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The effect of space allocation on barrow and gilt performance^1,2,3

Haskell Agricultural Laboratory, University of Nebraska, Concord 68728

	Abstract

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Two experiments were conducted to determine the variation in response to space allocation between barrows and gilts and to examine an alternative allocation regimen for barrows and gilts. Experimental space allocations in both experiments were achieved by varying the number of pigs per pen in a fully slatted facility. In Exp. 1, barrows were given 0.58 and 0.65 m²/pig (nine and eight pigs per pen, respectively) and gilts were given 0.65 and 0.74 m²/pig (eight and seven pigs per pen, respectively). In addition, barrows at 0.58 m²/pig were fed diets formulated for barrows or diets formulated for gilts. Barrows grew 4.8% slower (P = 0.031) and ate 3.1% less feed daily (P = 0.062) at 0.58 vs. 0.65 m²/pig from 22 to 115 kg BW, with no difference in feed conversion, daily lean gain, carcass lean percent, or variation in weight within the pen at time of first pig removal to slaughter. There was no improvement in daily gain, feed intake, feed efficiency, lean gain, or carcass lean percent when gilts were given 0.74 vs. 0.65 m²/pig from 22 to 115 kg BW. There was no difference in performance between the population that consisted of barrows and gilts at 0.65 m²/pig vs. the population of barrows at 0.58 m²/pig and gilts at 0.74 m²/pig. There was no difference in performance by barrows at 0.58 m²/pig when fed either barrow or gilt diets, except for a slight increase (P = 0.078) in within-pen weight variation when the first pig was removed for slaughter for the barrows fed gilt diets. In Exp. 2, barrows and gilts were given 0.58 m²/pig or 0.74 m²/pig (18 vs. 14 pigs per pen) from weaning (mean age 17 d) to slaughter on d 168 postweaning. There were no interactions between space allocation and gender. Daily gain and feed intake were decreased by 2.8% (P = 0.037) and 2.9% (P = 0.084), respectively, with no effect on feed conversion or standardized fat-free lean daily gain for the 0.58 vs. the 0.74 m²/pig treatment, whereas total live weight gain per pen was increased 20.8% (P < 0.001). Results of Exp. 1 suggest that space allocation can be used to achieve similar growth rates between barrows and gilts, and results of Exp. 2 suggest that the response to space allocation is similar for barrows and gilts. The difference in magnitude of response to space allocation between experiments may be due in part to when the social group was formed, with a smaller difference in performance in Exp. 2 associated with a stable social group from weaning to slaughter.

Key Words: Gender • Pigs • Space Allocation

	Introduction

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One frustration for many pork producers using all-in/all-out management in growing-finishing facilities is that barrows generally grow faster than littermate gilts (Castell et al., 1994; Anderson and Pedersen, 1996). This faster rate of gain results in facilities that may have up to 50% of the pen space unoccupied for 1 to 2 wk until the slower-growing gilts achieve weights similar to barrows. In smaller facilities, this also means increased discounts at slaughter for over- and/or underweight pigs if truckload lots (180 to 200 pigs) are a delivery requirement.

Brumm and Gonyou (2001) suggested that a major response to restrictions in space is a decrease in feed intake, but no mention was made of possible interactions between space allocation and sex, although one of the known sex effects is decreased feed intake for gilts vs. barrows (Reese et al., 2000). Thus, it is possible that gilts may respond more to limitations in space allocation than barrows because of the possible additive effects on feed intake.

The following experiments were conducted to determine the response to space allocation between barrows and gilts and to examine an alternative space allocation regimen that could result in barrows and gilts attaining slaughter weight at similar times.

	Materials and Methods

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The experiments were conducted in a naturally ventilated, fully slatted facility located at the University of Nebraska’s Haskell Agricultural Laboratory, near Concord, NE, with the approval of the University of Nebraska–Lincoln Institutional Animal Care and Use Committee. The facility had fresh water, under-slat flushing for daily manure removal. Further details are available in Brumm et al. (2002).

