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In vitro gas production was measured to investigate associative effects (AE) of different mixtures of corn silage (CS), Chinese wildrye hay (CWR), and alfalfa hay (AH). The mixtures were incubated in vitro to measure gas production (GP) and microbial protein yield, in order to approach to the mechanism causing associative effects in ruminants.In trail 1, CS, CWR, and AH were mixed at proportion of 0/100, 25/75, 50/50, 75/25, 100/0, respectively. The mixtures were incubated in vitro according to Menke’s gas production technique. The GP was recorded at 0, 2, 4, 6, 9, 12, 24, 36, 48 and 72h of incubation. The associative effects were expressed as the percentage difference between the observed GP from the mixtures and calculated values from GP of substances incubated individually.When the GP at 24h of incubation was used as the evaluation index, positive associative effect existed between CS and CWR, with the greatest value (9.3%, P<0.05) at proportion of 25/75. The AE between different CS/AH combinations was -0.5, -0.1 and 0.7%, respectively (P>0.05). The AE between different CWR/AH combinations was 0.0, -0.5 and -3.1%, respectively, with the maximum AE in group of 75/25 (P＜0.05). In trail 2, the mixture of CS/CWR (25/75) was used to combine with alfalfa hay at proportion of 0/100, 25/75, 50/50, 75/25 and 100/0, respectively. Protein content increased by increasing AH, but there was no remarkable associative effect between the forages.In trail 3, 500 mg of substrates were incubated to determine microbial protein content (MP), pH, and ammonia nitrogen content of CS/CWR combination, and AE was estimated by using the index of GP, MP, and Synthetically Associative Effect (AEs) that combined the information from GP and MP, respectively. The AE of CS/CWR mixtures was 9.4, -1.1 and 2.7%, respectively, similar to the result of experiment 1, when used the GP as the evaluating index, and -16.4, -18.0 and 2.0%, respectively, by using MP. There was great difference between the two results. When the index of AEs was used, the result was 4.1, -1.2 and 1.4%, respectively, similar to that depending on the GP. The pH of the CS/CWR varied between 6.65-6.72, being in the feasible range (6.6-6.8) to microbes. This may be due to the powerful buffer ability of the gas production system.It is inferred that positive associative effect occurs between corn silage and Chinese wild ryegrass, with the maximum value when two forages were mixed at the proportion of 25/75.
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