- Email: [email protected]
Flushing of mink. Effects of level of preceding feed restriction and length of flushing period on reproductive performance
Animal Reproduction Science, 17 (1988) 243-250
243
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Flushing of Mink. Effects of Level of Preceding Feed Restriction and Length of Flushing Period on Reproductive Performance ANNE-HELENE TAUSON
Fur Animal Division, Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Funbo-LSvsta Research Station, S-755 97 Uppsala (Sweden) ( Accepted 28 March 1988)
ABSTRACT Tauson, A.-H., 1988. Flushing of mink. Effects of level of preceding feed restriction and length of flushing period on reproductive performance. Anita. Reprod. Sci., 17: 243-250. In an experiment with 200 standard mink females (five groups of 40 females) the effect of flushing (a period of restriction followed by refeeding preceding the mating season) on reproductive performance was evaluated. The results of a non-flushed control group were compared with flushing from 20 February or 4 March until mating. For each date for start of flushing, the flushing period was preceded by a 2-week period of either moderate or severe restriction. Reproductive results confirm earlier data in that flushing from March 4, preceded by a 2-week period of moderate restriction, resulted in improved litter sizes (with 1.2 kits on average). Flushing from February 20 was less efficient. When comparing level of restriction, severely restricted females responded less to flushing than did moderately restricted females. The experimental treatment did not affect readiness to mate and the frequency of barren females was not conclusively affected. The rate of stillborn kits was not significantly affected but the highest rate was recorded in the group with superior litter size. In this investigation, both yearlings and adult females responded similarly to flushing.
INTRODUCTION
Reproductive results of multiparous species may be positively affected by flushing. However, as shown in ewes, positive effects of flushing are related to condition (Coop, 1966; Killeen, 1967; Gunn et al., 1969). In sows and gilts, ovulation rate was increased by flushing and, in thin sows, litter size was improved as an effect of an elevated plane of nutrition from weaning to mating (Brooks and Cooper, 1972). Reproductive results in does (Van den Broeck and Lampo, 1977) and in semi-starved and refed rats (Lintern-Moore et al., 1981 ) were improved by flushing. In the rats the numbers of growing follicles and corpora lutea (CL) were also increased. 0378-4320/88/$03.50
© 1988 Elsevier Science Publishers B.V.
244
Litter size in mink is determined by the number of ova shed and fertilized and by the embryonic and perinatal mortality. Hansson (1947) reported that 83.7% of the eggs ovulated were implanted, and 50.2% resulted in kits. This indicates a high rate of fetal losses, but the negative correlation found between litter size and length of gestation was explained by an increased mortality before implantation. Thus, whereas flushing affects ovulation and implantation rates, the length of the embryonic diapause and the rate of embryonic and fetal mortality must also be considered when evaluating the reproductive results. Kiiskinen and Mfikel~ (1974, 1977) failed to improve reproductive performance by increasing the dietary energy density throughout the mating season. Tauson (1985a), however, reported increased litter size, particularly in yearling females, as a result of flushing, by ad libitum feeding from 4 to 5 days prior to the start of the mating season until the individual females were mated. The effect of flushing was related to the weight gain of the females during flushing according to a quadratic regression. Ovulation rate tended to increase in flushed females. Sando Lund (1986) also, found positive effects of flushing on reproduction. MATERIALS AND METHODS
The experiment was carried out at the Research Fur Farm, Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Funbo-LSvsta, Uppsala, Sweden, from late January until whelping in 1985. TABLE1 Experimental design for evaluating the effect of degree of restriction and length of the flushing period on reproductive performance in mink Group 1 No. of animals 40 No. of yearlings 13 Degree of restriction Feed restriction started Flushing started Feed allowance, MJ female- ' day- 1 Prior to restriction 1.01 (100) a During restriction 1.01 (100) During flushing 0.96 (100) After flushing 0.96 (100)
2
3
4
5
40 13 Moderate Febr. 4 Febr. 20
40 13 Severe Febr. 4 Febr. 20
40 13 Moderate Febr. 15 March 4
40 13 Severe Febr. 15 March 4
1.01 (100) 0.80 (79) 1.39 (145) 0.96 (100)
1.01 0.64 1.39 0.96
(100) (63) (145) (100)
aFigures in parenthesis are relative values, where Group 1 is 100.
