Composition and cross-sectional area of muscle fibre types in relation to daily gain and lean and fat content of carcass in Landrace and Yorkshire pigs

The muscle fibre-type properties of longissimus were compared between Landrace and Yorkshire breeds and between the sexes in an attempt to shed light on the relationship of these histochemical parameters to animal growth and carcass composition. Muscle fibres were classified into three groups, type I, type lIA and type 118, using the myosin ATPase method. At a given live weight, the crosssectional area of type I fibres (CSA,) was smaller (p<0.01) and the cross-sectional area of type 118 fibres (CSA 11B ) larger (p<0.01) in longissimus of Landrace than in that of Yorkshire. The CSA 118 (pcO.Ol) was larger in gilts than in castrated males. At an average live weight of 97 kg, the Landrace pigs were significantly younger (p<0.01) than the Yorkshire pigs. The pH! value of Landrace was lower (p<0.01) than that ofYorkshire. The percentage of high-value cuts in carcass with head (M%) was lower (pcO.Ol) and the percentage of fat in back and loin (F%) higher (p<0.01) in castrated males than in gilts. The cross-sectional area of loin (CSA|o,n ) was larger (pcO.Ol) in gilts than in castrated males. All the histochemical and other traits varied considerably between the animals. The correlations between histochemical properties and growth and histochemical properties and carcass composition were rather low.


ntroduction
The pig breeds or crosses used for pork production tend to vary from country to country. The breeding goals also vary, and hence there may be genetic differences in the same breed between countries. The traditional pig breeds reared in Finland are Landrace and Yorkshire. Both have been bred intensively and separately for many years, although with similar breeding goals. The most important selection criteria in pig breed-ing have been high daily gain and carcass leanness. Genetic selection is thought to influence muscle fibre composition by increasing the number and area of type II fibres in domestic pigs (Staun 1972, Swatland 1977, Rede et al. 1986). The muscles of wild pigs have a higher percentage of type I and type lIA fibres and a lower percentage of type 118 fibres than do domestic pigs, and also the CSA of muscle fibres is smaller in wild pigs than in domestic pigs at the same live weight (Bader 1983).
Muscle fibre composition affects both growth and lean meat content. Miller et al. (1975) found in longissimus and Dwyer et al. (1993) in semitendinosus that faster growing pigs appeared to possess more, but smaller, fibres than slower growing pigs at the same live weight. Staun (1972) and Miller et al. (1975) established that the total number of muscle fibres in longissimus was more closely related to muscle mass than was fibre diameter. Dwyer et al. (1993) reported similar results in their study on porcine semitendinosus. A positive correlation has also been found between the percentage of type 118 fibres in longissimus and the CSA of pork loin (Bader 1981,Fiedler and Otto 1982, Wegner and Ender 1990,Fewson et al. 1993). Tornberg et al. (1993) pointed out that in lean carcasses, the CSA of muscle fibres is larger than that in fat carcasses of the same weight.
This study sought to compare the histochemical properties ofFinnish Landrace and Yorkshire breeds, to establish the differences in these properties between the sexes and also between pigs within the same breeds, and to investigate the relationships between these histochemical properties and animal growth and carcass composition.

Material and methods
The material consisted of 107 pure-bred pigs, 54 Finnish Landrace and 53 Yorkshire (N=s3), raised at four different test stations. The proge-ny of 39 Landrace and 41 Yorkshire sires were used. The pigs were slaughtered at an average live weight of 97 kg. Before slaughter, the pigs were allowed to rest overnight. They were then stunned with carbon dioxide. The day after slaughter, the left side of the carcass was dissected.

