Comparison of the in vitro gas production technique and the nylon bag degradability technique to measure short- and long-term processes of degradation of maize silage in dairy cows

Short-term (2–8 h) and long-term (8–48 h) processes of degradation of samples of 10 different maize silages were compared using two different techniques: (i) Hohenheimer in vitro gas production technique (GP) and (ii) the nylon bag degradability technique. Measurements of short-term degradability were complemented with determination of non-degraded dry matter loss from nylon bags. The objectives of the work were to (i) study the influence of sample processing on degradability and (ii) compare the nylon bag and the gas production techniques, for determining the nutritive value of maize silage. The maize silage samples for in situ degradability were in the form of either fresh approximately 5 mm chop length (F), F dried (FD), FD ground to pass through a 3 mm screen (FD3mm) and for in vitro gas production, FD ground to pass either a 1 or a 3 mm screens (FD3mm and FD1mm). Incubations were carried out for 2, 4, 8, 24 and 48 h (with a supplementary measurement at 16 h in situ). Non-degraded dry matter loss from nylon bags was higher for FD3mm than for FD or F (240.9 > 179.1, 156.3 g/kg respectively, P < 0.05). The effective DM degradability (DMDEf) was 17% higher for FD3mm than for FD or F (524.6 > 448.1, 449.6 g/kg respectively; P < 0.05). Grinding of maize silage increased the soluble fraction. However, it did not decrease the undegradable fraction of maize silage. In vitro gas production after 48 h was lower for FD3mm compared with FD1mm (38.23 versus 39.68 ml; P < 0.01). In a Pearson matrix correlation which included all incubation periods, all maize samples, DM disappearance, non-degraded loss of DM, and rate of gas production, (39 parameters in total yielding 416 correlation coefficients), only 63 (15%) of the correlations were significant (P < 0.05; r > 0.63). Correlations of DM disappearance of FD samples during the first 8 h of the incubation with gas production of FD3mm samples for the same incubation intervals gave the best correlations (r = 0.70 for values at 2 h, 0.75 for values at 4 h, 0.67 for values at 8 h). Processes of degradation were more correlated with parameters of the short term than the long term (P < 0.05). The nylon bag technique and Hohenheimer technique did not describe the degradation of maize silage in the same manner. This may be explained by loss of non degraded DM which occurred only with the nylon bag and which was not implicated in the GP technique. Thus, dry matter degradability and GP did not give the same predictions of nutritive value.