Characterizing microbial dynamics in continuous cultures and lactation performance of cows fed gamagrass

by 1965- Eun, Jong-Su

EUN, JONG-SU. Characterizing microbial dynamics in continuous cultures and lactation performance of cows fed gamagrass. (Under the direction of Vivek Fellner and Jerry W. Spears) Three experiments were performed to investigate the dynamics of fermentation in continuous cultures and lactation performance of cows fed gamagrass. We first studied the effects of dilution rate and forage to concentrate ratio on fermentation by rumen microbes cultured in fermentors. Methane production, when calculated from stoichiometric equations, did not change with dilution rate or forage to concentrate ratio. When measured from gas taken from fermentor headspace, methane production increased with dilution rate and level of forage in the diet. Compared to actual concentrations, stoichiometric equations consistently underestimated methane output at higher dilution rates and with high forage diets. Higher dilution rates resulted in an increase in microbial yield and microbial efficiency. Increasing the level of concentrate in the diet only increased microbial efficiency. Overall results show that dilution rate and forage to concentrate ratio can alter the partitioning of substrate by rumen microbes. Further that gas production, in particular methane, may not be accurately estimated using stoichiometrics of end product appearance. Twenty lactating Holstein cows were fed gamagrass hay or silage without or with supplemental corn to determine their effects on milk production. Milk yield did not differ among cows fed gamagrass hay or gamagrass silage. Feeding supplemental corn increased milk yield but only at the medium and high levels of corn inclusion. Gamagrass silage increased the conversion of feed nitrogen to milk nitrogen compared to gamagrass hay. Contrary to expectation, gamagrass silage lowered milk urea nitrogen compared to hay. Supplemental corn further reduced milk urea nitrogen. In the last experiment, we investigated the characteristics of microbial fermentation of gamagrass. Increasing corn supplementation in gamagrass silage linearly decreased culture pH whereas ammonia nitrogen was similar across treatments. Corn supplementation to gamagrass silage was an effective strategy to increase microbial capture of rumen degradable protein from gamagrass via enhanced availability of ruminal fermentable energy. However, gamagrass silage without corn supplementation resulted in higher efficiency of microbial growth but at the expense of microbial yield.