Processing soybeans of different origins : response of a Chinese and a western pig breed to dietary inclusion

Soybeans (Glycine max) have high nutritional value for domestic animals, due to their protein and energy contents. The feeding effects of full-fat soybeans for non-ruminant and immature ruminant animals, however, are limited by the presence of some antinutritional factors (ANFs). Therefore, processing is needed to inactivate these ANFs before the soybeans are used to feed animals. The nature and levels of ANFs may be quite variable between soybeans of different origin, and this can influence the effects of soybean processing. Thus, a similar processing procedure may result in different effects for different soybean origins.

It has been reported that there are differences in the digestive capacity between pig breeds. But, it was not clear whether the responses to soybean ANFs also differ between pig breeds or genotypes.

If soybeans of different origins respond differently to processing, and different pig breeds have different tolerances to dietary ANFs, it is of great importance to establish optimal processing procedures for a specific soybean origin and for a specific pig herd. The objectives of the present studies were to investigate the potential differences between soybean origins in response to heat processing, and the differences between pig breeds in response to dietary soybean treatments.

In Chapter 2, the literature on ANFs of different soybean and soybean products, and on the use of full-fat soybeans in pig diets is reviewed. It is shown that the ANFs (particularly trypsin inhibitor activity, TIA) vary greatly among different soybean origins and soybean products. Literature also shows that there exists a large variation among the physiological stages of pigs in tolerating dietary soybean ANFs. The ability to tolerate soybean ANFs increases with the age of animals. Pregnant sows can tolerate higher dietary levels of raw soybeans than lactating sows.

The weight and/or the size of digestive organs, and dietary nutrient digestibilities vary among breeds or genotypes of pigs according to the results from the literature (Chapter 3). For example, Chinese native pig breeds can tolerate higher levels of dietary fibre than Western pig breeds.
Argentine and Chinese commercial soybeans were used for the present studies. The TIA for Argentine and Chinese soybeans were 15.2 and 20.6 mg/g, respectively. The two origins of soybeans were steam-heated under various conditions (temperature and time combinations). The heating effects on the two soybean origins were evaluated by both kinetic studies and animal trials. In kinetic studies (Chapter 4 and 5), the TIA and protein dispersibility index (PDI) of the soybeans from both Argentine and Chinese origin decreased in a logarithmic pattern with prolongation of heating time when they were heated at a given temperature. The TIA and PDI of the Chinese soybeans processed under various conditions, however, were higher than that of the Argentine soybeans processed under the same conditions.

In the digestion and balance trials, it was found that the digestibilities of dry matter (DM), crude protein (CP) and fat of the diet containing raw Chinese soybeans were lower than that containing raw Argentine soybeans (Chapter 7). The CP digestibility of the Chinese soybeans heated at 100 °C/40 min., 118 °C/5 min. and 136 °C/1.5 min. was also lower compared with the Argentine soybeans heated under the corresponding conditions (Chapter 6). The highest protein biological value (PBV) and net protein utilization (NPU) were obtained at the heating condition of 100 °C/40 min. for Argentine soybeans, and 136 °C/1.5 min. for Chinese soybeans. The CP digestibility and NPU of soybeans were correlated with their TIA (R=-0.8118; P<0.05).

Processing can also improve the fat digestibilities of soybeans (Chapter 6) and of the diet containing soybeans (Chapter 7). The effect of processing on the improvement of fat digestion, however, is different between Argentine and Chinese soybeans. A significant (P<0.05) interaction was found between soybean origin and processing condition (Chapter 6 and 7). In Chapter 7, the fat digestibility of the diet with raw Chinese soybeans was much lower than that of the diet with raw Argentine soybeans (50.9 vs 62.8%). After the soybeans of the two origins were processed at 118 °C/7.5 min. however, the fat digestibility of the Chinese soybean diet was much higher than for the Argentine soybean diet (74.4 vs 60.9%). In Chapter 6, the fat digestibility of soybeans determined by difference method showed that the optimal processing condition for improving fat digestion differs with soybean origin. The highest digestibility of fat for Argentine soybeans was found in those heated at 100 °C/40 min.; for Chinese soybeans in those heated at 136 °C/1.5 min..

Feeding raw soybean diets may cause pathological changes in some organs of piglets (Chapter 7). The extent of the pathological changes varied with the soybean origin. The pigs fed the diet with raw Chinese soybeans had a heavier small intestine while pancreas weight was decreased compared to those fed the diet with raw Argentine soybeans as measured as g per kg live-weight. The light and electron microscopy examinations showed that the intestinal mucosa is more severely damaged in animals fed raw Chinese soybean diet than for those fed raw Argentine soybeans.

The comparison made between different pig breeds (Landrace and Min pig) indicated that the digestibilities of dietary DM, CP and fat in Min pigs (a native pig breed in north-east China) were higher than in Landrace pigs. When the two breeds of pigs were fed on the diets containing soybeans of any origin, the small intestine of Landrace pigs, however, was longer and heavier than that of Min pigs. Histological examination also showed that the villi of the small intestinal mucosa of Landrace pigs were more seriously damaged by feeding the raw Chinese soybean diet compared with that of Min pigs.

It is concluded that the ANFs, at least TIA, in soybeans vary greatly among soybean origins; Chinese soybeans have higher levels of TIA and PDI than Argentine soybeans as studied in the present experiments. TIA and PDI decrease with longer duration of heating at a given temperature in both Argentine and Chinese soybeans. The residual level of TIA and PDI in the processed Chinese soybeans are higher than that of the Argentine soybeans processed under the same conditions (except at the extreme heating conditions). The best improvement of N and fat digestibilities and net protein utilization (NPU) is obtained at the heating condition of low temperature/long time (100 °C/40 min.) for Argentine soybeans and high temperature/short time (136 °C/1.5 min.) for Chinese soybeans. With respect to intestinal enlargement and histological examination, raw soybean diet can have a more deleterious influence on Landrace than on Min pigs. Thus, soybean heat processing is more important for the Landrace than for the Min pigs when the soybeans are to be used in the diets of these animals.