Histochemical properties
Muscle samples for histochemical analysis were taken from the right side of the carcass about 30-40 min after exsanguination from longissimus between the 13th and 14th ribs. The muscle sample, about 20 g, was cut into pieces measuring 0.5x0.5x1.0 cm, which were frozen in liquid nitrogen and stored at -80°C until analysed.
The fibres were classified into three groups, type I, type lIA and type 118, with the myosin ATPase method (Brooke and Kaiser 1970) and an acid preincubation solution (pH 4.6). Stained sections were examined with an image analysis system using CUE 2 Planomorphometry software (Olympus, Germany). The magnification was 160x. The fibres were counted in an area of about 0.9 mm 2 , containing about 150-200 fibres. Three different pieces of each muscle sample were stained, and thus 450-600 fibres were analysed. The following parameters were computed: percentage of type I (I% number ), type lIA (lIA% )  bres, percentage of the total area of type I (I% area ), type lIA (lIA% ami ) and type 118 (118% area ) muscle fibres, and the cross-sectional area of each fibre type (|4m 2 ) (CSA,, CSA i|a and CSA I|B ).
The number of muscle fibres in longissimus per whole loin area (Num fjbre ) was calculated as the loin area (at last rib) (CSA |ojn ) divided by the average fibre cross-sectional area (CSA fibre ).

Other traits
The following station test traits were used in the study: age at slaughter (age), live weight at the end of the test (LW), average daily gain during the test period adjusted to a weight of 25-100 kg (ADG), carcass weight (CW), loin area at 13-14 rib (CSA |oin ), percentage of fat in back and loin (F%) and percentage of high-value cuts in carcass with head (M%).
The pH value of longissimus was measured between the 13th and 14th ribs at 45 min and 24 h (pH 2 ) post mortem on each carcass (Knick Portamess pH meter 752 equipped with a Xerolyte electrode; Ingold Xerolyt L0T406-M 6, Germany).

Statistical methods
Data were analysed with the GLM procedure and the CORR procedure of the SAS/STAT program (SAS Institute 1990). The effects of breed and sex were studied using the following statistical model: The relationship between histochemical properties and animal growth and between histochemical properties and carcass composition was described with correlation coefficients. The effect of breed and sex was taken into account.

Results and discussion
Difference in histochemical properties between breeds and sexes The muscle fibre composition of different breed has been compared in only a few studies. Here, longissimus ofLandrace contained a significantly higher percentage of type I fibres and a lower percentage of type 118 fibres (pcO.Ol) than did that of Yorkshire (Table 1). The percentages of muscle fibre types ofanimals within both breeds varied, however, more than did those between the breeds on average. On the basis ofhistochemical properties, similar pigs can be found in both breeds. Miller and coworkers (1975) did not find any differences in fibre type composition in longissimus between Hampshire, Yorkshire and Hampshire x Yorkshire pigs, and Essen-Gustavsson and Fjelkner-Modig (1985) found none between Swedish Landrace, Yorkshire and Hampshire pigs. Here, no significant difference (p>0.05) was found in fibre type composition between the sexes (Table 1).
At the same live weight, however, the CSA, was smaller (p<0.01) in longissimus ofLandrace than in that ofYorkshire, but the CSA, m was larger (p<0.01) ( Table 1). In Danish Landrace-Yorkshire crossbred pigs the CSAs of type I and type 118 fibres were 2824 and 6120 pm 2 , respectively at a live weight of 90 kg (Oksbjerg et al. 1990), and in Swedish Landrace-Swedish Yorkshire crossbred pigs 3200 and 5600 pm 2 , respectively at a live weight of 100 kg (Essen-Gustavsson et al. 1992). The CSAs of different fibre types reported in various studies are difficult to compare because the muscle samples were excised from the carcasses at different times post mortem, and the CSA measurements were made on sections stained with different methods.
In this study, no significant difference was found in the estimated Num r . between Landrace fibre and Yorkshire or between sexes. The CSA |IB (5051 pm 2 ) (p<0.01) and the CSA fibre (4476 pm 2 ) (p<0.05) were larger in gilts than in castrated males (4593 and 4153 pm 2 , respectively). Karlsson et al. (1994) also showed that the CSA of all fibre types was significantly smaller in longissimus of intact Yorkshire male pigs than in gilts at a live weight of 103 kg.

