- The Kleiber Ratio as a possible selection for Afrino Sire Selection
|Last update: August 18, 2011 09:46:36 AM|
The Kleiber Ratio as a possible selection for Afrino Sire Selection
Grootfontein, Agricultural College
Middelburg, (CP) 5900
The selection aims of the Afrino can be summarised as follows. The ewes must be adapted to the natural environment where they must produce and reproduce. This means that the rate of reproduction must be optimal, lambs must be marketable at an early age and the ewe must produce wool of an acceptable quality and quantity.
There is, however, uncertainty about the selection criteria to be applied to achieve the above selection aims. For this reason the most suitable selection criteria for the Afrino were investigated. Laubscher (1965), as quoted by De Lange (1981), is of the opinion that the selection methods for rams and ewes should be separated and that the sexes should be selected on different traits. Effective ewe selection is of great economical implication for the existing flock, yet in the next generation it could only contribute on a small scale. Conversely, ram selection may realise an insignificant economical influence on the existing flock, while contributing towards a vast genetic improvement in the following generation. It means, therefore, that ewe selection should be aimed at increasing production (i.e. total mass of Iambs and wool produced) in the existing flock, while ram selection should be aimed at effectively increasing production in the next generation.
Possible selection criteria were, consequently, investigated for Afrino ram and ewe selection. In respect of ewe selection, ewe production, i.e. the ability of the ewe to wean heavy Iambs under veld conditions, was investigated as selection criterion. Ram selection, on the other hand, should preferably concentrate on growth ratter than fertility aspects. However, a problem that prevails among most mutton and dual purpose sheep breeds is that there is a lack of knowledge about the most suitable selection criteria for growth traits. In this discussion the Kleiber ratio will be evaluated as possible selection criterion for ram selection.
What does Kleiber Ratio mean?
The Kleiber ratio is formulated as follows:
where ADG = Average daily gain, expressed in g/day
W = Body weight, expressed in kg.
W0.75 = Metabolic mass
ADG is calculated as follows: suppose the ADG from wean to one year of age must be calculated. For this the weaning mass, the year old mass and the dates of weighing weaning mass and year-old mass are required. The dates are necessary in order that the exact number of days can be calculated between taking the weaning mass and the year-old mass.
Year-old mass - Weaning mass
Note that the initial mass, in this case the weaning mass, must always be deducted from the end mass - the year-old mass. Whenever the Kleiber ratio is calculated, W is always the end mass, in other words in this case W will be the year-old mass. Therefore, if we want to calculate the Kleiber ratio for the growth period from weaning to one-year, it will look like this:
|Kleiber Weaning year =||
ADG (as calculated above)
Year-old mass 0.75
The same procedure is followed when calculating the Kleiber ratio for the period from birth to wean. In this case the birth mass and the birth date as well as the weaning mass and weaning date must be known. As already stated, W 0,75 is the metabolic mass. Now, what does metabolic mass mean and why is it used?
A portion of feed intake is utilised in the animal's body for the normal, essential body functions - i.e. the functions necessary to survive. This need is referred to as the animal's maintenance need. The remaining nutrients can be utilised for production functions such as growth, wool production and milk production.
When two animals, one weighing 60kg and the other weighing 40kg, are compared, the maintenance need is bigger for the larger animal than for the smaller one. The larger animal's feed intake would therefore be more than that of the smaller animal before it would have extra nutrients available for growth or wool production. It appears therefore that the larger animal is less effective in the sense that it must take in larger quantities of feed to reflect the same body mass gain as the smaller animal. The larger animal can therefore be at a disadvantage. Some method would therefore have to be used whereby the larger and smaller animals can be compared independently from their body size. This is where the metabolic mass (W0. 75) is applied. Where use is made of the metabolic mass, large and small animals can be compared without the one having an advantage over the other.
Why do we consider the Kleiber ratio?
The Afrino is a dual purpose sheep deriving 80% of its income from mutton; therefore it is obvious that amongst others, one should select for growth and mutton traits. Research results show that extended selection for mass or growth rate can result in the following undesirably correlated responses: a decrease in fertility and longevity and an increased fat deposit in the body. The latter is undesirable as an excessive fat deposit in the body can disturb the homeostatic balance in the body which in turn can lead to lower fertility. Selection for increased mass per age can also give rise to high birth masses, resulting in difficult births. Care should therefore be exercised against too high a birth mass (difficult births) and too high a mature mass (decreased fertility).