Experiment 1
After a 75-km transport, crossbred (326 x C15, PIC Inc., Franklin, KY) barrows and gilts were given an ear tag, weighed, and assigned to pens on the basis of arrival weight outcome groups. Pen size was 2.44 x 2.13 m, with space allocation achieved by varying the number of pigs per pen. In the event of pig removal for poor performance or death, pen size was adjusted to maintain stocking density. There was one nipple drinker and one two-hole feeder per pen and six pens per experimental treatment.

The experimental treatments in a randomized complete block design with arrival weight as the blocking criteria were as follows: barrows at 0.65 m²/pig (eight pigs per pen), gilts at 0.65 m²/pig (eight pigs per pen), gilts at 0.74 m²/pig (seven pigs per pen), barrows at 0.58 m²/pig (nine pigs per pen), and barrows at 0.58 m²/pig (nine pigs per pen) fed gilt diets.

Diets were formulated with corn and soybean meal according to recommendations (Reese et al., 1995) for barrows and gilts of high lean gain potential (Table 1). Diets were switched on the week when individual pens averaged 36, 59, and 86 kg BW. All diets met or exceeded NRC (1998) recommendations for vitamin and trace mineral additions.

Experiment 2
After a 300-km transport at weaning (17 d mean age), crossbred barrows and gilts (EB x GPK348, Monsanto Choice Genetics, St. Louis, MO) were given an ear tag, weighed, and assigned to experimental treatments on the basis of BW outcome groups. Weight blocks were not used in assignment of pigs to experimental treatments (O’Quinn et al., 2001), and all pens had a similar CV for BW. Pen size was 2.44 x 4.27 m with space allocation achieved by varying the number of pigs per pen. In the event of pig removal for poor performance or pig death, pen size was not adjusted.

Experimental treatments in a 2 x 2 factorial arrangement were as follows: sex—barrow or gilt, and space—14 or 18 pigs per pen (0.74 or 0.58 m²/pig).

Each pen contained one two-hole wean-finish feeder (FarmWeld Jumbo, Teutopolis, IL) and one wean-finish cup drinker. Each pen was provided with a 1- x 1-m rubberized mat and 250-W heat lamp for supplemental zone heating for the first 4 wk after arrival. Sprinklers were used for summer heat relief, with the thermostat set to begin intermittent sprinkling anytime that the air temperature in the research facility was greater than 27°C. There were four pens per each combination of experimental treatments.

Diets were formulated with corn and soybean meal according to the lysine recommendations of the genetic supplier (Table 2). They were switched on the week individual pens achieved target weights. All diets met or exceeded NRC (1998) recommendations for vitamin and trace mineral additions.

Statistical Analyses
The pen of pigs was the experimental unit for all statistical evaluations except death loss. Statistical evaluations were conducted using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC).

In Exp. 1, the model included weight block and treatment. Means were separated using the following nonorthogonal contrasts: 1) barrows at 0.65 vs. 0.58 m²/pig fed barrow diets to determine the effect of space allocation on barrow performance, 2) gilts at 0.65 vs. 0.74 m²/pig to determine the effect of space allocation on gilt performance, 3) barrow and gilt at 0.65 m²/pig vs. barrow at 0.58 and gilt at 0.74 m²/pig to determine the effect of altered space allocation on pig performance, and 4) barrows at 0.58 m²/pig fed barrow diets vs. barrows at 0.58 m²/pig fed gilt diets to determine the effect of diet on pig performance when space is restricted. In Exp. 2, the model included sex, space, and the two-way interaction. Death loss and pig removal were evaluated by ² analyses.

	Results

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Experiment 1
Decreasing space allocation for barrows from 0.65 to 0.58 m²/pig resulted in a decrease in daily gain (P = 0.031) and daily feed (P = 0.062), and an increase in dressing percent (P = 0.009), with no effect on feed conversion, lean gain, or carcass lean percent (Table 3).

Barrows and gilts at 0.65 m²/pig were compared with barrows at 0.58 m² and gilts at 0.74 m²/pig to examine whether an alteration in space allocation between barrows and gilts in a facility affected overall pig performance and thereby facility utilization. There was no difference in performance for any trait examined.