Difference in growth and carcass traits between breeds and sexes
When pigs grow, the CSA of all fibre types grows as well, but the area of type lIA and type 118 is drace pigs were significantly younger (p<0.01) when they reached the average live weight of 97 kg than were the Yorkshire pigs, although there was no significant difference in ADG between the breeds during the test period. The castrated males grew faster (p<0.01) than the gilts. The live weight was the same in both sexes, but M% was lower (p<0.01) and F% higher (pcO.Ol) in castrated males than in gilts (Table 1). Also, the CSA |oin was larger (p<0.01) in gilts than in castrated males. Similarly, in their study of German Landrace Niirnberg and Ender (1990) found that carcass meat content was lower, fat content higher and the cross-sectional area of loin smaller in castrated males than in gilts at the same live weight.
The pH, value of Landrace was lower (p<0.01) than that of Yorkshire pigs (Table 1. ) On the basis of pH, values (<5.8), there were only two PSE carcasses among the pigs studied, one in each breed. This is a very low number (1.3%), due possibly to the fact that the pigs were always allowed to rest overnight before slaughter.
The pH, value found here was lower and the CSA |ib larger in Landrace than in Yorkshire pigs.
A large fibre CSA may result in limited diffusion of muscle lactate into blood in stress situations. Lactate can thus rapidly accumulate in the muscle and give rise to low pH values. In porcine longissimus, glycogen is normally stored in type 118 fibres (Swatland 1975, Karlsson et al. 1994. If the content of type 118 fibres is high and that of type I fibres in longissimus low, there is also a high glycogen content, and thus more lactate may be formed. In dark porcine muscles with a high content of type I fibres it is impossible for the ultimate pH value to go as low as in light muscles because of the lower glycogen stores in dark muscles. Neither in this study nor in the work of Niirnberg and Ender (1990) were any significant differences found in pH! or pH, values between the sexes.
The relation of histochemical properties to growth and carcass parameters The correlation coefficients for growth and carcass traits and for the histochemical parameters measured for porcine longissimus are listed in Table 2. The finding of Sosnicki (1987) that a negative correlation exists between the percentage of BR fibres and age was not confirmed here (p>0.05). Nor was our study able to confirm the finding of Miller et al. (1975) that faster growing pigs appeared to possess more, but smaller, fibres in longissimus than did slower growing pigs at the same live weight. We did, however, find a positive correlation between 1% . and r number LW (p<0.05) and between CSA i|a and LW (p<0.00l), and between CSA I(a and CW (p<0.001) and between CSA |m and CW (p<0.05). We also found a negative correlation between lIA% , (p<0.05) and CW. The positive correnumber v ' ' r lation between live weight and ADG (r=.414, p<.001) makes it difficult to come to any definite conclusion regarding the relationship between histochemical parameters and growth rate. Some researchers have found a positive correla-tion between the percentage of type 118 fibres in longissimus and the CSA of pork loin (Bader 1981, Fiedler and Otto 1982, Wegner and Ender 1990,Fewson et al. 1993), but we could not confirm this finding (p>0.05). Our findings were, however, consistent with those of Staun (1972) and Miller et al. (1975), namely that a positive correlation exists between the Num,. and fibre CSA |oin and between the Num tlbre and M%. A negative correlation was found between Num rk and fibre F%.

Conclusions
Comparisons of all histochemical traits reveal significant variance between animals. Landrace longissimus shows a lower number of type 118 fibres with a larger fibre CSA than does Yorkshire longissimus. The CSA of type 118 fibres is larger in gilts than in castrates. There are differences in growth and carcass parameters between the sexes. The large variation in histochemical and other traits between animals suggests that it should be possible to select animals with the desired characteristics for breeding. Further investigations are needed to establish the criteria for good meat quality parameters and the heritability of these properties.