Abstract: The objective of this study was to examine whether growth performance, nutrient digestion and N-balance of growing minks were affected by different dietary protein levels from good protein feed resource. Eighty young male minks were allotted to five treatment groups (P36, P32, P28, P24 and P20) provided with diets containing approximately 36, 32, 28, 24 and 20% protein in Dry Matter (DM), respectively. From July to middle September the Body Weight (BW) was lower in group P20 than other groups (p<0.05). No significant difference of dietary protein on minks’ BW and average daily gain (ADG) between groups P36 and P32 (p>0.05). The performance and digestibility of DM, Crude Protein (CP) and Crude Fat (CF) were similar when the dietary protein level decreased from P36 to P32 (p>0.05). N intake, urinary N excretion and Blood Urinary Nitrogen (BUN) increased linearly with increasing dietary protein level (p<0.05). No significant difference was found of serum Total Protein (TP) among all groups (p>0.05). Considering of all the factors, when minks fed good protein resource the level of dietary protein should be about 32% of DM (29.78% of ME) of diet could meet the requirement of growing minks. Furthermore, the urinary N could decrease 6.06% in growing period of minks.
INTRODUCTION
Considering today’s growing environmental concerns and increasing feed costs, feeding appropriate dietary crude protein concentrations is of the utmost importance in the feedlot industry. For most animal species, the protein requirement is met with as little as 10% of the digestible energy (Crampton, 1964). The mink, as an obligate carnivore, has a demand for a large proportion of protein in its diet (MacDonald et al., 1984). An animal’s requirement for protein depends on many factors including energy intake, level of activity, physiological status, growth capacity, previous nutrition and genetic differences etc. An estimated protein requirement always depends on the physiological activity for which the requirements is met since the protein level (quantity and quality) required for top performance in one physiological process may not necessarily be sufficient for another physiological process.
The National Research Council provided the earliest recommendation of specific levels of protein for mink of the ranch year (Harris et al., 1953). Multiple studies have been done on the protein requirements of the mink but the protein level of requirement for minks had a big range. (Basset et al., 1948, 1951; Sinclair et al., 1962; Allen et al., 1964; Skrede and Herstad, 1978; Tyopponen et al., 1987; Kerminen-Hakkio et al., 2000; Rasmussen and Borsting, 2000; Sandbol et al., 2004). Earlier studies obviously indicated that the quality of protein in the mink’s diet will determine the exact level of percentage of ME as protein required for top performance of the mink for the growth phase of the mink ranch year.
The objective of the present study was to observe if the minks feed good quality of protein diet, did it can get better performance and decrease N excretion and to investigate the effects of dietary protein levels on growth performance, nutrient digestion metabolism and serum biochemical parameters in growing mink and to find the optimal protein level.
MATERIALS AND METHODS
The experiment was carried out at the fur animal farm of the institute of special economic animals and plants, Chinese Academy of Agricultural Science in Northeast of China. The experimental minks were housed in the standard sheds with open sides.
Animals and experiment design: The experiment began on 9 July with a total of 80 weaned mink males. The average (±SD) age and weight of the animals at start were 57±3d and 820±36 g, respectively. The animals were allocated to the five treatment groups and 16 animals each. The experiment was preceded by 1 week adjustment period, during which the animals were accustomed to the experimental feed. The ingredients and compositions of experiment diets were shown in Table 1. Animals had free access to drink water and were fed twice a day with the experimental diets. All training and testing of the animals was performed by the same person. BW of animals was determined each 15 days. ADG, Average Daily Feed Intake (ADFI) and feed: gain ratio (F/G) was calculated at the end of study.
Blood samples were collected from 8 minks per experimental group during the growing period and brought to lab quickly and centrifuged for 10 minutes at 5000 rpm/min. then serum separated from blood and transferred into Eppendorf centrifuge tubes and kept at-20°C until analysis.
N-balance experiments: The N-balance experiments were carried out using nine 14-weeks old male kits from the respective treatment groups. The feces and urine collection period lasted three days. The animals were kept in metabolism cages constructed for separate collection of feces and urine, in principle as described by Jorgensen and Glem-Hansen (1973) and Glem-Hansen (1992). Feces and urine were collected quantitatively daily and kept frozen pending analyses. To avoid ammonia evaporation from the urine, 20 mL sulphuric acid (5% solution) was added to the urine collection bottles and the urine collection trays were sprayed with citric acid (20% solution) once per day. In the N-balance calculations, retained N was determined as ingested N-(fecal N+urinary N).