1.01 0.80 1.39 0.96
(100) (79) (145) (100)
1.01 0.64 1.39 0.96
(100) (63) (145) (100)
245
Experimental animals and management Mink females of the standard colour type, free from plasmacytosis {tested according to Hansen, 1974), were used. The experiment comprised five groups of 40 females, of which 13 in each group were yearlings. The animals were grouped according to age and live weight in mid-January. Housing was as described by Johansson (1976).
Experimental designl diets and feeding routines The experimental design is given in Table 1. Flushing started on 20 February or 4 March and, for each date, moderate and severe restriction during a 2-week TABLE2 Dietary composition and results of chemical analyses in flushing experiment with mink
Ingredients, % Cod offal Baltic herring Filleting scrap of herring Poultry waste Slaughter offal from cattle Commercial, extruded cereal mixture Potato mash powder Steamed, rolled oats Wheat germ Wheat bran Soy protein concentrate Vitamins Water Chemical composition, % No. of samples analyzed Dry matter Crude protein Fat Carbohydrate Ash ME, MJ/kg ME, MJ/kg DM Percentage of calculated ME from Protein Fat Carbohydrate
JanuaryFebruary
March
35 8 8 8 8 5 2.5 2.5 1 1 0.7 up to 100
31 4 12 8 8 5 2.5 2.5 1 1 1 0.7 up to 100
6 33.1 13.2 5.3 10.8 3.7 5.34 16.13
3 29.8 13.0 5.8 7.5 3.5 5.06 16.98
40 34 26
41 40 19
246
period preceding flushing were compared. Flushing was individually terminated when mating of each female was completed. The animals were fed the standard ration of the farm (Table 2 ) and feeding routines and feed consumption were recorded as described by Tauson (1985b).
Mating routines and data collection The mating routines were as earlier described by Tauson (1985a, b). The animals were weighed weekly from the start of the experiment until the start of the mating season. The animals were weighed again after first mating and, if remated, after 8-10 days also after the second mating. On these occasions, the condition of the females was subjectively graded as lean, medium or fat. Within 24 h after parturition, the date and number of live- and stillborn kits were recorded, and females and live-born kits were weighed.
Chemical analyses Samples of the diets were taken daily, pooled once a week and analyzed as described by Tauson and Alden ( 1984 ).
Statistical analyses Statistical analyses were carried out according to the GLM procedure of the Statistical Analysis System (SAS Institute Inc., 1982). The effects of the experimental treatment on animal live weights and reproductive traits were evaluated according to a model comprising the effect of treatment group. The results reported are least-squares means and statistically significant differences between treatment groups are indicated. RESULTS
Daily feed consumption The composition of the diets conformed to Scandinavian standards (Rimesl~tten, 1964) and was much the same as those in earlier investigations (Tauson, 1985a). The chemical analyses revealed that the percentage of metabolizable energy (ME) from fat was higher in the diet used in March than in the one fed in January-February. Daily feed consumption reflected changes in ambient temperature. Thus, feed refusals increased on days with a temperature below - 10 ° C. During the first days of flushing, the animals consumed the total allowance of 275 g (1.39 MJ) per animal and day but the feed intake decreased somewhat from the start of the mating season (March 7 ). However,
247 TABLE 3 A n i m a l live weights a n d reproductive results of m i n k as affected by degree of r e s t r i c t i o n a nd l e ngt h of the flushing period Group 1
Live weig ht J a n u a r y 24 February 7 February 14 February 21 March 6 At first m a t i n g At second m a t i n g At p a r t u r i t i o n
2
3
4
5
F l u s h i n g from Febr. 20 Restriction
F l u s h i n g from M a r c h 4 Restriction
Control
Moderate
Severe
Mode ra t e
Severe
1042 d 987 d 941 d 917 d 939 d 968 d 998 d 1105 d
1074 a 988 d 935 d 929 d 1059 e.f 1056 e 1108 e 1155 d'~
1040 d 979 d 917 d 910 d 1053 ~ 1079 e 1108 e 1159 ~
1039 d 984 d 925 d 905 d 1 0 0 F 'f 1031 ~'f 110Y 1147 d'~
1038 d 970 d 938 d 912 d 936 d 998 d'f 108W 1136 d'~
Date of first mating, M a r c h
9d
No. of m a t i n g s per female B a r r e n females, % K i t s per m a t e d female b L i t t e r size b Stillborn kits, % L e n g t h of gestation, days c
2.6 d 5d 5.1 d 5.3 d 6 d'e 45.9 d
9d 2.5 d 20 e 4.9 d 6.1 d'e 3d 45.6 d
10d 2.6 d 0d 5.8 d'f 5.8 d'e 7 d'e 46.1 d
9d 2.5 d 3d 6.C 'f 6.5 e 9e 45.3 d
10 d 2.C 8d 5.1 d 5.5 d 4d 45.7 d
P-value; effect of group"
0.84 0.97 0.90 0.92
0.001 0.001 O.001 0.26 0.25 0.88
0.006 0.03 0.06 0.16 0.74
"Figures in italics indicate s t a t i s t i c a l l y s i g n i f i c a n t differences. bIncluding stillborn kits. CFrom last m a t i n g to p a r t u r i t i o n . d'e'~Figures t h a t share no c o m m o n superscript are significant l y different ( P < 0.05 ).
