Determination of microbial protein in perennial ryegrass silage

Abstract

The microbial matter fraction was determined in perennial ryegrass silages of different dry‐matter (DM) contents, ensiled with or without Lactobacillus plantarum. 15N‐Leucine and the bacterial cell wall constituent diaminopimelic acid (DAPA) were used as markers for microbial‐N. Perennial ryegrass crops with DM contents of 202, 280 or 366 g kg−1 fresh weight were ensiled in laboratory‐scale silos and stored for 3 to 4 months. At different times after ensiling, silages were analysed and microbial fractions were isolated. Microbial‐N concentration determined with 15N‐leucine reached a maximum during the first week of ensilage. It remained unchanged thereafter, except in silage with a DM content of 280 g kg−1 in which it decreased (P < 0·01) by 32% during storage. After 3 to 4 months ensilage, microbial‐N concentration varied from ≈0·3 to ≈1·7 g kg−1 DM. A negative relationship was observed between microbial‐N concentration and silage DM content. Inoculation resulted in an approximately twofold increase (P < 0·001) in microbial‐N concentration. Microbial‐N concentrations determined with DAPA were 1·14–2·07 times higher than those determined with 15N‐leucine. However, 19–35% of the DAPA in silage occurred in a soluble form, indicating that this fraction of DAPA was not associated with intact bacteria.