Animals

Five pairs of siblings were chosen (n= 10). Each consisted of one low birth weight (LBW) piglet weighing less than one kilogram at birth (0.912 ± 0.040 kg), and one normal birth weight (NBW) piglet (1.612 ± 0.223 kg) selected from among the siblings weighing nearest the average litter weight. The animals were selected from five litters containing more than twelve live-born piglets. All ten piglets were fed normally and remained on the farm under normal management conditions until ninety days of age. After this period, they were moved to the Metabolic Unit to a room with controlled temperature (19 to 22 °C). Each animal was housed individually in a metabolic cage with a dedicated drinking trough and feeder. A mesh allowed separation and collection of feces, and urine was collected through funnels under the cage floor.

When the NBW pigs had reached 50 kg weight, the first nitrogen and energy balance assays were done, and only involved the NBW animals. Nitrogen and energy balance assays were done of the LBW pigs when they reached 50 kg weight, and the NBW were assayed again. This approach produced data for both groups at 50 kg weight, as well as for both groups at the same age.

Experimental diet and feed management

At the beginning of the adaptation period to the experimental diet and throughout the experimental period the pigs were fed based on their metabolic weight (555 kcal DM x Kg^0.60); they also became accustomed to consuming the allotted food within one hour¹⁸. The experimental diet (Table 1) was a sorghum-soybean diet enriched with vitamins and minerals and provided the requirements recommended by the NRC for this productive stage¹⁹.

Nitrogen and energy balance

During the balance assays, water provided to the pigs was limited to 3 L of water per kilo dry matter consumed. This functioned to control water intake for total collection of urine. Each experimental period consisted of five days of adaptation. At the beginning of the experimental period the feed contained a ferric oxide marker included at a rate of 3 g/kg. Total feces collection was done every 12 hours beginning from the moment the feed was marked. On the sixth day (the first day after the experimental period), the feed was again marked with ferric oxide (3 g/kg). Feces collection ended when marked feces was excreted again. All collected feces were stored at -20 °C until processing. Urine was collected twice daily for five days. The urine collection container contained 40 ml 6M HCl to acidify the urine and prevent ammonia loss by volatilization. Each day of the experiment the collected urine was filtered through gauze and fiberglass, weighed, and a 5 % aliquot was taken and stored at -20 °C until analysis.

Laboratory analysis

Feces samples were partially dried at 55 °C and ground in a laboratory mill (Arthur H. Thomas Co., Philadelphia, PA) until passing through a 0.5 mm mesh. Established methods were used to quantify dry matter (DM) content (934.01) and crude protein (CP) (976.05) in the raw materials, experimental diets and feces²⁰. Energy content was measured with an adiabatic calorimetric pump (model 1281, Parr, Moline, IL). Urine nitrogen content was quantified following an established protocol (976.05)²⁰. The energy content of lyophilized urine was estimated according to Le Bellego²¹.

Data analysis

Daily feed intake was recorded to calculate intake of DM (g/day), nitrogen (N) (g/d) and energy (E) (Kcal/day) by multiplying feed intake by each nutrient’s diet concentration. Excretion of DM (g/day), nitrogen (g/d) and energy (Kcal/d) in feces was estimated by multiplying the amount of feces produced on a dry basis by the nutrient concentration in the feces. Excretion of nitrogen (g/d) and energy (Kcal/d) in urine was estimated by multiplying the total urine produced by nutrient concentration in the urine. Fecal digestibility of DM, nitrogen and energy was estimated using the following equation proposed by Adeola²²:

Where ATD = apparent total digestibility; NC= nutrient consumed (g/d); and NX = nutrient excreted (g/d). Nitrogen retention (g/d) and digestible and metabolizable energy (Kcal/d) were calculated by subtracting the amount of nutrients excreted in feces and urine from the amount of nutrients consumed.

Statistical analysis

Data were analyzed using a completely random block design (each sibling pair was a block) and two comparisons: N and E balances at same weight (50 kg); and N and E balances at same age (when LBW pigs weighed 50 kg)²³. Statistical analyses were run using the GLM procedure in the SAS statistical package²⁴. Significance was set at P<0.05, and a trend identified when 0.05<P<0.10.

