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Calibration of net energy for fat by growth assay in early and late phases of growth in pigs
Growth assays are required to calibrate ingredient net energy (NE) values. This study was conducted to determine the NE for CWG fat in two phases of growth for pigs fed ad libitum. Literature estimates are too variable. A total of 2344 gilts and castrates (37.9 ± 0.9 kg, PIC genetics) were used in two 28 d assays, involving 98 pens of 23-24 pigs each. Diets involved a corn-soy (C-S) NE reference diet, and a C-S fat (4.50%) control to compare response by sex. Four diets were used to estimate fat NE by replacing corn with fat, amino acids and diluent (washed fine sand, bentonite). Fat was assumed to have 8.000, 7.000, 6,000 or 5.000 mcal NE/kg fat, with respective diets increasingly diluted to deliver equivalent NE to the C-S reference. These diets formed a standard curve with G:F ratio regressed to intersect the C-S G:F response; thereby estimating fat NE. All diets had equivalent content of soy, corn DDGS and other ingredients. Nutrients met or exceeded NRC (1998) minimums, with nutrient to calorie ratios held constant. CWG fat was chemically described: FFA, 5.9%; GE 9.395 mcal/kg; U:S ratio 1.60; 18:2n-6, 15.7%: MIU, 0.79% and peroxide. Pigs were acclimated to diets and pens were allotted by sex and weight to treatment. Both sexes responded similarly to the fat control, in both phases (sex x diet, P>0.40). G:F ratio for early growth (38-67 kg) was 0.439 and 0.478 for pigs fed C-S and Fat control diets (P<0.001). Fat titration diets averaged 0.440, 0.443, 0.450, 0.455 (± 0.003, P<0.001), which suggests that CWG fat was approximately 8.000 mcal NE/kg; lower NE estimates (7.000–5.000) were increasingly incorrect. Pigs were placed on a common diet for 12 d (4.6% fat), pens were weighed and re-allotted to the diet format. A total of 2250 gilts and castrates (78.6 ± 1.4 lbs, in 96 pens) were used for a second assay (end weight, 106.5 ± 1.2 kg). G:F ratio was 0.328 and 0.354 for pigs fed C-S and fat control diets. Productive energy use (G:F ratio) improved (linear, P<0.001) with the progressive decline in assumed fat NE, (0.325, 0.333, 0.342, 0.345 ± 0.003 respectively). Assuming the C-S reference G:F ratio and that body composition was not altered, fat NE approximated 8.000 mcal/kg. Regression estimates yielded 8.059 mcal NE/kg for 38-67 kg and 8.502 for 79-107 kg phases. Approximately 85% of GE (9.395) is productively useful. Our NE estimate is 14% higher than NRC (2012) estimates of 7.148 to 7.374 mcal NE/kg (similar U:S ratio, 18:2n-6 %).
Keywords: net energy, growth, fat