Providing barrows at 0.58 m²/pig a diet sequence designed for the lower daily feed intake of gilts did not improve daily gain, feed conversion, daily lean gain, or carcass lean percent compared with barrows fed the recommended diet sequence. Carcass dressing percent was increased (P = 0.021) for barrows fed the barrow sequence and the coefficient of variation for within-pen weight was decreased slightly (P = 0.078).

Total gain per area was calculated based on the original pen dimensions. Thus, this measure of facility utilization includes the effect of death loss or removal. There was no effect of experimental treatments on gain per unit of pen space area.

Experiment 2
There were no interactions between space allocation and sex; therefore, the results of Exp. 2 are presented as main effects in Table 4.

The decrease in pig weight and daily gain for the first 52 d after weaning for the crowded pigs was accompanied by a nonsignificant (P = 0.142) reduction in daily feed. During the typical grower-finisher phase (d 52 to 168) and overall, there was a tendency for crowded pigs to consume less feed. There was no effect of space allocation on feed conversion.

Crowded barrows and gilts had a higher (P = 0.006) carcass standardized fat-free lean content. The higher fat-free lean for crowded pigs, when combined with the lower daily live weight gain resulted in no effect of space allocation on daily lean gain (P = 0.231).

In this experiment, barrows grew faster than gilts from d 52 to slaughter (P < 0.001), with a higher final weight (P = 0.002), daily feed intake (P = 0.001), and lower feed conversion (P = 0.017). Barrow carcasses had a lower standardized fat-free lean (P = 0.001), but because of their higher daily BW gain, they had a higher (P = 0.003) daily fat-free lean gain.

Although pen sizes were not adjusted in the event of pig death or removal for injury or poor performance, in this experiment the maximum space at slaughter for the 0.58 m²/pig treatment was 0.65 m²/pig in one barrow and one gilt pen. The three other barrow pens, and two of the other three gilt pens, had a final density of 0.61 m²/pig.

	Discussion

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Ssu et al. (1998) examined the use of dietary feather meal to decrease barrow rate of gain and improve carcass lean content as one method for altering barrow performance. Diets containing 10% and 20% feather meal reduced gain when the additions began at 36 and 86 kg BW with no effect on carcass lean content. However, the daily gain by barrows was still greater than that by gilts fed diets containing no feather meal.

In Exp. 1, providing barrows 0.58 m² and gilts 0.74 m² of space per pig resulted in similar rates of BW gain from purchase to slaughter. However, carcasses from barrows were still lower than gilts in lean content.

In Exp. 1, decreasing space for barrows by 11% (0.58 vs. 0.65 m²/pig) resulted in a 4.8% decrease in daily gain. Increasing space by 14% for gilts (0.65 vs. 0.74 m²/pig) resulted in a 3.3% increase in daily gain. In Exp. 2, a 22% decrease in space allocation (0.58 vs. 0.74 m²/pig) resulted in a 2.8% decrease in daily gain overall and 2.2% reduction during the grow-finish phase.

Although group sizes differed between the experiments, there are little data supporting a possible interaction between group size and space allocation for groups of 20 or fewer pigs (Randolph et al., 1981; Gonyou and Stricklin, 1998). However, McGlone and Newby (1994) suggested that as group size increased from 20 to 40, the space needs per pig decreased slightly.

The smaller reduction in daily gain that was due to crowding in Exp. 2 vs. Exp. 1 supports the hypothesis of Brumm et al. (2001) that the response to space allocation differs depending on when the social group (pen) is formed. In their experiments, a 28% decrease in space (0.56 vs. 0.74 m²/pig) during the grow-finish phase resulted in a 6.9% decrease in daily gain when the social group was created when the pigs were moved from nursery to the grow-finish facility. However, if the social group was maintained (pen identity preserved) at the move, a 19% decrease in space (0.60 vs. 0.74 m²/pig) resulted in no effect on grow-finish daily gain.