it remained above the control feed allowance until reduced after the females had been mated.
Animal live weights Animal live weights decreased from late January to February 21 (Table 3 ), but there were no significant differences between groups. From February 21 to March 6, live weights increased in all groups, the increase being greatest in Groups 2 and 3 which were flushed from February 20. From March 6 until the second mating, animal live weights increased further, resulting in an average live weight of about 100 g higher in the flushed than in the control group. In Groups 4 and 5 (flushed from March 4), the increase was higher during this period than in Groups 2 and 3 (flushed from February 20). The two groups,
248 which were strongly restricted, reached the same or about the same live weight during flushing, as did the moderately restricted groups.
Mating and reproductiveperformance Matings were performed at a similar rate in all groups and the average date of first mating was March 9-10 (Table 3). Frequency of barreness was significantly different between groups, the rate being highest in Group 2. In all other groups, the frequency of barren females was low (Table 3). Reproductive performance, expressed as number of kits per mated female, was superior in Group 4 (moderate restriction, flushing from March 4) in which litter size also was most improved by the experimental treatment. Litter size was superior to that of the control group in all flushed groups with the exception of Group 5 (strong restriction, flushing from March 4). The rate of stillborn kits was not significantly affected by the experimental treatment but the highest frequency, although still on a normal level, was recorded in Group 4 with the largest litter size (Table 3). Length of gestation was not dependent on the experimental treatment and ranged from 45.3 days (Group 4) to 46.1 days (Group 3). Relating the reproductive traits to the weight change during the flushing period did not reveal any statistically significant differences. DISCUSSION The experimental treatment in the present investigation did not affect the animals' readiness to mate. The average date of first mating was March 9-10 and the average number of matings per female was similar in all groups. A more severe restriction might have impaired mating performance. This investigation confirms earlier results (Tauson, 1985a) that increased litter size in mink occurs as an effect of flushing from 3 to 4 days before the start of the mating season, provided the flushing was preceded by a 2-week period of moderately restricted feeding. The data by Tauson (1985a) indicated that the effect of flushing was most pronounced in yearling females. However, in the present investigation, adults reacted positively as well. Barren frequency was not conclusively affected by flushing either in the present study or in the earlier work (Tauson, 1985a). It appears that both the length of the flushing period and the level of the preceding restriction are decisive for the effect of the treatment. Thus, as shown by Tauson (1985a), flushing from February 20 was less efficient than flushing from early March. Litter size increased more when flushing was preceded by a 2-week period of moderate restriction than when the animals were not restricted prior to flushing (Tauson, 1985a). In the present investigation a more severe restriction prior to flushing gave inferior results compared with moderately restricted animals. Group 5, particularly, with a short but severe re-
249
striction period prior to flushing from March 4, showed little effect of flushing on litter size. Thus, it seems likely that the effect of flushing depends on several factors such as age of female {Tauson, 1985a), length of flushing period (Tauson, 1985a, and the present investigation), and the level and length of restriction prior to flushing. Moreover, ambient temperature may influence feed intake, thereby affecting performance. As pointed out earlier (Tauson, 1985a ), the condition of the female might determine the effect of flushing. The rate of stillborn kits was not significantly affected by the experimental treatment, but improved litter size is likely to increase the rate of stillborn kits as shown by Udris (1968) and also as indicated by data from Tauson (1985a). Our present findings verify previous results. An increase in litter size could possibly result in decreased length of gestation (Hansson, 1947) but, in the present investigation, gestation length was similar in all groups. The physiological background to the effects of flushing is still obscure. Some females in the present investigation were blood-sampled during gestation and killed after parturition, and the results indicate that flushing increased the number of CL and implantation sites as well as litter size (Tauson et al., in press). Plasma estradiol prior to mating and plasma progesterone throughout gestation were not significantly affected by the experimental treatment. However, the interval in days between increase in plasma progesterone over basal levels and plasma progesterone peak decreased (Tauson et al., in press). To reach a better understanding of the physiological mechanisms involved in flushing, more data on plasma estradiol, plasma progesterone and data on FSH and LH as well as more data on ovulation rate and implementation rate are needed.
