The effect of crude protein and metabolizable energy levels on the efficiency of nitrogen retention and energy utilization by the weanling pig

A metabolism trial was conducted with young pigs to evaluate the effect of crude protein and metabolizable energy (ME) levels on the efficiency of protein and energy utilization. In each of three replicates, six littermate barrows, with weights averaging 14.6, 18.4 and 27.1 kg, respectively, were individually fed one of six diets. The diets, representing a factorial arrangement of two levels of crude protein (150 and 180 g/kg) and three levels of energy (3000, 3300 and 3600 kcal ME/kg), were formulated to maintain a constant ratio of fat calories to carbohydrate calories across all treatment effects. Each replicate, randomly assigned to one of six identical metabolism cages, was subjected to a 5-day preliminary period and a succeeding 5-day period of collection of urine and feces.

The intake of feed and gross energy (GE) was equal across all energy treatments. Pigs fed higher levels of ME retained more GE than did pigs fed lower levels of ME. Nitrogen (N) intake, excretion and balance were equal among all energy subcells, as was the percent of ingested N that was retained. Nitrogen retention efficiency (NRE, g N retained per Meal GE metabolized) decreased inversely with ME content of the diet. The energetic efficiency of ME for fat deposition was greatest when the magnitude of the calorie:protein ratio was highest. Feed and GE intake were not affected by the protein content of the diet. The fractional amount of GE digested and metabolized did not differ between the two protein levels. The amount of GE retained increased within each protein level as the caloric density of the diets increased. The amount of N consumed, excreted and retained was less in pigs fed the 15 percent protein diets. The fractional amount of ingested N that was retained did not differ between protein levels. Pigs fed the 18 percent protein diets had higher NRE values than did pigs fed the 15 percent protein diets.

Nitrogen retention efficiency was maximized when the calorie:protein ratio was minimized, such that NRE was greatest when pigs were fed diets containing 18 percent protein and 3000 kcal ME. Pigs fed treatments differing in the amount of energy and protein per kg of diet, but still made-up to contain equivalent calorie:protein ratios, had equal measures of NRE.

Amounts of feed, GE intake, GE retained (metabolized energy), N intake, N excretion and N balance increased with increasing pig liveweight. Pigs weighing 14.6 kg, on a percentage basis, retained more ingested N than did pigs weighing 18.4 or 27.1 kg. Both the fractional amount of GE which was consumed and metabolized and the values for NRE were not affected by pig weight.