Chemical analyses: The chemical composition of feed and feces was analysed by standard methods. DM, ash and crude protein (CP: Kjeldahl-Nx6.25), calcium and phosphorus contents were analyzed according to AOAC (2003) procedures. Crude Carbohydrate (CC) was calculated as the difference by subtracting ash, CP and EE from the DM content. The calculation of ME content and the proportional composition of ME were based on the digestibility coefficients achieved and the following values of ME: protein 18.8 MJ kg-1, fat: 39.8 MJ kg-1 and carbohydrate 17.6 MJ kg-1 (Hansen et al., 1991).
| Table 1: | Ingredient and composition of experiment diets |
| aThe premix provided the following per kg of diet: Ca (as CaHPO4•2H2O), 6.4 mg, P(as Ca(H2PO4)2 •H2O), 4.4 mg, Mg (MgO), 1.6 mg, Na (NaCl), 24 mg, Fe (as FeSO4•H2O), 16 mg, Cu (as CuSO4•5H2O), 4.0 mg, Zn (as ZnSO4•H2O), 10 mg, Mn (as MnSO4 •H2O), 12 mg, VA, 300 IU, VB1, 0.15 mg, VB2, 0.40 mg, VB6, 0.30 mg, folic acid, 0.30 mg, nicotinic acid, 1.60 mg, D-pantothenic acid, 1.3 mg, bME calculated according to Hansen et al. (1991) | |
Serum urea nitrogen concentration and serum total protein were measured according to the method of Bradford (1976) using kit (Nanjing Jiancheng Biotechnology Co., Ltd, Jiangsu, China) as the standard.
Statistical analyses: All data were analyzed using the GLM procedure of SAS (2002). A level of p<0.05 was set as the criterion for statistical significance. Data were represented as Mean±SD.
RESULTS
Performance and Nutrient digestibility: Effects of different dietary protein levels on performance and nutrient digestibility were shown in Table 2. The final weights and ADG were affected by different level of diets (p<0.05), the value decreased with dietary protein level reducing. The ADFI were not affected by different dietary protein levels (p>0.05), F/G had significant difference among all groups (p<0.05) but the ADFI and F/G were all increased with declining dietary protein levels. Digestibility of DM was similar among the groups. The CP digestibility increased with dietary CP level increasing and found the highest in P36 (90.07%) (p<0.05). the digestible protein level of 5 diets were 327.04, 294.83, 260.89, 216.87 and 188.47 g kg-1 DM. Apparent digestibility of CF was impaired with decreasing dietary protein level (p<0.05), reduced the dietary protein level decreased the digestibility of CF.
| Table 2: | Effects of different dietary protein levels on performance and nutrient digestibility of minks |
| The values are the Mean±SD, for method of estimation, see text, Values within rows with different letters differ significantly (p<0.05) | |
| Table 3: | Effects of different dietary protein levels on N-balance of minks |
| The values are the Mean±SD, for method of estimation, Values within rows with different letters differ significantly (p<0.05) | |
N-balance: Effects of different dietary protein levels on N-balance were shown in Table 3. N intake and urinary N excretion increased linearly with increasing dietary protein level (p<0.05). In daily retention of N (g per mink) there were significant difference (p<0.05) but N retained was no significant difference among P36, P32 and P28 (p>0.05), N retained decreased with dietary protein level reducing, the values were 2.31 g day-1 per mink, 2.17 g day-1 and 1.96 g day-1, respectively. Effects of different dietary protein levels on BUN and serum total protein (TP) were shown in Table 3. BUN were affected by different dietary protein levels (p<0.05). BUN decreased linearly with declining dietary protein level, the values were 236.69, 219.63, 216.28 and 180.32 mg L-1, respectively. No significant difference was found of TP among all groups.
DISCUSSION
Conventional mink diets are largely based on by-products from the fishing industry and abattoirs and, therefore, are rich in protein and other N-containing constituents. The mink has a very short intestine (Kainer, 1954) and a feed passage time of only 4-6 hours (Hansen, 1978; Szymeczko and Skrede, 1990). Thus, ingested dietary protein is rapidly digested and absorbed, so we used good quality protein feed in currently study. The decline of the protein was achieved with a proportional decline of all protein-sources and they were regulated with maize starch.