REFERENCES Brooks, P.H. and Cooper, K.J., 1972. Short term nutrition and litter size. In: D.J.A. Cole (Editor), Pig Production. Butterworths, London, pp. 385-398. Coop, J.E., 1966. Effect of flushing on reproductive performance of ewes. J. Agric. Sci., 67: 305323. Emerson, D.D. and Henricks, D.M., 1977. The effects of low levels of PMSG and flush feeding upon embryonic survival in gilts. Theriogenology, 8 (5): 281-291. Gunn, R.G., Doney, J.M. and Russell, A.J.F., 1969. Fertility in Scottish Blackface ewes as influenced by nutrition and body condition at mating. J. Agric. Sci., 73: 289-294. Hansen, M., 1974. Ny og bedre metode til p~visning av plasmacytose. Dan. Pelsdyravl, 37: 209211. Hansson, A., 1947. The physiology of reproduction in mink (Mustela vison schreb. ) with special reference to delayed implantation. Acta Zool., 28: 1-136. Johansson, A.-H., 1976. Bur- och lytyper i P~ilsdjursfdrs~ksg~rden. V~ra Piilsdjur, 47: 178, 181. Kiiskinen, T. and M~ikel~i, J., 1974. FlushingF6rsSk med mink p~ Helves f6rs6ksfarm, 1973-1974. Fin. P~ilstidskr., 46: 495-500. Kiiskinen, T. and M~ikelii, J., 1977. II. Flushing in k~iyttSii koskevat jatkotutkimukset minkeill~i. Lantbrukets Forskningscentral, Rapport nr 10, 1977, pp. 12-21.
250 Killeen, J.O., 1967. The effects of body weight and level of nutrition before, during and after mating on ewe fertility. Aust. J. Exp. Agric. Anita. Husb., 7: 126. Lintern-Moore, S., Everitt, A.V., Mariana, J.-C. and Maul~on, P., 1981. The effect of restricted food intake and refeeding on the ovarian follicle population of the pre-puberal Wistar rat. Reprod. Nutr. Ddv., 21 (5A): 611-620. Rimesl~tten, H., 1964. Normer for protein, fett og kullhydrater i fSret til mink. Nord. Jordbrugsforsk., Suppl., II: 483-486. Sando Lund, R., 1986. Flushingforsog. Dan. Pelsdyravl, 49: 847, 849. SAS Institute Inc., 1982. SAS User's Guide: Statistics. 1982 Edition. SAS Institute Inc., Cary, N.C., 1982. 584 pp. Tauson, A.-H.,1985a. Effects of flushing on reproductive performance, ovulation rate, implantation rate and plasma progesterone levels in mink. Acta Agric. Scand., 35: 295-309. Tauson, A.-H., 1985b. Effects of lactic acid bacteria as feed additive on reproductive performance and early kit growth rate in mink and blue foxes. Acta Agric. Scand., 34: 485-506. Tauson, A.-H. and Alden, E., 1984. Pre-mating body weight changes and reproductive performance in female mink. Acta Agric. Scand., 34: 177-187. Tauson, A.-H., Gustafsson, H. and Jones, I., 1989. Flushing of mink. Effect on plasma progesterone, plasma estradiol, implantation rate and reproductive performance. Acta Agric. Scand., in press. Udris, A., 1968. ValpdSdlighet hos mink. Lantbrukshiigsk. Medd. Ser. A, 99: 1-28. Van den Broeck, L. and Lampo, P., 1977. Influence de trois niveaux d'alimentation de lapines futures r~productives sur rardeur sexuelle et la fertilitd ~ 4 mois. Ann. Zootech., 25: 565-574.
