Costs of growing pigs from weaning to market weight are determined largely by the amount of feed they consume and their rate of growth. Average daily feed intake is an expression of appetite. When carcass weights are within optimum ranges, value is determined by percentage lean. Therefore, rapid and efficient lean growth is important in pig production systems.
Protein requirements of pigs decrease as they grow. Thus, phase-feeding, or adjusting dietary protein intake during intervals of the growing period, is commonly used to reduce feed costs. Further, protein requirements differ for barrows and gilts because the sexes differ in appetite and in rate and composition of growth. Both barrows and gilts become increasingly less efficient in converting feed to weight as they get heavier. Thus, dietary protein concentrations and slaughter weights for optimum efficiency of lean growth are different for barrows and gilts.
Different selection strategies can produce genetic lines with different lean and fat growth curves and different appetites. Heavy emphasis on average daily gain will increase appetite. Whereas, heavy emphasis on reducing backfat will decrease appetite. Furthermore, selection strictly for decreased feed conversion ratio (feed/gain) will improve feed/gain ratios and decrease rate of fat deposition but will also decrease appetite and rate of gain.
Most selection strategies do not emphasize only one trait, but rather utilize an index of several traits weighted for some overall breeding objective. However, these indexes are usually designed uniquely for the role of each breed or line in a crossbreeding system. These different selection strategies, employed in breed and line development, cause genetic lines to have different appetites, different growth rates, and different rates of backfat and lean deposition. Therefore, the feeding strategy and slaughter weight that optimizes economic efficiency of lean growth may also differ across breeds and genetic lines of pigs.
In this article, feed intake, growth and efficiency of growth of pigs in the Quality Lean Growth Modeling (QLGM) project are presented. Genetic lines are characterized according to their biological type - mainly by their appetite (daily feed intake), rate of growth, and percentage carcass lean at different slaughter weights. Effects of diet and slaughter weight on these characteristics are presented.
Procedural Review Description of the genetic lines, the diets used, and management of the pigs are described in Goodwin's article (beginning on page 18). But a brief review is helpful to better understand the data in this report.
The pigs were samples from six genetic lines, as described in Tables 1 and 2, page 20. Sampling of lines was not done in a way required for valid genetic comparisons among lines. Therefore, each line is identified by a letter, A through F. To maintain anonymity they were shuffled and placed in the order listed in Goodwin's Table 4 (page 22) and abbreviated in the adjoining Key to Genetic Line Classification.
The six lines were selected, based on previous studies, to provide a relatively broad range of biological types for appetite, rate and composition of growth, and meat quality traits.
For example, line A was known to have a large appetite and to be fatter than other lines, so they ranked "high" for appetite and "low" for their less desirable backfat.
Line D was known to have appetite similar to that of line A, but to have greater growth rate and less fat. Thus, lines A and D were chosen because they differ in rate and efficiency of lean growth, but have similar appetites.
Line B was known to be lean with low appetite and moderate growth rate, and thus, to have good rate and efficiency of lean growth.
Lines C, E and F were expected to differ in component traits such as average daily gain, backfat and perhaps feed intake. The lines also are known to differ in muscle quality characteristics but these differences are not pertinent to discussions in this article.
For the most part, lines performed as expected based on previous data, but some deviations occurred, presumably because the sampling process did not duplicate that used in previous evaluations.
Diets are described in Table 7, page 24. They differed in amount of lysine. Beginning when pigs were placed on test at 90 lb., the high-lysine diet contained 1.25% lysine and lysine was decreased in increments of .15% to the low-lysine diet with .8% lysine. This reduction in lysine was accomplished by reducing the amount of soybean meal and increasing corn in the diets. Further, the amount of lysine in each diet was reduced by .15% as pigs attained pre-specified weights.
From the start of the test at 90 lb., pigs were fed the respective diet with the most lysine until they weighed 140 lb. Incremental reductions in dietary lysine occurred at gain intervals of 50 lb. - in other words, when the pigs reached weights of 140, 190 and 240 lb. Thus, there were four feeding periods for each pig with a .15% reduction in dietary lysine between each period, the last period being from 240 lb. to final off-test weight.
Pigs within a pen were all of one genetic line, but contained approximately an equal number of barrows and gilts. Within pen and sex, pigs were randomly assigned to be slaughtered at 250, 290 or 330 lb. They were individually removed from pens as they attained target slaughter weights.
The FIRE system (Feed Intake Recording Equipment from Osborne Industries) was used to record daily feed intake of each pig. Also, each pig was weighed at regular intervals beginning at 90 lb. until it attained its designated slaughter weight. Pen feed consumption was recorded for each period that corresponded to the dietary changes. The FIRE data were not available for preparation of this report. Therefore, analyses were done on a pen basis. Because pen averages were used in analyses, the effects of sex could not be evaluated. Also, because the average off-test weight was approximately the same for pigs in each pen, the effects of weight were evaluated by considering changes across the specific weight intervals corresponding to the four distinct periods.