In Exp. 1, the social group was formed following transport at the end of the nursery phase. For these pigs, this was the second social group formation following weaning. In Exp. 2, the social group was formed at weaning and remained until slaughter.

These results also agree with those of Hamilton et al. (2003a), who reported no effect of gender on the response to space allocation. These authors reported that barrows had greater variation in weight (CV) than gilts at time of slaughter (8.2% vs. 6.5%). In Exp. 2, while not significant (P = 0.153), barrows had a 9.1% CV in final weight within a pen vs. 8.3% for gilts.

Unlike the results of Exp. 2, Hamilton et al. (2003b) reported a sex x space allocation interaction for last-rib backfat depth. They reported that gilts were leaner in a crowded environment and barrows were leaner in an uncrowded environment. In Exp. 1, there was no effect of space allocation on carcass lean percent within gender.

Hamilton et al. (2003b) further speculated that higher dressing percents for pigs in crowded environments can be anticipated because of lower gut fill due to the reduction in daily feed intake. Results of Exp. 1 support this conclusion. The lack of response in Exp. 1 by crowded barrows to diets higher in lysine and other essential amino acids agrees with the results of Brumm and Miller (1996) and Edmonds et al. (1998), who reported no response to elevated lysine, energy, or amino acids by crowded pigs that had a decrease in daily feed intake. More recently, Ferguson et al. (2001) have proposed a decrease in protein retention as an explanation for why crowded pigs do not respond to diets with increased amounts of limiting nutrients. However, in Exp. 1, there was no difference in daily lean gain (an estimate of protein retention rate) between crowded barrows fed the barrow or gilt diets.

The relationship between space allowance (A) and BW can be expressed as A = k x BW^0.67, where k = an empirical coefficient (Petherick, 1983). Brumm and Gonyou (2001) concluded that maximum growth will occur at k-values between 0.035 and 0.040. Reduction of space to that determined by k = 0.030 will result in a 5% decrease in growth. At 0.74 m²/pig, space was predicted to be adequate until 97 kg BW. At 0.65 m², space was adequate until 80 kg BW, and it would become limiting at 67 kg, when space was 0.58 m²/pig.

For the barrows in Exp. 1, at 80 kg BW, the corresponding space restriction of 0.58 m² vs. the adequate 0.65 m²/pig resulted in a k-value of 0.031. In this experiment, daily gain for barrows was decreased 4.8%, very close to the predicted decline. However, in Exp. 2, 0.74 m²/pig was predicted to be adequate to 97 kg. Nonetheless, daily gain during the grow-finish phase was only decreased 2.2% for the crowded pigs. This supports the hypothesis that knowing when the social group is formed is necessary to adequately predict the effects of space allocation on daily gain.

The reduction in daily gain in Exp. 2 during the first 52 d for the crowded pigs is most likely a result of the increased number of pigs per pen (and thus pigs per feeder and drinker) and not a result of space allocation (Kornegay and Notter, 1984).

	Implications

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Results of Exp. 1 suggest that space allocation can be used to modify the growth rate of barrows so it is similar to that of gilts. This has implications for facility utilization and the marketing of pigs from all-in/all-out production facilities. Results of Exp. 2 support the conclusion that the response to space allocation is similar for barrows and gilts. Results from both experiments suggest that the response to space allocation may depend on when the social group is formed. This concept merits further investigation in order to better predict the response to space allocation in a variety of management systems.

	Footnotes

 
¹ A contribution of the University of Nebraska Agric. Res. Div., Lincoln 68583. Journal Series No. 14208.

² The author acknowledges D. Forsberg for animal care and J. Dahlquist and S. Colgan for data collection and statistical analyses.

³ Supported in part by grants from PIC Inc., Franklin, KY, and Monsanto Choice Genetics, St. Louis, MO.

⁴ Correspondence: 57905 866 Road (phone: 402-584-2816; fax: 402-584-2859; e-mail: mbrumm1{at}unl.edu).

Received for publication November 20, 2003. Accepted for publication April 16, 2004.

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