243
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Flushing of Mink. Effects of Level of Preceding Feed Restriction and Length of Flushing Period on Reproductive Performance ANNE-HELENE TAUSON
Fur Animal Division, Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Funbo-LSvsta Research Station, S-755 97 Uppsala (Sweden) ( Accepted 28 March 1988)
ABSTRACT Tauson, A.-H., 1988. Flushing of mink. Effects of level of preceding feed restriction and length of flushing period on reproductive performance. Anita. Reprod. Sci., 17: 243-250. In an experiment with 200 standard mink females (five groups of 40 females) the effect of flushing (a period of restriction followed by refeeding preceding the mating season) on reproductive performance was evaluated. The results of a non-flushed control group were compared with flushing from 20 February or 4 March until mating. For each date for start of flushing, the flushing period was preceded by a 2-week period of either moderate or severe restriction. Reproductive results confirm earlier data in that flushing from March 4, preceded by a 2-week period of moderate restriction, resulted in improved litter sizes (with 1.2 kits on average). Flushing from February 20 was less efficient. When comparing level of restriction, severely restricted females responded less to flushing than did moderately restricted females. The experimental treatment did not affect readiness to mate and the frequency of barren females was not conclusively affected. The rate of stillborn kits was not significantly affected but the highest rate was recorded in the group with superior litter size. In this investigation, both yearlings and adult females responded similarly to flushing.
INTRODUCTION
Reproductive results of multiparous species may be positively affected by flushing. However, as shown in ewes, positive effects of flushing are related to condition (Coop, 1966; Killeen, 1967; Gunn et al., 1969). In sows and gilts, ovulation rate was increased by flushing and, in thin sows, litter size was improved as an effect of an elevated plane of nutrition from weaning to mating (Brooks and Cooper, 1972). Reproductive results in does (Van den Broeck and Lampo, 1977) and in semi-starved and refed rats (Lintern-Moore et al., 1981 ) were improved by flushing. In the rats the numbers of growing follicles and corpora lutea (CL) were also increased. 0378-4320/88/$03.50
© 1988 Elsevier Science Publishers B.V.
244
Litter size in mink is determined by the number of ova shed and fertilized and by the embryonic and perinatal mortality. Hansson (1947) reported that 83.7% of the eggs ovulated were implanted, and 50.2% resulted in kits. This indicates a high rate of fetal losses, but the negative correlation found between litter size and length of gestation was explained by an increased mortality before implantation. Thus, whereas flushing affects ovulation and implantation rates, the length of the embryonic diapause and the rate of embryonic and fetal mortality must also be considered when evaluating the reproductive results. Kiiskinen and Mfikel~ (1974, 1977) failed to improve reproductive performance by increasing the dietary energy density throughout the mating season. Tauson (1985a), however, reported increased litter size, particularly in yearling females, as a result of flushing, by ad libitum feeding from 4 to 5 days prior to the start of the mating season until the individual females were mated. The effect of flushing was related to the weight gain of the females during flushing according to a quadratic regression. Ovulation rate tended to increase in flushed females. Sando Lund (1986) also, found positive effects of flushing on reproduction. MATERIALS AND METHODS
The experiment was carried out at the Research Fur Farm, Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Funbo-LSvsta, Uppsala, Sweden, from late January until whelping in 1985. TABLE1 Experimental design for evaluating the effect of degree of restriction and length of the flushing period on reproductive performance in mink Group 1 No. of animals 40 No. of yearlings 13 Degree of restriction Feed restriction started Flushing started Feed allowance, MJ female- ' day- 1 Prior to restriction 1.01 (100) a During restriction 1.01 (100) During flushing 0.96 (100) After flushing 0.96 (100)
2
3
4
5
40 13 Moderate Febr. 4 Febr. 20
40 13 Severe Febr. 4 Febr. 20
40 13 Moderate Febr. 15 March 4
40 13 Severe Febr. 15 March 4
1.01 (100) 0.80 (79) 1.39 (145) 0.96 (100)
1.01 0.64 1.39 0.96
(100) (63) (145) (100)
aFigures in parenthesis are relative values, where Group 1 is 100.
