Animal, Wildlife and Grassland Sciences

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Browsing Animal, Wildlife and Grassland Sciences by Author "Bester, Nena"

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    Effect of different dietary energy levels on productive and reproductive traits in Dorper rams
    (University of the Free State, 2008-02-19) Bester, Nena; Schwalbach, L. M. J.; Greyling, J. P. C.; Van der Merwe, H. J.
    English: A study was conducted to evaluate the effects of different dietary energy levels on the productive (ADG, FCR, body weight, carcass) and reproductive (scrotum, testes, semen) characteristics of young Dorper rams. The study was carried out in two phases at the University of the Free State campus in Bloemfont ein, South Africa. Out of a group of 60 rams, 36 animals that were successfully trained for semen collection with the aid of an artificial vagina (AV), were selected to be part of this study and housed in individual metabolic cages. During phase 1 of this study, 36 (11 to 12 months old) Dorper rams with a mean initial body weight of 42.0 ± 0.52kg were randomly allocated to 3 groups (n=12 per group). Each group was randomly assigned to one of 3 experimental diets (treatments), formulated on a crude protein, degradable protein, calcium and phosphorus equivalent basis. A medium energy (Me) diet was formulated according to the National Research Council standards (NRC) for young growing rams to serve as the control diet. The metabolisable energy (ME) content of this diet (Me) was increased by 15 % for the high energy (He) and decreased by 15% for the low energy (Le) diets respectively. The actual ME levels for the Le, Me, He: were 6.52, 8.09 and 9.39 MJ/kg, respectively. The rams received the diets and fresh water ad libitum during a 127 day trial period and were weighed once a week. Their ADG, FCR and ME/kg weight gain calculated and compared amongst groups. During this trial, a digestibility trial was conducted for 7 days (during week 14 of the trial), using 5 randomly selected animals from each treatment group. The amount of feed offered and consumed by each animal as well as the feaces excreted daily were recorded. The chemical composition of the diets and the feaces (dry matter, energy, crude and degradable protein) were determined. Semen was collected from each ram, every forthnight for 3 consecutive days, with the aid of an AV and evaluated for volume, overall sperm motility, forward progression, sperm concentration, % live sperm and % normal sperm. The semen samples were then diluted (using a one-step dilution method with a cryodiluent containing 5% glycerol), packed in 0.25ml plastic straws, equilibrated for 4 hours and cryoperserved (frozen) in liquid nitrogen vapour (-70 ºC). One week after freezing, the semen was thawed (38 ºC for 30 seconds) and evaluated for the same qualitative parameters as the fresh semen. The results were compared amongst groups for each collection time, using ANOVA for repeated measures analysis procedures of SAS. The energy level of the diet and the collection week as well as their interaction was used as independent variables, while the different semen parameters considered were the dependent variables in the model. When means differed significantly, the Tukey method was used to compare means. At the end of this trial period, 6 rams per group were randomly selected and slaughtered at a commercial abbatoir. After slaughtering the carcass characteristics (cold carcass weight, dressing percentage, carcass grade, shoulder circumference, buttock circumference, backfat thickness, eye muscle area, marbeling and kidney fat) were recorded and compared amongst groups. The scrotum (total weight, skin weight, fat and circumference), testes (volume and weight) and epidydimis (weight and volume) characteristics were also recorded and compared between groups, using ANOVA procedures of SAS. When means differed significantly the Tukey method was used to compare means. From the independent variables considered in the model, only the diet (ME level) had a significant effect on the parameters considered. In general, an increase in dietary energy level resulted in higher growth rates, better feed conversion rates, heavier carcasses, higher dressing percentages and greater fat deposition, both in the carcass, around the kidneys and in the scrotum (particularly in the neck region, over the testicular vascular cone). It seems that the energy requirements recommended by the NRC (1985) are not applicable for early mature Dorper replacement ram lambs, as those in the He group, (real ME level similar to NRC recommendations) deposited excessive fat in their body. Despite the significant differences induced by the He on the growth performance, carcass characteristics as well as on the scrotal and testicular characteristics of Dorper rams, no detrimental effects were recorded in their semen quantity, quality and cryotolerance. During the following 90 days (phase 2), the remaining rams (n=6 per group) were further maintained on the Le diet for 90 days. This was done to simulate the situation when over-conditioned (fat) rams bought at auctions are placed on low energy diets (veld). Other objectives of this trial phase were to evaluate if the fat deposits accumulated in the scrota of rams fed on high energy diets are mobilized during subsequent periods of moderate nutritional restriction and if these nutritional restrictions induce any changes on the semen characteristics of rams. During this phase of the trial, semen was collected, with the aid of an AV, every 3 weeks during 2 consecutive days. The semen was evaluated using the same procedures described for phase 1 of this study. The rams were slaughtered at the end of this trial period (90 days) at a commercial abattoir. After slaughtering, the carcass characteristics (cold carcass weight, dressing percentage, carcass grade, shoulder circumference, buttock circumference, backfat thickness, eye muscle area, marbeling and kidney fat), the testicular (volume and weight), the scrotal (weight, circumference, skin weight and fat) and the epididymis characteristics (weight and volume) were also recorded and compared amongst groups. Data was analysed statistically using the same procedures as described for phase 1. The results of this trial clearly indicated that the scrotal fat deposits of rams previously fed on the He diet are mobilized (at least partially) during subsequent periods of moderate dietary energy restrictions without adverse effects on semen quantity and quality. This is the first time that such finding is reported. Further research on the effect of high energy diets on subsequent ram fertility (short and long terms), as well as their reversibility are warranted. Different age groups, feeding regimes and other management practices must also be associated to high energy diets in order to fully evaluate the effect of these common practices on subsequent ram fertility.