Nitrogen balance

At the same age N intake was higher (P<0.001) in the NBW pigs (48.9 g/d) than in the LBW pigs (42.4 g/d) (Table 3). At the same weight (50 kg) N digestibility tended (P<0.10) to be higher in the NBW pigs (77.6) than in the LBW pigs (76.7) but at the same age it was significantly higher (P<0.05) in the NBW pigs (78.0) than in the LBW pigs (76.7). Nitrogen excretion in urine was 8.3 g N/d in both groups regardless of age and weight (P>0.10). At 50 kg the N retained as a percentage of feed intake (57.4 %) did not differ (P>0.10) between groups, but at the same age it was higher (P<0.01) in the NBW pigs (61.1 %) than in the LBW pigs (57.7 %). This difference remained (P<0.10) in the N retained as a percentage of absorbed N, with the NBW pigs having higher values (78.4 %) than the LBW pigs (75.2 %).

Energy balance

At the same age energy intake was higher (P<0.001) for the NBW pigs (8,289 kcal/day) than the LBW pigs (7,183 kcal/d)(Table 4). At the same weight energy digestibility was higher (P<0.05) in the NBW pigs (85.1 %) than in the LBW pigs (84.1 %), which was more significant (P<0.01) at the same age (85.4 % [NBW] vs 84.1 % [LBW]). The energy excreted in the urine was 153 kcal/day and did not differ (P>0.10) between the groups. At 50 kg, metabolizable energy (ME) was higher (P<0.05) in the NBW pigs (83.0 %) than in the LBW pigs (82.0 %). At the same age ME differed even more (P<0.01) (83.5 % [NBW] vs 82.0 % [LBW]).

Growth

Low birth weight (LBW) pigs were lighter at weaning and throughout the growth stage, the result of overall lower daily weight gain. The currently higher frequency of LBW piglets in pig production is due to selection for prolificity in females. This has generated a greater frequency of LBW piglets (<1 kg)^2-4; indeed, among all domestic species the pig exhibits the highest rate of low weight births⁷. Piglets born with low weight are characterized by a higher mortality rate, lower weight gain during lactation and consequent lower weight at weaning²⁵. Their post-weaning growth rate is also lower than siblings born with normal weight^3,25,26, which leads to lower carcass quality²⁶.

Digestibility

The lower N and E digestibility observed in the LBW piglets has been reported mainly in piglets^11,27-31. Various explanations for this phenomenon have been proposed and include lower digestive capacity due to a shorter GI tract length and area in piglets^11,28,30; compromised intestinal histological structure in piglets¹⁰; lower lactase and aminopeptidase concentrations and smaller pancreas size, which may reduce digestive enzyme secretion capacity²⁷; and lower neutral amino acid transporter concentrations²⁹. Studies in this area are fewer for growing animals, although there are reports of decreased aminopeptidase enzyme concentrations and a lower relative GI tract weight in pigs³¹, and smaller GI tract size in cattle³².

Energy and nitrogen balances

The LBW pigs exhibited lower nutrient absorption efficiency, as shown by the lower N retention and ME levels in the LBW pigs when both groups were the same age. Perhaps some enzymes and metabolites were altered in the LBW animals consequently affecting intermediate metabolism. For example, increased plasma concentrations of fructosamine and cholesterol in conjunction with low-density lipoproteins have been reported in LBW piglets, indicating a tendency towards insulin resistance in these animals²⁶. Low birth weight piglets also exhibit lower serum concentrations of serotonin and tryptophan³³, a lower protein synthesis rate³⁴, a higher protein degradation rate³⁵, and a smaller number of muscle cells^2,36-38. It is not known if these characteristics continue during post-weaning growth. All the aforementioned factors may affect metabolic use of nutrients since lean tissue (protein) deposition capacity is reduced under these conditions. This would imply that weight gain would have a higher metabolic cost, which would agree with the higher relative fat content of these animals²⁶.

Feed intake

During the balance phase at the same age, feed intake was lower in the LBW, which was a result of the 555 kcal DMS per kg^0.60 ration for both groups. Nonetheless, LBW pigs also tend to eat less throughout their lives than NBW piglets^6,36-38. In piglets, feed intake is affected by diet digestibility³⁹ and LBW animals have lower digestion capacity. Some animals have fewer ghrelin-secreting gastric cells; this hormone stimulates feed intake by generating the hunger sensation⁴⁰. All the above characteristics help explain the lower feed intake observed throughout their lives in the LBW animals.