Introduction

Both genetics and dietary improvements have contributed significantly to increased growth performance of pigs over the last 25 years. Genetic selection has increased the amount and efficiency of lean growth, which allows more nutrients to be deposited in tissue and fewer nutrients excreted in manure. Many changes have taken place in the design of diets for pigs compared to 25 years ago. These include the use of enzymes, crystalline amino acids, improved nutrient balance, improved feed processing, e.g. pelleting, and more sophisticated phase feeding programmes.

The objectives of the study study were to compare the effects of genotype and diet on nutrient digestion and retention, ammonia emission and odour in manure of growing-finishing pigs.

Materials and Methods

First parity white line females were obtained from an unselected commercial population formed in 1980 that has been maintained at NCSU since 1989. These sows were mated using frozen semen of Hampshire or Duroc boars that were typical of those used in 1980. Pigs representative of 2005 genotype of similar age were obtained from a North Carolina swine production company.

All pigs were reared at the North Carolina Swine Evaluation Station in Clayton. After farrowing, 28 piglets (14 pigs of the 1980 genotype and 14 pigs of the 2005 genotype) were selected and allotted to a 2 × 2 factorial randomised complete block design. Factors included: 1) genotype representative of 1980 and 2005, and 2) feeding programme representative of 1980 and 2005. The characteristics of the feeding programmes are shown in Table 1.

The nutrient levels for the four-phase 1980 feeding programme (e.g. lysine from 1.05 to 0.62 per cent and ME from 3,262 to 3,317 kcal/kg) were based on formulations from the 1978 Pork Industry Handbook (PIH). The 2005 feeding programme consisted of a seven-phase programme using pelleted diets similar to diets used by North Carolina pork producers, e.g. lysine from 1.51 to 0.73 per cent and ME from 3,428 to 3,651 kcal/kg.

Pigs (n=28) were transferred to metabolism crates once they reached a body weight of approximately 65 kg. The 1980 diet (corn-soybean meal, meal form) and the 2005 diet (pelleted, supplemented with amino acids and phytase) fed during the metabolism period contained, respectively 13.3 versus 14.7 per cent crude protein, 3,317 versus 3,655 kcal/kg ME, 0.67 versus 0.43 per cent calcium, 0.56 versus 0.41 per cent phosphorus, and 0.62 versus 0.94 per cent total lysine. After an adaptation period of seven days, faeces and urine were collected quantitatively for three days. A portion of the faeces and urine was then mixed together and homogenised within respective animal at the rates they were produced to form fresh manure. Half of this fresh manure was aged anaerobically for 21 days to create aged manure.

Both fresh and aged manure were sampled for odour evaluation by a professional odour panel. The panelists were asked to smell each sample individually and assign a designation of degree of pleasantness or unpleasantness according to a -10 to +10 hedonic tone scale, with 0 being neutral. They also were asked to assign a score for strength of odour by smelling the sample and comparing it to standards of n-butanol at increasing concentrations to generate a 1 to 5 scale (very faint, faint, moderate, strong and very strong).

Ammonia emission of the manure samples was determined by placing 400ml of the manure mixture in a rectangular container. Air was drawn through the container with manure, and then through a gas dispersion tube placed in a 500ml Erlenmeyer flask containing dilute sulphuric acid in order to trap the ammonia released from the manure. This sulphuric acid solution was sampled at 12, 24, 36, 48, 72 and 96 hours and analysed for ammonia.

Results

Pig average daily gain was greater for the modern genetics and feeding programme, resulting in heavier body weight when the metabolism study was initiated (69.9 versus 63.5 kg, P=0.04; and 73.1 versus 60.3 kg, P<0.001, respectively).

To account for the differences in body weight, all other data were expressed per unit of metabolic body weight, which eliminates bias due to weight difference of pigs.

Intake of nitrogen tended (P=0.07) to be greater (1.15 versus 1.02 g/BW^-0.75/day) and intake of phosphorus was lower (P<0.001; 0.20 versus 0.26 g/BW^-0.75/day) for the 2005 diet compared to the 1980 diet (Table 2).

Faeces (15.1 versus 20.0 g/BW^-0.75/day) and urine (37.7 versus 48.3 g/BW^-0.75/day) production were lower (P<0.01), faecal nitrogen excretion (0.168 versus 0.196 g/BW^-0.75/day) tended to be lower (P=0.08) and faecal phosphorus excretion was lower (P<0.001; 0.117 versus 0.162 g/BW^-0.75/day) for the 2005 diet when compared to the 1980 diet.

Digestibility of nitrogen (85.3 versus 80.3 per cent) and gross energy (88.9 versus 85.3 per cent) was greater (P<0.05) for the 2005 diet compared to the 1980 diet (Table 2).

Urinary N excretion was greater in 1980 pigs when fed 2005 diets compared to 1980 diets (0.35 versus 0.16 g/BW^-0.75/day) but no differences between diets were observed in 2005 genetic pigs.

Cumulative ammonia emission in fresh manure was greater (P=0.05) for 2005 pigs at 24 hours but not at any of the other time points.

In aged manure (Figure 1), ammonia emission was 86, 52, 29, 18 and 12 per cent greater (P<0.05) for the 2005 diet at 12, 24, 36, 48 and 96 hours, respectively. No differences in manure odour were observed.

Figure 1. Effect of genotype (1980 versus 2005) and nutrition programme (1980 versus 2005) on in vitro cumulative ammonia emission manure aged for 21 days

Conclusion

Consistent with improvements in growth rate in the present study, the researchers have previously demonstrated a 15 per cent reduction in days to slaughter and a 30 per cent improvement in feed efficiency due to improvements in genetics and nutrition programmes.

These positive impacts on pork production can be calculated to result in a reduction in nutrient excretion of approximately 23 per cent.

In addition, the researchers observed improved nutrient utilisation for the 2005 diet, which will further reduce manure output and nutrient excretion. However, they also demonstrated that modern feeding programmes may increase ammonia emission when measured in vitro at one point in time.

November 2009