comparison of two methods of ruminal digestibility determination (in situ and in vitro) of pomegranate seed pulp

introduction using of agro-industrial by-products in diet of livestock not only reduces the production costs but also can dwindle the use of human foods in animal nutrition and competition between human and livestock. thus, proper use of these by-products in animal nutrition and identification of new and low cost feed resources may be one of the priorities in livestock husbandry of our country. pomegranate seed pulp (psp) is a by-product of the industrial decoction of pomegranate. iran with annually production of more than 900 tons of pomegranates is one of the most important regions of pomegranate cultivation and psp produced from pomegranate processing factories may be extensively used in animal nutrition. digestibility determination of feeds is one of the most effective ways to evaluate their nutritional value. in addition, there is a strong relationship between feeds digestibility and performance of animal. there are in vivo, in vitro and in situ methods to determine the digestibility of feeds. although in vivo methods are reference for digestibility values of feeds and are of high precision but they are usually expensive and time consuming. in addition, these methods do not provide any information related to ruminal degradability kinetic of nutrients. the aim of this study was comparison of two methods of ruminal degradability determination methods namely, in situ (nylon bag) and in vitro (daisy incubator), using dried and ensiled pomegranate seed pulp (psp). materials and methods the psp used in this study was prepared from anaryan co. in ferdows, iran. decocted pomegranate was a mixture of yazd varieties which were harvested at early autumn of 1389. two types of psp as dried and ensiled were used to compare the methods of ruminal digestibility determination, pervasively. the psp silage was prepared by ensiling of wet psp (containing 475 g/kg dm) in 3 kg bins (4 replicates)and with density of 650 kg wet psp per cubic meter for 60 days and dried psp was prepared by drying of wet psp in oven at 60ºc. the digestibility of each feed was determined by both in situ and in vitro methods using two holstein fistulated heifers and daisy incubator, respectively. animals were fed a total mixed ration containing 1.8 kg/d of alfalfa hay, 1.8 kg/d of concentrate, 0.5 kg/d of corn silage and 1.8 kg/d of wheat straw (dm basis) at two meals. the ingredients of concentrate were barley grain (35%), corn grain (18%), soybean meal (10%), canola meal (15%), wheat bran (11.5%), molasses (7%), vitamin-mineral supplement (1%), oyster shell (2%) and salt (0.5%) (dm basis). comparison between treatments for each of digestibility methods at 24 and 48 h of incubation was done separately based on completely randomized design using sas software. correlation between digestibility determination methods was estimated using corr proc of sas and regression coefficient and also regression equation of in situ digestibility method on in vitro digestibility method was determined using reg proc of sas. results and discussion the results showed that dm digestibility of psp (both ensiled and dried) was estimated higher when measured by in vitro than in situ method at both incubation times. the correlation between two methods of digestibility determination for dried and ensiled psp after 24h incubation was 0.81 and 0.96, respectively and after 48h incubation was 0.99 and 0.75, respectively. the regression equations of dm in situ digestibility estimation from dm in vitro digestibility data were of high accuracy. tagliapietra et al. (27) compared the in vitro and in situ dm digestibility of 11 different feeds and found a high correlation coefficient of 0.90 (p <0.01) between two methods. other researchers also reported higher digestibility estimation from in vitro method than in situ. one reason for this, which also may be applicable for the data obtained from present study, is that in in situ method some of ruminal microbes cannot penetrate to bags because in rumen each microbe type is located at a distinct phase while in in vitro method of digestibility all microbes can participate in digestion actively and this will increase the dm digestibility. conclusion it was concluded that two studied methods of digestibility determination were highly correlated and regression equation may be used to estimate the in situ dm digestibility of experimental feed samples from in vitro digestibility data precisely.