Rumen digesta and products of fermentation in cows fed varying proportions of fodder beet (Beta vulgaris) with fresh pasture or silage or straw

Fodder beet (FB) is gaining popularity as a feed for dairy cows in autumn (late lactation) and during winter (dry; non-lactating) in the New Zealand seasonal grazing systems. High yields, high digestibility, low nitrogen (N) content in the dry matter (DM) and easy feeding systems (in situ with controlled access by electric fence) have contributed to increasing usage. However, reports of acidosis have required research to understand appropriate feeding regimens to ensure cows fed FB remain healthy. Therefore, the focus of research presented here is to understand aspects of rumen function when FB is fed in conditions representative of practical feeding systems. Two indoor experiments were undertaken; one with Holstein/Friesian lactating cows fed fresh perennial ryegrass with three proportions of FB (0, 0.23 and 0.45) and another with dry (non-lactating) cows fed pasture silage and 0.65 FB and barley straw with 0.86 FB. Lactating cows were fed at about 0800 and 1530 h daily and dry cows at 0800h, in accordance with normal farm practice. Measurements were part of trials addressing nitrogen dynamics, and include rumen pH, short-chain fatty acid (SCFA) and ammonia concentrations determined at 2h intervals over 24h, as well as intakes, estimates of microbial growth and rumen dynamics, including particle size distribution. Inclusion of 0, 0.23 and 0.45 FB with fresh pasture did not affect daily DM intakes (~14.6 kg), milk yield (~10.7 kg), microbial synthesis (129 g of N/d) or fractional outflow rates of digesta (0.16/h; 11.2 L/h). The diet comprising FB with straw, advocated by some advisors, was inappropriate and resulted in low intakes, insufficient dietary N and microbial growth was about one third of that in cows fed pasture silage with 0.65 FB. The minimum ruminal pH (averaged for cows within treatments) was 5.6-5.9 in all treatments where FB was fed, and was observed 7-8 h after feed was first offered each day. Rumen ammonia concentrations averaged 4.4 mmol/L in cows fed pasture but was sometimes undetectable in lactating cows fed 0.45 FB and in dry cows, and the amount of FB in the diet corresponded with the circadian pattern of changes in molar proportions of SCFA. Contrary to expectations there was a greater proportion of large particulate DM (unable to pass a sieve with a 2 mm aperture) in cows fed diets with FB, than pasture alone, and we suggest chewing may be an important factor contributing to acidosis in some cows and not others.