By Elsje Pieterse, Francois Siebrits, FK Gloy and Louw Hoffman
Sound business principals and rules of survival demand that producers aim at maximum profit or, put differently, minimum cumulative cost per income. This boils down to the difference between expenses and income.Income of the one hand is a function of carcass weight and composition (lean meat percentage) which in turn is dependant on genotype and nutritional status. Expenses can be divided into fixed and variable cost.
Although all cost can, in time, be variable the variable cost referred to in will include cost that vary on the short term. For practical purposes this can be grouped as cost of producing a weaner pig and the cost incurred in the growing out and final marketing of this animal. The cost of producing a weaner pig is influenced by sow cost and boar cost. Sow cost mainly includes factors like the purchase price of the gilt or the cost of growing the gilt to reproductive age.
Boar cost includes the cost of keeping a boar and/or the cost of artificial insemination. These costs make up the cost of the piglet at birth. This cost is influenced by a number of factors including number born alive, pre-weaning mortality, number of litters per sow per year, farrowing rate, replacement rate, number of empty days, etc.
These factors can be manipulated through management and can sometimes be very difficult to change and/or maintain at acceptable levels. A variable that is, however, relatively easy to change is slaughter weight. With this in mind a trial was designed to look at the effect of increased slaughter weight on efficiency of pork production on farm as well as the effect of increased slaughter weight on carcass, meat quality, processing, yield, chemical and sensory characteristics.
In total 250 variables were measured on 192 animals/carcasses and analysed for 95 effects, variables were compared within and between sex types (castrate, boar and gilt) and five genotypes. Because of the high efficiency of growth of boars the cost of producing boars are much lower than that of either gilts or castrates. The main reason for this is the change in feed conversion ratio (kg feed needed per kilogram live weight gain). As can be seen from the graph the position of line of the graph is different for different sex types as well as the rate at which the slope increases (shape of the curve).
Feed conversion ratio is only one example of differences in absolute values and rates of change of different production parameters. The aim of the study was to quantify these absolute values and rates of change of the different parameters. These analysis will then be used in a form of a mathematical/economical model to predict optimum slaughter weight on farm using on farm data. In the experimental phase it was found that the effect of genotype was small but that sex type and slaughter age/weight had a large effect on production parameters.
The effect on meat quality parameters was however small or positive. In summary it can be said that boars performed the best in terms of average daily gain, and feed conversion after 18 weeks of age. They did the same as gilts but better than castrates for feed conversion before 18 weeks of age, P2 measurement (live animals), P2 at slaughter, subcutaneous fat area, fat thickness (over loin area) and predicted percentage lean (Hennessy grading probe). In terms of meat quality characteristics very little differences were observed and most of these differences are of little practical value.
The effect of slaughter weight can be summed up as follows. Average daily gain peaked at an average live weight of 112 kg (castrates 101 kg, gilts 113 kg and boars 125 kg). The rate of change in feed conversion ratio is of great importance and it was found that the rate of change is fast up to 75 kg live weight, it then slows up to 125 kg live weight. As most producers either produce or aim to produce within this range of live weights the effect of sex type on FCR is expected to be greater than the effect of live weight/age.
Other positive factors associated with increased slaughter weight are an increase in dressing percentage, eye muscle area, muscle depth, carcass measurements, absolute yields, water holding capacity, drip loss and colour. The main disadvantage of increased slaughter weight is an increase in subcutaneous fat.
Another factor that must be kept in mind is the incidence and occurrence of taints. The incidence of taints (androstenone and skatole) were low in the trial and skatole only became a problem in carcasses of animals slaughtered above 139 kg live weight.
In conclusion it can be said that the researchers were successful in the description of rates of changes of different parameters and that the regressions developed in this way can be used effectively in the development of a predictive model that can be used by the pork producer as a decision support system for optimising slaughter weight. It can however also be used as an analytical tool to identify and ultimately rectify areas of inefficiency. On the abattoir and processor side this model can predict cut yields and yield of processable lean meat given certain carcass characteristics