1.01 0.80 1.39 0.96
(100) (79) (145) (100)
1.01 0.64 1.39 0.96
(100) (63) (145) (100)
245
Experimental animals and management Mink females of the standard colour type, free from plasmacytosis {tested according to Hansen, 1974), were used. The experiment comprised five groups of 40 females, of which 13 in each group were yearlings. The animals were grouped according to age and live weight in mid-January. Housing was as described by Johansson (1976).
Experimental designl diets and feeding routines The experimental design is given in Table 1. Flushing started on 20 February or 4 March and, for each date, moderate and severe restriction during a 2-week TABLE2 Dietary composition and results of chemical analyses in flushing experiment with mink
Ingredients, % Cod offal Baltic herring Filleting scrap of herring Poultry waste Slaughter offal from cattle Commercial, extruded cereal mixture Potato mash powder Steamed, rolled oats Wheat germ Wheat bran Soy protein concentrate Vitamins Water Chemical composition, % No. of samples analyzed Dry matter Crude protein Fat Carbohydrate Ash ME, MJ/kg ME, MJ/kg DM Percentage of calculated ME from Protein Fat Carbohydrate
JanuaryFebruary
March
35 8 8 8 8 5 2.5 2.5 1 1 0.7 up to 100
31 4 12 8 8 5 2.5 2.5 1 1 1 0.7 up to 100
6 33.1 13.2 5.3 10.8 3.7 5.34 16.13
3 29.8 13.0 5.8 7.5 3.5 5.06 16.98
40 34 26
41 40 19
246
period preceding flushing were compared. Flushing was individually terminated when mating of each female was completed. The animals were fed the standard ration of the farm (Table 2 ) and feeding routines and feed consumption were recorded as described by Tauson (1985b).
Mating routines and data collection The mating routines were as earlier described by Tauson (1985a, b). The animals were weighed weekly from the start of the experiment until the start of the mating season. The animals were weighed again after first mating and, if remated, after 8-10 days also after the second mating. On these occasions, the condition of the females was subjectively graded as lean, medium or fat. Within 24 h after parturition, the date and number of live- and stillborn kits were recorded, and females and live-born kits were weighed.
Chemical analyses Samples of the diets were taken daily, pooled once a week and analyzed as described by Tauson and Alden ( 1984 ).
Statistical analyses Statistical analyses were carried out according to the GLM procedure of the Statistical Analysis System (SAS Institute Inc., 1982). The effects of the experimental treatment on animal live weights and reproductive traits were evaluated according to a model comprising the effect of treatment group. The results reported are least-squares means and statistically significant differences between treatment groups are indicated. RESULTS
Daily feed consumption The composition of the diets conformed to Scandinavian standards (Rimesl~tten, 1964) and was much the same as those in earlier investigations (Tauson, 1985a). The chemical analyses revealed that the percentage of metabolizable energy (ME) from fat was higher in the diet used in March than in the one fed in January-February. Daily feed consumption reflected changes in ambient temperature. Thus, feed refusals increased on days with a temperature below - 10 ° C. During the first days of flushing, the animals consumed the total allowance of 275 g (1.39 MJ) per animal and day but the feed intake decreased somewhat from the start of the mating season (March 7 ). However,
247 TABLE 3 A n i m a l live weights a n d reproductive results of m i n k as affected by degree of r e s t r i c t i o n a nd l e ngt h of the flushing period Group 1
Live weig ht J a n u a r y 24 February 7 February 14 February 21 March 6 At first m a t i n g At second m a t i n g At p a r t u r i t i o n
2
3
4
5
F l u s h i n g from Febr. 20 Restriction
F l u s h i n g from M a r c h 4 Restriction
Control
Moderate
Severe
Mode ra t e
Severe
1042 d 987 d 941 d 917 d 939 d 968 d 998 d 1105 d
1074 a 988 d 935 d 929 d 1059 e.f 1056 e 1108 e 1155 d'~
1040 d 979 d 917 d 910 d 1053 ~ 1079 e 1108 e 1159 ~
1039 d 984 d 925 d 905 d 1 0 0 F 'f 1031 ~'f 110Y 1147 d'~
1038 d 970 d 938 d 912 d 936 d 998 d'f 108W 1136 d'~
Date of first mating, M a r c h
9d
No. of m a t i n g s per female B a r r e n females, % K i t s per m a t e d female b L i t t e r size b Stillborn kits, % L e n g t h of gestation, days c
2.6 d 5d 5.1 d 5.3 d 6 d'e 45.9 d
9d 2.5 d 20 e 4.9 d 6.1 d'e 3d 45.6 d
10d 2.6 d 0d 5.8 d'f 5.8 d'e 7 d'e 46.1 d
9d 2.5 d 3d 6.C 'f 6.5 e 9e 45.3 d
10 d 2.C 8d 5.1 d 5.5 d 4d 45.7 d
P-value; effect of group"
0.84 0.97 0.90 0.92
0.001 0.001 O.001 0.26 0.25 0.88
0.006 0.03 0.06 0.16 0.74
"Figures in italics indicate s t a t i s t i c a l l y s i g n i f i c a n t differences. bIncluding stillborn kits. CFrom last m a t i n g to p a r t u r i t i o n . d'e'~Figures t h a t share no c o m m o n superscript are significant l y different ( P < 0.05 ).
