GENERAL DISCUSSION, RECOMMENDATIONS AND OVERALL CONCLUSION
5.1 General Discussion
The overall aim of this thesis was to develop an energy evaluation protocol to be used in South Africa for horse feeds. Although a protocol was not as such 'developed', much investigative work was conducted that enabled any problem area to be identified so that any further research can now move forward quicker and with a greater amount of accuracy.
In Chapter Two, a preliminary trial was conducted to investigate miniature horses as possible pilot trial animals for use in vivo for feed evaluation.Their quiet temperament, lower feed intakes, reduced faecal output and lower overall costs makes them an ideal animal to use in digestion trials as opposed to large horses.Feeding 2% ofthe horses' body weight met the horses' digestible energy requirements but not the digestible crude protein requirements, this being due to the low crude protein quality in the poor quality hay that was fed.The DE results obtained from using miniature horses compared favourably to that of overseas results where large horses were used. Comparisons were based on DE equations developed overseas through large horse digestibility trial work. An equation developed by Fonnesbeck (1981) used digestible nutrients in the development of his equation and this equation showed the best fit to our digestibility results for both hay and concentrate.No significant differences were shown between DE results obtained by the male and female or the different ages. The method to calculate the digestibility of concentrates by the difference method gave questionable results, and it is believed that the assumption that the digestibility of feedstuffs are the same when fed together as when fed alone may be incorrect, and requires further testing.
Rate of passage of feedstuffs was investigated so as to determine the correct length of collection period to use when conducting digestion trials with miniature horses. Two methods were used to analyse these data namely, Grovum and Williams (1973) and Castle (1956). The method by Castle (1956) was preferred in this study due to its simplicity and ease at which results were calculated from cumulative excretion curves. The time- independent model of Grovum and Williams (1973) has also been shown to not fit
ruminant faecal excretion data very well (pond et al., 1988). Moore-Colyer (2000) has shown as well that rate of feedstuff passage through the digestive tract of equids is a time- dependent process.
A marker, Celite ®, was administered to the horses and the acid-insoluble ash method was employed to analyse the horses' faeces in the laboratory. It appears from these two methods that two and a half days is enough time to clear the digestive tract. Five days is recommended so as to ensure faeces collected are truly representative of the test diet.
These two models also allow us to estimate the rates of passage through the foregut and the hindgut of the horse.This warrants further investigation as this could prove very valuable information when formulating speciality diets for horses with specific needs.
Miniature horses appeared to be the perfect in vivo digestibility trial animal. Using this information it was decided in Chapter Three to compare the miniature horse to other possible pilot trial animals, the sheep and rabbit. The sheep was investigated, as current equine diet formulations in South Africa are based on ruminant TDN values and the accuracy of such assumptions is unknown. Rabbits, like horses, have very similar digestive systems both being non-ruminant hindgut fermenters and would be an ideal pilot trial animal due to their size, ease of handling and reduced overall cost. Unfortunately the rabbits gave too many problems in this trial to validate their use. It is believed that the hay may have been the underlying cause of all the problems as it was quiet dusty and rabbits are very sensitive to dust. They found the hay unpalatable, therefore large hay refusals were reported and most of the rabbits lost a lot of weight. Many contracted Pasteurellosis and died. Feed intake was increased and proportion of hay to concentrate provided was reduced so as totry and alleviate these problems, but these did not help.
Sheep were found to be accurate predictors of digestibility for the fibre components of a horse feed, but not for other nutrients, indicating to us that there is definite room for improvement in equine diet formulation.
In Chapter Four the use of in vitro digestion methods as a means of predicting the digestible energy of horse feeds. As no fistulated horses were available, rumen fluid and horse faeces as sources of inoculum were compared. No significant difference was found between the two inoculums. The decision would ultimately be based on situation and animal availability as to inoculum choice.
Significant differences were found between in vivo and in vitro results. Although this perhaps shows that in vitro cannot be used as a predictor of digestible nutrients for horse feeds, more research is needed as the method used to remove supernatant between the two stages of the in vitro method was not reliable and removed too many fine hay particles to warrant a decision on the validity of the in vitro method.No significant difference was found between the rates and maximum gas production by the inoculums used.However, there was a significant difference between the maximum gas produced by the different diets. This should be investigated further as if we had the ability in this country to predict the fermentability of individual ingredients as well as the fermentability resulting from combinations ofvarious ingredients, then we could perhaps formulate designer diets more specific to an individual horses needs and requirements.