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Commercial producers are considering health management changes to improve both the rate and efficiency of lean growth. Substantial differences in performance exist between different environments and health management strategies. In a recent Purdue trial, pigs with minimal diseases via segregated early weaning (SEW) and fed a series of nonlimiting diets achieved 230 lb. at 136 days of age and 264 lb. at 151 days of age. Pigs raised on the original commercial farm conventionally weaned with all-in, all-out production required 180 days to attain 230 lb. liveweight.

Substantial genetic variation for lean growth rate, lean efficiency, carcass percent lean and feed intake exists between different genetic populations of pigs. Lean growth trials conducted in the late 1980's at Purdue University and the University of Kentucky, found large amounts of variation for lean growth between different genetic populations or genotypes of pigs. In the early 1990's, seedstock were imported from Canada and Europe resulting in additional genetic variation in percent lean and feed intake.

Commercial producers who have implemented or are considering implementing SEW to improve health status, may want to reconsider their genetic choices. The best combination of traits for high health status pork production must be based on swine growth concepts.

Concepts of Pig Growth

The first concept is that lean growth increases after 40 lb. liveweight, reaches a plateau and then declines. In general, high lean growth genotypes achieve their maximum lean growth rate later and maintain higher lean growth rates to heavier weights (Figure 1). The second concept of swine growth is that as feed intake increases, a linear response in lean growth and fat rate occurs (Figure 2). The change in lean growth (or protein accretion) per unit increase in energy intake is called the slope. As energy intake increases, lean growth increases until a plateau occurs. The plateau is achieved when the energy required for maximum lean growth is reached. Energy supplied in excess of a pig's maximum lean growth rate requirements will be utilized for fat deposition. Pigs with high lean growth rate potentials respond to higher energy intakes by increasing lean growth rate (Figure 2).

To optimize lean feed conversion, producers must aim to achieve high lean growth rates without excessive fat deposition (Table 1). Animals with reduced intakes achieve lower lean growth rates, grow slower and allocate a higher proportion of their energy intake to maintenance. As feed intake is increased in the linear response range, lean growth increases with only small decreases in the ratio of lean gain to fat gain. In the linear feed intake response range, backfat marginally increases with increased feed intake. As energy intake increases above that needed for maximum lean growth, increases occur in the ratio of fat:lean deposition, backfat thickness and lean feed conversion. The most efficient lean growth is achieved when pigs consume enough energy to achieve 95-100% of their lean growth potential.

Table 1. Growth of gilts at different energy intakes *

	Energy Intake, Mcal ME/Day
	7.7	8.3	8.9	9.5
Carcass lean gain (g/day)	322	363	390	391
Carcass fat gain (g/day)	150	163	204	231
Lean gain:fat gain	2.16	2.15	1.91	1.63
Backfat, 10th rib (cm)	1.88	1.93	2.00	2.23
Average daily gain (kg/day)	0.72	0.78	0.83	0.87
Kg feed/kg lean gain	6.80	6.28	6.28	6.62

* 58-104 kg liveweight, maximum lean growth achieved at an intake of 8.7 Mcal ME/day; 36 gilts per energy intake.

The slope of lean gain (or protein accretion) on energy intake determines the extent to which energy intake is partitioned into lean versus fat gain. Pigs with moderate feed intakes during this time deposit a high proportion of lean and little fat. For this reason, improving management to increase feed intake at these liveweights can be very cost effective because the additional nutrients will efficiently be used to increase lean gain. As a pig grows, the slope becomes less steep and the partitioning of energy changes so that at moderate energy intakes 70- 80% of the intake is needed for maximum lean growth, the ratio of lean gain to fat gain declines. The slope of lean gain to energy intake decreases rapidly at heavier liveweights (150-240 lbs.) as the pig's maximum lean growth rate declines. In other words, once a pig matures, and its lean growth rate declines, the partitioning of energy shifts so that even at moderate energy intakes the ratio of lean gain to fat gain decreases. Therefore, genotypes that decline rapidly in lean growth at lighter weights will also have more fat and less lean deposition at moderate feed intakes. This makes it very difficult to produce uniform lean carcasses from early maturing low lean growth genotypes.

Another pig growth concept is that fat requires substantially more energy to deposit that lean. The energy cost of protein gain is 4.78 Mcal/lb., the energy cost of fat accretion is 5.73 Mcal/lb. For each pound of protein accretion there is an accompanying 2.5 lbs. of fat free lean (water), therefore fat accretion requires three times more energy per lb. than fat-free lean growth. For this reason, lean lines depositing a high proportion of lean due to steep slopes of protein accretion on energy intake require less energy to achieve the same lean growth rate. These genotypes have the potential to achieve very efficient lean growth due to their high ratio of lean to fat growth.

However, if these lean genotypes will also respond differently to environments which limit feed intake. Because of their already low feed intakes under ideal conditions and the fact that they are gaining a higher proportion lean, these genotypes will respond to intake limiting environments with larger absolute and percentage drops in liveweight and lean growth.

In the future, U.S. pork processors will probably pay premiums on 250-270 lb. pigs that produce heavier lean cuts. It is very difficult to produce efficiently lean 270 lb. barrows unless the genotype maintains a high lean growth rate to heavier weights have higher growth rates and continue to deposit a high ratio of lean to fat (1.6-2.0:1). Thus, to produce lean pork efficiently, commercial producers should identify high lean growth genotypes. The pigs must also have adequate feed intakes under commercial conditions in order to achieve a high percentage of their genetic potential for lean growth. Such pigs must also be able to maintain high lean growth rates to heavier weights, resulting in improved lean efficiency from 220 lb. to market weight.