it remained above the control feed allowance until reduced after the females had been mated.
Animal live weights Animal live weights decreased from late January to February 21 (Table 3 ), but there were no significant differences between groups. From February 21 to March 6, live weights increased in all groups, the increase being greatest in Groups 2 and 3 which were flushed from February 20. From March 6 until the second mating, animal live weights increased further, resulting in an average live weight of about 100 g higher in the flushed than in the control group. In Groups 4 and 5 (flushed from March 4), the increase was higher during this period than in Groups 2 and 3 (flushed from February 20). The two groups,
248 which were strongly restricted, reached the same or about the same live weight during flushing, as did the moderately restricted groups.
Mating and reproductiveperformance Matings were performed at a similar rate in all groups and the average date of first mating was March 9-10 (Table 3). Frequency of barreness was significantly different between groups, the rate being highest in Group 2. In all other groups, the frequency of barren females was low (Table 3). Reproductive performance, expressed as number of kits per mated female, was superior in Group 4 (moderate restriction, flushing from March 4) in which litter size also was most improved by the experimental treatment. Litter size was superior to that of the control group in all flushed groups with the exception of Group 5 (strong restriction, flushing from March 4). The rate of stillborn kits was not significantly affected by the experimental treatment but the highest frequency, although still on a normal level, was recorded in Group 4 with the largest litter size (Table 3). Length of gestation was not dependent on the experimental treatment and ranged from 45.3 days (Group 4) to 46.1 days (Group 3). Relating the reproductive traits to the weight change during the flushing period did not reveal any statistically significant differences. DISCUSSION The experimental treatment in the present investigation did not affect the animals' readiness to mate. The average date of first mating was March 9-10 and the average number of matings per female was similar in all groups. A more severe restriction might have impaired mating performance. This investigation confirms earlier results (Tauson, 1985a) that increased litter size in mink occurs as an effect of flushing from 3 to 4 days before the start of the mating season, provided the flushing was preceded by a 2-week period of moderately restricted feeding. The data by Tauson (1985a) indicated that the effect of flushing was most pronounced in yearling females. However, in the present investigation, adults reacted positively as well. Barren frequency was not conclusively affected by flushing either in the present study or in the earlier work (Tauson, 1985a). It appears that both the length of the flushing period and the level of the preceding restriction are decisive for the effect of the treatment. Thus, as shown by Tauson (1985a), flushing from February 20 was less efficient than flushing from early March. Litter size increased more when flushing was preceded by a 2-week period of moderate restriction than when the animals were not restricted prior to flushing (Tauson, 1985a). In the present investigation a more severe restriction prior to flushing gave inferior results compared with moderately restricted animals. Group 5, particularly, with a short but severe re-
249
striction period prior to flushing from March 4, showed little effect of flushing on litter size. Thus, it seems likely that the effect of flushing depends on several factors such as age of female {Tauson, 1985a), length of flushing period (Tauson, 1985a, and the present investigation), and the level and length of restriction prior to flushing. Moreover, ambient temperature may influence feed intake, thereby affecting performance. As pointed out earlier (Tauson, 1985a ), the condition of the female might determine the effect of flushing. The rate of stillborn kits was not significantly affected by the experimental treatment, but improved litter size is likely to increase the rate of stillborn kits as shown by Udris (1968) and also as indicated by data from Tauson (1985a). Our present findings verify previous results. An increase in litter size could possibly result in decreased length of gestation (Hansson, 1947) but, in the present investigation, gestation length was similar in all groups. The physiological background to the effects of flushing is still obscure. Some females in the present investigation were blood-sampled during gestation and killed after parturition, and the results indicate that flushing increased the number of CL and implantation sites as well as litter size (Tauson et al., in press). Plasma estradiol prior to mating and plasma progesterone throughout gestation were not significantly affected by the experimental treatment. However, the interval in days between increase in plasma progesterone over basal levels and plasma progesterone peak decreased (Tauson et al., in press). To reach a better understanding of the physiological mechanisms involved in flushing, more data on plasma estradiol, plasma progesterone and data on FSH and LH as well as more data on ovulation rate and implementation rate are needed.