Only two carcass traits are considered in this report, tenth rib backfat, measured off the midline on chilled carcasses, and percentage carcass lean, which was predicted from Fat-O-Meater measurements on chilled carcasses. These traits were used to relate differences in rate of growth, feed intake and feed conversion ratio to composition of the pigs.
Analyzing Results The data considered in this article include pen average daily feed intake, pen average daily gain, and pen feed conversion ratio (feed/gain) during each of the three test groups (from March 1996 through November 1997), and tenth rib backfat and percentage lean in carcasses.
There were a total of 394 pen/period records. Pen/period records included 3 test groups x 35 pens/group x 4 test periods = 420. Of the 420 records, hospital pens and a few pens with incomplete data brought the total down to 394.
Tenth rib backfat data were analyzed on 1,554 pigs and carcass lean was analyzed for 1,195 pigs. Analyses of pen averages were conducted to investigate effects of genetic line, diet, growth period (which corresponded to intervals of body weight change), and the interaction among these effects. Growth periods were 90-140 lb., 141-190 lb., 191-240 lb. and 241 to end weight for each pen since data is on a pen basis.
Backfat and percentage carcass lean were subjected to similar analyses, but effects of actual slaughter weights of 250, 290 and 330 lb., rather than the period of growth, were considered. Results are presented in tables and graphs to illustrate responses across these combinations of effects.
Feed Intake Analysis Average daily feed intake is an expression of appetite (Figures 1, 2 and 3). Figure 1 illustrates feed intake for each genetic line averaged across diets during each growth period. Figure 2 shows each diet averaged across genetic lines during each growth period. And, Figure 3 illustrates each line-by-diet combination averaged across periods.
Effects of line, diet and growth period were all significant (P<.01), and there was a significant interaction between genetic lines and periods.
There was a linear increase in daily feed intake as pigs increased in weight. The pattern was similar for pigs of each line, but the increase was greatest for lines A and D, the high appetite lines.
Daily feed intake reached a plateau in line E pigs as they ate the same amount of feed during periods 3 and 4 (P3, P4).
On average, pigs consumed 4.1, 5.3, 6.0 and 6.7 lb. of feed, respectively, during the four periods. Rate of increase in daily consumption was less for lines with lower appetite (Figure 1), but the proportional increase above average intake in the preceding period was similar for each line with the exception noted for line E.
Pigs fed diet 1, the high lysine diet, consumed more feed per day than pigs eating other diets (Figure 2). The average across periods was 5.8 lb. of feed per day for diet 1, and 5.5, 5.5, and 5.3 lb. per day for diets 2, 3 and 4, respectively. The increase in daily feed intake as weights increased was similar for pigs consuming each diet.
Pigs of line A, a high appetite line, consumed more of diet 4, the low lysine diet, than of the other diets (Figure 3). Lines B, D, E and F tended to consume less feed per day as lysine in the diet decreased. This pattern looks much like an interaction. However, the test of the line-by-diet interaction did not approach significance (P>.3). Therefore, the observed differences among lines on the different diets are probably due to sampling and not to real biological differences. The conclusion is that line differences in feed intake were consistent across diets.
Average Daily Gain Overall, line D had significantly greater average daily gain than all other lines (Figure 4). The other lines did not differ significantly.
Daily gain also differed significantly across periods. It was equal during P1 and P2 (1.69 lb./day), and then decreased in P3 (1.57 lb./day) and P4 (1.53 lb./day). Although it looks as though there might have been an interaction because line A had the greatest gain during period 4 and daily gain of line F started to decline in period 2, this interaction was not significant.
The conclusion is that, on average, pigs of all lines make their most rapid gains during weight intervals that correspond to periods 1 and 2 (90 to 190 lb.), and rate of gain declines thereafter as weight increases.
Average daily gains for pigs on each diet are illustrated in Figure 5. The pattern across periods is the same as described for Figure 4. Daily gains differed significantly for pigs on diets 3 and 4. Pigs eating diet 3 gained fastest (1.66 lb./day) and pigs eating diet 4 gained the least (1.56 lb./day) - a tenth of a pound difference, which has both economic and pig flow implications. Daily gain for pigs eating diets 1 and 2 were intermediate (1.63 lb./day) and did not differ significantly from gains for pigs on either diets 3 or 4.
Average daily gain for pigs of each line eating each diet are illustrated in Figure 6. The line-by-diet interaction was not significant. Therefore, differences between lines were consistent across diets. This finding is somewhat surprising as pigs with quite different appetites and composition of growth might be expected to have different average daily gains as dietary lysine changes. Specific tests of differences among lines on the different diets also did not reveal any interactions. Therefore, the observed variation in average daily gain for lines across diets was probably due to sampling. Pigs of all lines responded similarly to the dietary protein changes used in this experiment.
Feed Conversion Feed-to-gain ratios for pigs of each line at each period are illustrated in Figure 7. There were significant differences among genetic lines, and all conversion ratios at each growth period differed statistically from each other.
