ileal digesta, and ileal apparent digestibility of the nutrients after feeding pigs with Thr-deficient and Thr-supplemented diets