REFERENCES Brooks, P.H. and Cooper, K.J., 1972. Short term nutrition and litter size. In: D.J.A. Cole (Editor), Pig Production. Butterworths, London, pp. 385-398. Coop, J.E., 1966. Effect of flushing on reproductive performance of ewes. J. Agric. Sci., 67: 305323. Emerson, D.D. and Henricks, D.M., 1977. The effects of low levels of PMSG and flush feeding upon embryonic survival in gilts. Theriogenology, 8 (5): 281-291. Gunn, R.G., Doney, J.M. and Russell, A.J.F., 1969. Fertility in Scottish Blackface ewes as influenced by nutrition and body condition at mating. J. Agric. Sci., 73: 289-294. Hansen, M., 1974. Ny og bedre metode til p~visning av plasmacytose. Dan. Pelsdyravl, 37: 209211. Hansson, A., 1947. The physiology of reproduction in mink (Mustela vison schreb. ) with special reference to delayed implantation. Acta Zool., 28: 1-136. Johansson, A.-H., 1976. Bur- och lytyper i P~ilsdjursfdrs~ksg~rden. V~ra Piilsdjur, 47: 178, 181. Kiiskinen, T. and M~ikel~i, J., 1974. FlushingF6rsSk med mink p~ Helves f6rs6ksfarm, 1973-1974. Fin. P~ilstidskr., 46: 495-500. Kiiskinen, T. and M~ikelii, J., 1977. II. Flushing in k~iyttSii koskevat jatkotutkimukset minkeill~i. Lantbrukets Forskningscentral, Rapport nr 10, 1977, pp. 12-21.
250 Killeen, J.O., 1967. The effects of body weight and level of nutrition before, during and after mating on ewe fertility. Aust. J. Exp. Agric. Anita. Husb., 7: 126. Lintern-Moore, S., Everitt, A.V., Mariana, J.-C. and Maul~on, P., 1981. The effect of restricted food intake and refeeding on the ovarian follicle population of the pre-puberal Wistar rat. Reprod. Nutr. Ddv., 21 (5A): 611-620. Rimesl~tten, H., 1964. Normer for protein, fett og kullhydrater i fSret til mink. Nord. Jordbrugsforsk., Suppl., II: 483-486. Sando Lund, R., 1986. Flushingforsog. Dan. Pelsdyravl, 49: 847, 849. SAS Institute Inc., 1982. SAS User's Guide: Statistics. 1982 Edition. SAS Institute Inc., Cary, N.C., 1982. 584 pp. Tauson, A.-H.,1985a. Effects of flushing on reproductive performance, ovulation rate, implantation rate and plasma progesterone levels in mink. Acta Agric. Scand., 35: 295-309. Tauson, A.-H., 1985b. Effects of lactic acid bacteria as feed additive on reproductive performance and early kit growth rate in mink and blue foxes. Acta Agric. Scand., 34: 485-506. Tauson, A.-H. and Alden, E., 1984. Pre-mating body weight changes and reproductive performance in female mink. Acta Agric. Scand., 34: 177-187. Tauson, A.-H., Gustafsson, H. and Jones, I., 1989. Flushing of mink. Effect on plasma progesterone, plasma estradiol, implantation rate and reproductive performance. Acta Agric. Scand., in press. Udris, A., 1968. ValpdSdlighet hos mink. Lantbrukshiigsk. Medd. Ser. A, 99: 1-28. Van den Broeck, L. and Lampo, P., 1977. Influence de trois niveaux d'alimentation de lapines futures r~productives sur rardeur sexuelle et la fertilitd ~ 4 mois. Ann. Zootech., 25: 565-574.