Lines E and B were most efficient, averaging 3.35 and 3.36 lb. feed/lb. of gain. Their average was statistically better than that of line A (average = 3.70), line C (average = 3.61) and line F (average = 3.57). Although the average for line D was greater (3.46 lb. feed/lb. gain) than for lines E and B, the difference was not significant, nor did the line D differ from lines F and C, but line D was significantly more efficient than line A. Average efficiency for lines A, C and F did not differ significantly.
Feed efficiency increased (got worse) linearly over periods. It averaged 2.48, 3.25, 3.87 and 4.45 lb. feed/lb. gain in periods 1 to 4, respectively.
Diet interactions with feed conversion were similar to diet interactions with average daily gain (Figure 8). Pigs eating diet 3 gained weight most efficiently (3.38 lb. feed/lb. gain). However, their efficiency did not differ significantly from that of pigs eating diet 2 (3.43), but was statistically better than that of pigs eating diet 4 (3.54) and diet 1 (3.68). Efficiency of pigs eating diets 2 and 4 did not differ significantly, but pigs eating diet 1 were significantly less efficient than pigs eating diets 2, 3 or 4. There was no interaction as differences among diets were very consistent across periods.
Feed conversion for pigs of each line eating each diet are illustrated in Figure 9. Again, line by diet interactions were not significant, as differences among lines were consistent across all diets.
Carcass Lean, Backfat Percentage carcass lean and tenth rib backfat for pigs of each line and diet at each slaughter weight are in Table 1 (page 46). Lines ranked similarly in percentage lean at each weight.
Lines B and E were consistently leanest; line A was fattest. The other lines were intermediate to these two groups. Differences between lines of pigs in percentage lean can be expected to be similar at slaughter weights between 250 and 330 lb.
Final slaughter weight had a significant effect on percentage carcass lean and on backfat thickness. The average percentage lean was 52.2% at 250 lb., 50.8% at 290 lb. and 49.6% at 330 lb. Tenth rib backfat average was 1.00, 1.17 and 1.28 in., respectively, at the three slaughter weights.
There was a linear increase in backfat and a linear decrease in percentage lean as slaughter weight increased above 250 lb. Each additional 40 lb. of live weight brought with it an average increase of .14 in. backfat and a decrease of 1.3% in carcass lean. The increase in backfat and decrease in percentage lean with increasing slaughter weight was similar in all lines.
Diets significantly affected percentage carcass lean and backfat thickness. Carcasses of pigs eating diets 1, 2 and 3 had similar backfat and percentage lean at all three slaughter weights. Diet 4 produced significantly fatter carcasses at all weights.
The only significant line-by-diet interaction was for percentage carcass lean at 290-lb. slaughter weight. The exact cause of this interaction is not clear. It appears to be mostly a change in magnitude of differences among lines, as lines ranked similarly on each diet. If this interaction were of biological importance we would expect to see a similar interaction at either, or both, of the 250 and 330 lb. slaughter weights. Also, the interaction was not significant for tenth rib backfat, adding additional evidence that it may not be biologically important.
What We've Learned Differences among genetic lines in feed intake, growth rate, feed conversion and carcass composition, as pigs increased in weight and while consuming diets with different amounts of lysine, were remarkably consistent.
Pigs consumed more feed per day when fed diet 1, the diet with the most lysine. But, the pigs neither grew faster nor developed carcasses with more lean while eating this diet. This result was consistent across lines and over all weight intervals. Therefore, it seems that diet 1 contained excess lysine for pigs of all genetic lines and, as such, did not promote greater nor more efficient lean growth in any of them.
Feed conversion was markedly poorer when pigs consumed diet 4. Also, these pigs had carcasses with more backfat and less lean than pigs consuming the other diets. Therefore, although efficiency of lean growth was not studied here, it was most certainly less for pigs of all lines when they received diet 4.
It appears that when all traits are considered, optimum performance for pigs of all lines was achieved when they consumed diets 2 and 3. Additional economic analyses of these data using individual feed intake records to determine whether there is an optimum diet and slaughter weight for each genetic line are needed.
Optimum slaughter weight depends on market price, costs of each additional increment of weight gain and the effect increasing weight has on composition. No attempt to determine this optimum for pigs of each line was made in this study.
However, pigs became quite inefficient at converting feed to live weight as weights increased above 250 lb. Further, daily rate of gain declined for pigs of all lines as weights increased above 190 lb. Even with inexpensive feed, it appears that costs of gain increase substantially as pigs are taken to heavy slaughter weights. The problem is further compounded by the fact that percentage carcass lean decreases with increasing weight.
Optimum slaughter weight is certainly different for pigs of the different genetic lines - lower for fat lines than for leaner lines.
Data reported here are averages for barrows and gilts. Additional analyses of individual feed intake to determine optimum diets and slaughter weights for barrows and gilts of each genetic line are needed. However, the data reported here can be used as a guide by producers. By matching performance of the line of pigs used by the producer to those of one of the biological types studied here, the effect of changing dietary lysine and the extra time and feed needed to grow pigs to heavy weights can be estimated.