Effects of High Health Status

Pigs of improved health status have higher growth rates, feed intakes and lean growth rates. The changes observed in carcass composition and backfat thickness will be highly dependent upon the characteristics of the genotype. Pigs with high lean growth potential and primarily gaining lean (low lipid:protein accretion as the result of a steep slope of lean growth on energy intake) will respond to improved health status and increased feed intakes with drastically increased lean growth rates. The pigs will become substantially more feed efficient (i.e., 3.2 to 2.6). Backfat depths will be similar. Loin eye areas will be approximately .5 square inches larger. Pigs with a low-medium lean growth potential and average percent lean will respond differently to improved health status. Feed intake will increase by 20-30 percent. The pigs, especially barrows, will have too high feed intake after approximately 200 lbs. liveweight. As lean growth declines, feed intake becomes excessive, above that needed for the pigs maximum genetic potential for lean growth. For this reason, fat accretion rapidly increases and backfat depth rapidly increases. When low to medium lean growth genotypes have grown 1.9-2.0 lb. per day, backfat thickness has increased from .8-1.0 inches at 220 lbs. to 1.1-1.4 inches at 250 lbs. liveweight.

Based on concepts of pig growth, lean genotypes with high lean growth potentials and moderate feed intakes will respond most favorably to improved health status and environmental conditions. The genotypes best suited to high health status, high level management conditions are low-moderate in feed intake.

Example Genotypes

A number of genotypes were evaluated in the 1993 lean growth trial. The pigs were reared under near optimal conditions (18 ft2 per pig). In the 1993 lean growth trial, genotypes 7 and 1 had moderate feed intakes and high lean growth rates (Tables 2, 3 and 4). Line 8 had high lean growth, low feed intake and had a high ratio of lean gain to fat gain. Under average environmental-health status conditions, with 20-30% lower feed intakes, genotype 8 pigs would grow slowly due to limited feed intakes. Line 8 pigs are consuming just enough under ideal conditions to achieve high lean growth rates and subsequently are very efficient in converting feed to lean. For this reason, line 8 would only be recommended in the best managed, high health status production units. When reared under good conditions, high lean growth genotypes offered a substantial increase in profitability (Table 5).

Table 2. Means for growth and performance traits (1993 trial).

			ADG, (lb/day)		Feed Intake, (lb/day)		Feed Conversion
Genotype	Sex	N	Mean	SE	Mean	SE	Mean	SE
7	F	32	2.00	.03	5.15	.06	2.50	.05
8	F	32	1.88	.03	4.59	.06	2.46	.06
9C	F	16	1.98	.06	5.57	.09	2.82	.07
1	B	28	2.23	.04	5.49	.07	2.47	.06
2	B	16	2.07	.05	5.90	.09	2.82	.08
7	B	32	2.11	.03	5.78	.06	2.78	.05
8	B	32	2.04	.03	4.90	.06	2.41	.05
9C	B	16	2.03	.05	5.73	.09	2.84	.07

Table 3. Means for fat standardized lean gain*, carcass fat gain, and lean feed conversion (1993 trial).

* Lean gain is standardized to contain 10% fat in the dissected lean.

Table 4. Means for carcass measurements (1993 trial).

		Fat Depth, 10th Rib, in		Loin Eye Area, in2		Backfat Last Rib, in
Genotype	Sex	Mean	SE	Mean	SE	Mean	SE
7	F	.79	.04	6.79	.14	.93	.04
8	F	.85	.05	6.30	.15	.85	.04
9C	F	1.11	.06	5.99	.20	1.06	.06
1	B	.89	.05	6.43	.16	1.01	.05
2	B	1.44	.07	5.08	.22	1.45	.06
7	B	1.04	.04	5.99	.14	1.13	.04
8	B	.98	.04	5.95	.14	.97	.04
9C	B	1.27	.06	5.49	.20	1.20	.06

Table 5. Economic returns above feed costs for the three highest and four lowest genotypes with either liveweight or carcass value marketing.

					$ Economic Return *
Lean Growth Genotype	Average Daily Gain	Feed Intake (lb/day)	Lean Gain (lb/day)	Fat Gain (lb/day)	Live Market	Carcass Value
High	2.17	5.36	.817	.610	.666	.760
Low	2.02	5.74	.654	.721	.567	.550

* Daily return of either liveweight gain ($48.00/cwt) or carcass value (lean at $124.00/cwt, fat at $20/cwt). Feed cost is $7.00/cwt.

To evaluate genetic by health status interactions, a trial has been intiated. Two genotypes will be evaluated, one high lean growth low-medium feed intake and one low-medium lean growth with medium to high feed intake. Each genotype will be reared under two health- management environments, SEW and 10 ft2 per pig in grow-finish versus conventional weaning and continuous flow grow-finish with 7 ft2 per pig. The pigs are expected to farrow in early April, 1996. Measures of immune system activation, growth factors and gene expression for muscle growth will be evaluated to investigate the underlying biological changes between genotypes and health status.

Future of Lean Pork Production

In the future, pork production will become more intensively managed. Improved health status and facilities will allow pigs to grow faster with higher feed intakes. Producers will also consider high lean growth genotype's feed intakes to maximize profitability per head per day.

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