In present experiment, the BW and ADG of all minks increased fleetly, the requirement for protein ( g day kg-1 live weight or g kg-1 dietary DM) is higher during the early than the late phases of growth, as demonstrated earlier in blue foxes (Rimeslatten, 1976) and in mink, too (Skrede, 1978; Damgaard, 1997). Our research indicated that fed diets with low protein level showed lower body weight gain than the animals fed higher protein diets, no significant affects of dietary protein on minks’ BW and ADG between groups P36 and P28. In the present study, we found that the content of dietary protein was not the higher the better, superfluous dietary protein would cause to waste, moreover, increased the N excretion. By contraries, deficient dietary protein could not satisfy the requirement of mink growth.
ADFI significantly increased with reducing dietary protein levels. ADG decreased linearly with reducing dietary protein level, by contraries, F/G increased linearly with declining dietary protein level. The results implicate that increasing dietary protein level had a consistent increase in ADG and a decrease in F/G of minks. Proteins are the major component in muscles. As previous study reported muscles and other tissues are primarily formed in the early growth phase of mink kits (Stryer, 1997).
Our research showed that the digestibility of DM, CP and CF were decreased with declining dietary protein level. The lower the dietary protein level, the lower was the digestibility of the DM, CP, CF and other nutrients in fox (Dahlman et al., 2002). The influence of protein level on the apparent digestibility of protein has been studied more. Research on ileal-fistulated blue foxes by Szymeczko and Skrede (1991) showed a sight reduction in the apparent digestibility of CP at lower protein levels. Similar results were obtained by Skrede (1979), working with mink. The influence of protein level on the apparent digestibility of protein has been under intensive research in other animals, pigs in particular. A number of studies suggest that reductions in dietary protein lead to increases in relative amount of endogenous N secretion, which, in turn, reduce the apparent digestibility of protein (Low, 1980; Baker, 1989). In contrast, in studies by Li et al. (1993) the apparent digestibility of protein was not affected by the dietary level of protein. In the present investigations, the performances and digestibility of nutrient was not affected when the dietary protein level decreased from P36 to P32, which means 294.83 g kg-1 DM digestible protein of diet could meet the requirement of growing minks.
N intake, UN and N retention decreased with reducing dietary protein level. As pointed out by Glem-Hansen (1980) the growing mink fed low-protein diets were able to compensate for previous low nitrogen retention during the growing-furring period. From an environmental point of view, a substantial reduction in N emissions can be achieved by decreasing the dietary protein level. According to the present research, N excretion in urine declines significantly when the protein level in the diet is lowered from P36 to P32 and did not affected the performance of minks. The results are consistent with earlier findings that N excretion declined noticeably along with a reduction in dietary protein pigs (Jongbloed and Lenis, 1992; Valaja et al., 1993). According to Valaja et al. (1993), a reduction in dietary protein from 180 to 120 ( g kg-1) resulted in a 38.3% decrease in urinary N excretion in growing pigs. Our research show that when the dietary protein level decreased from P36 to P32 resulted in a 6.06% decrease in urinary N excretion in growing minks.
BUN was affected by dietary protein levels. It was reduced linearly by declining CP level in the present study, which has also been found in plasma or serum in other reports (Kerr and Easter, 1995; Figueroa et al., 2002; Nyachoti et al., 2006; Yue and Qiao, 2008). No significant difference was found of serum total protein among all groups. The result are consistent with earlier findings, Blome et al. (2003) reported that TP concentration in plasma of dietary CP content was not significant in pig.
CONCLUSION
Based on the results from this experiment, we suggest that, in order to achieve normal growth and ensure low N emission, the level of dietary protein should be about 32% of DM (29.78% of ME) and the digestible protein about 29.4% of DM when the minks fed good protein resource. Furthermore, compare with the high protein level urinary N could decrease 6.06% in growing minks. BUN was affected by dietary protein levels. Serum total protein of minks was irrespective of dietary protein level.
ACKNOWLEDGMENTS
The financial support is from Special Fund for Public Welfare Technology Research of Agricultural Industry(200903014). The authors also thank all the crew at the fur farming of the institute of special economic animals and plants, Chinese academy of agricultural science.