A dose-response trial involving abomasal infusion of 2-hydroxy-4-(methylthio)butanoic acid (HMTBa) demonstrated linear responses for plasma methionine (Met), total sulfur amino acids (TSAA) and HMTBa. This trial was to further evaluate and identify remaining weakness in the approach by comparing abomasal infusion of HMTBa (Rhodimet AT88) vs feeding dl-2-hydroxy-4-(methylthio)butanoic acid isopropyl (HMBi) (MetaSmart), a known source of metabolizable Met. Five lactating Holstein cows were used in a 5 × 5 Latin square with 7-d periods. Treatments were negative control, 12 and 24 g/d of infused Met equivalents from HMTBa, and 24 and 48 g/d of fed Met equivalents from HMBi. Infusions were continuous except during milking. Cows were fed at 8-h intervals; HMBi treatments were divided into 3 equal portions, mixed with 1.5 kg of the basal TMR and eaten within 20 min. Blood samples were collected the last 3 d of each period at 2, 4, 6 and 8 h after the morning feeding, cooled, and centrifuged. Plasma was used for HMTBa analysis and deproteinized plasma for AA analysis. Data were analyzed using the PROC MIXED and PROC REG procedures of SAS with period and treatment by level as main effects. Increasing amounts of infused HMTBa resulted in linear increases in plasma Met, TSAA, and HMTBa, while increasing amounts of fed HMBi resulted in only a trend for linear increases in these metabolites (Table 1). Previous research indicated undetectable concentrations of HMTBa in plasma of control cows when infused with increasing amounts of HMTBa. Others have observed rapid increases in plasma HMTBa after bolus meals of HMTBa, with concentrations not always returning to baseline values by 8 h after the meal. We suggest a need for longer experimental periods and for mixing dietary treatments with the entire diet. Table 1 (Abstr. M189). Effect of infused HMTBa and fed HMBi treatments on plasma sulfur metabolites (µM)

Item	HMTBa				HMBi		SEM	Linear contrast P =
0	12	24	24	48	HMTBa	HMBi
Met	23.5	25.4	27.8		24.2	25.6	1.16	0.02	0.07
HMTBa	0.266	2.086	4.968		1.396	2.308	0.936	0.003	0.13
TSAA	99.9	105.2	108.5		103.9	107.6	2.95	0.02	0.06