Degradation of methionine by rumen contents in vitro and efficiency of its protection

Degradation of methionine (M), soya isolate (S) and casein hydrolysate (C) has been evaluated by incubation of these substrates (100 mg) with sheep rumen contents in vitro, in the presence of starch (250 mg). Five incubations were performed using one adult rumen fistulated sheep fed 300 g of hay and 300 g of concentrate daily. Total NH3 production at different sampling times (after 2, 4, 6, 8 and 24 h of incubation) was calculated by subtracting the net NH3-N production observed in the incubation containing starch only from that produced in the incubation containing the substrate. Protected M preparations, Smartamine (Sm) and Mepron (Mp), were also used. During two incubations of M, using four sheep on permanent pasture or fed four different hays, it was shown that the net disappearance of free M matches net NH3 production at both low (2 mM) and high (20 mM) M concentration. The results indicate that the initial M degradation rate (3 h of incubation) was 12–14% that of C and never exceeded 1.5 mmol h−1 l−1 of rumen contents. Adaptation of the sheep rumen by daily introduction of 5 or 20 g of DL-methionine for 21 and 8 days, respectively, did not change this result (three to four incubations with rumen contents of three sheep fed 300 g of hay and 300 g of two different concentrates). The results also indicate that protection against rumen degradation was effective for Sm but not for Mp. The ‘effective degradation rate’ of M was estimated as 0.30 that of protein. Considering the high price of protected M (four to five fold the price of non protected M), and the low degradation rate of M, it is suggested that the latter be used at 1.43 of the amount normally added as rumen protected M, to obtain the same result.