In pig diets based on cereal grains and oilseed meals, sulphur amino acids are usually the second, third or fourth limiting amino acids (Lewis, 2001). The other limiting amino acids are lysine, threonine and tryptophan. Because of this importance, numerous experiments have investigated the sulphur amino acid requirements of pigs. A comprehensive review
g kg-1 g kg-1 g kg-1 g kg-1 dl-Met dl-HMB dl-Met dl-HMB
Fig. 8.5. Average daily gain of 4-kg pigs fed a basal diet (containing 2.3 g kg 1 methionine and 4.8 g kg 1 cystine) supplemented with dl-methionine or 2-hydroxy-4-(methylthiol)butyric acid (HMB) (Knight et al., 1998). The amounts of HMB (0.57 and 1.14 g kg 1) are equimolarto 0.5 and 1.0 g kg 1 dl-methi onine. The contrasts basal vs. (0.50 g kg 1 methionine + 0.57 g kg 1 HMB) and (0.50 g kg 1 methionine + 0.57 g kg 1 HMB) vs. (1.0 g kg 1 methionine + 1.14 g kg 1 HMB) were significant (P< 0.05); there was no difference between dl-methionine and HMB.
of sulphur amino acid requirements was published by NRC (1998). A computer model is included that enables requirements to be calculated for growing pigs with specific weights and lean growth rates or for sows with specified levels of reproductive performance.
To adjust for differences in bioavailability in amino acids among different feedstuffs, the NRC computer model uses true (or standardized) ileal digestible amino acids. However, output is also provided in terms of apparent ileal digestible amino acids and also as total amino acids. The NRC requirements (on a true ileal digestible basis) for methionine + cystine decrease from 7.6 to 3.1 g kg-1 of the diet, as pigs increase in weight from the 3- to 5-kg range up to the 80- to 120-kg range. This is an increased requirement from 1.9 to 9.5 g day-1 over the same weight ranges. For sows, methionine + cystine requirements range from 3.1 to 3.3 g kg-1 (5.7-6.4 g day-1) during gestation and from 3.5 to 4.3 g kg-1 (13.9-26.0 g day-1) during lactation, depending on the level of production.
Some additional research on sulphur amino acid requirements of pigs is available that was not included in the NRC review. Kirchgessner et al. (1994a) reported that in 7- to 30-kg pigs the optimal ratio of methion ine + cystine:lysine depended on the dietary lysine concentration and thus the growth rates of the pigs. At the highest lysine concentrations fed (11 g kg-1) the optimal methionine + cystine:lysine ratio was 56-57%. This is identical to the NRC ratio for this weight range. In another experiment, Kirchgessner et al. (1994b) found that weight gain and feed efficiency of pigs weighing 20-60 kg were maximized when the total methionine content was 3.1 g kg-1 (3.4 g kg-1 of dry matter). For pigs weighing 60-95 kg, weight gains were maximized with 2.4 g kg-1 total methionine (2.6 g kg-1 of dry matter). These values are higher than the corresponding NRC values of 2.5 and 2.0 g kg \ respectively. However the estimates of methionine + cystine requirements (4.9 and 4.1 g kg-1) were somewhat lower than the NRC requirements (5.4 and 4.4 g kg-1). Obviously, this implies a discrepancy in the proportion of methionine + cystine requirement that can be provided by cystine. This issue will be discussed more in a later section.
Knowles et al. (1998) reported that for pigs from approximately 75 to 110 kg the ratio of total sulphur amino acids:lysine required was no greater than 47% to maximize growth performance and carcass muscling, although the ratio to minimize fat deposition was 65%. In comparison, the NRC value for 80-120 kg is 58%. Loughmiller et al. (1998) found that the apparent ileal digestible methionine requirement of gilts in the late finishing stages was 1.25 g kg-1 (-3.0 g of apparent ileal digestible methionine day-1) or 25% of the ileal digestible lysine requirement. These values are all lower than the NRC requirements of 1.3 g kg-1, 4.1 g day-1, and 28% of lysine for apparent ileal digestible methionine.
Recently, Matthews et al. (2001c) determined that the sulphur amino acid requirement of pigs from 5 to 10 kg was 6.4 g kg-1 on an apparent ileal digestible basis. This is almost identical to the NRC requirement of 6.3 g kg-1.
Thus, some of these additional results suggest requirements higher than those of the NRC and some suggest lower requirements. Overall, it seems that the NRC requirements are satisfactory, although there is some indication that the requirements during late finishing stages may be lower than indicated by NRC.
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