An alternative selection criterion other than mass itself should therefore be sought. Selection for efficiency, for example, will not decrease fertility or result in increased fat deposits in the body. It appears therefore that efficiency of feed conversion is a more acceptable selection criterion than mass itself for increased growth rate. It is however, both impossible and impractical to select for efficiency of feed conversion under extensive veld conditions, as the individual feed intake of each animal on the veld can not be determined. This is where the Kleiber ratio is applied, because this ratio is recommended by Kleiber (1936), Roux & Scholtz (1984) and Scholtz (1986) as an alternative selection criterion for efficiency.
The advantages of the Kleiber ratio are that it can be determined under veld conditions, it is positively correlated with ADG, compared to ADG it predicts feed conversion more accurately and it does not have a negative effect on fertility or longevity. In an experiment with Afrino ram Iambs under feedlot conditions a correlation of 0.87 has been found between the Kleiber ratio and efficiency of feed conversion, where efficiency of feed consumption was expressed as the increase in body weight per unit of feed consumed. It is also interesting to note that the correlation between ADG and efficiency of feed consumption during this test period was only 0.64 in comparison with the correlation of 0.87 between the Kleiber ratio and efficiency of feed consumption. In this case the Kleiber ratio therefore predicted feed conversion 36% more accurately than ADG.
It is evident from the results of the above experiment that when applied to the Afrino, the Kleiber ratio can indeed be used as a reliable indication of efficiency, especially under feedlot conditions. It is accepted that the Kleiber ratio is also accurate in predicting the efficiency of feed consumption of young, growing animals under veld conditions.
Heredity and Genetic correlations of the Kleiber ratio
Before the Kleiber ratio can be used as a selection criterion, one must first consider the heredity factor and its correlations with other of the Afrino's economically important traits. The heredities of and genetic correlations between birth mass, weaning mass, Kleiber from birth to wean (Kleib-birth-wean), year mass, Kleiber from wean to year-old (Kleib-wean-year) and mature mass are shown in Table 1.
As already mentioned care should be exercised against excessive birth mass and excessive mature mass. If one selects only for weaning mass, birth mass as well as mature mass will increase as is evident from Table 1, because the genetic correlation between weaning mass and birth mass is very high - 0.717 - and between weaning mass and mature mass it is 0.710. From these results it appears therefore that selection for weaning mass can result in undesirable genetic responses in birth mass and mature mass.
On the other hand the correlation between Kleiber-birth-wean and birth mass is lower than that of weaning mass and birth mass, namely 0.250. The correlation between Kleiber-birth-wean and mature mass is also lower, namely 0.604. It therefore appears that Kleiber-birth-wean is indeed a safer selection criterion than weaning mass inasmuch as it would not bring about any significant undesirably correlated response in birth mass or mature mass.
When looking at the correlation of Kleiber-wean-year with birth mass, it can be seen from Table 1 that birth mass will not change notably if one selects for Kleiber-wean-year, because the correlation is insignificantly negative, namely -0.119. The same principle applies to mature mass as this correlation is also insignificant, namely 0.104. It appears, therefore, that Kleiber-wean-year as selection criterion offers the best advantages to be exploited during selection.
One aspect that has to be borne in mind, however, is the negative relationship that exists between the pre- and post-weaning growth periods, as can be seen from the negative correlations between (1) weaning mass and Kleiber-wean-year (-0.512) and (2) Kleiber-birth-wean and Kleiber-wean-year (-0.564). If one therefore selects for Kleiber-wean-year, weaning mass or Kleiber-birth-wean should also be taken into consideration. Further tests are presently being carried out to evaluate these recommended selection criteria in practice. The Breed Society will keep breeders abreast of the progress and any new developments or results.
DE LANGE, A.O., 1981. Patrone in die wolskaapbedryf - twintig jaar later. Verrigtinge van die F.X. Laubscher-Herdenkingsimposium, Stellenbosch
KLEIBER, M., 1936. Problems involved in breeding for efficiency of food utilisation. Proc. Am. Soc. Anim. Prod.: 29th Annual Meeting: 247-258
ROUX, C.Z. 8t SCHOLTZ, M.M., 1984. Breeding goals for optimal total life cycle production systems. Proc. 2nd World Congress on Sheep and Beef Cattle Breeding. Pretoria.
SCHOLTZ, M.M., 1985. 'n Kritiese Evaluasie van Prestasietoetsing. Verrigtinge van 'n Simposium oor Skaap- en Vleisbeesteling. Dept. Veekunde, Universiteit van Pretoria, Pretoria.
Afrino Handleiding Vol 4 : 9-12