Recent changes in New Zealand dairying have included incorporation of fodder beet (FB) into rations. The present trial explored the impact of substituting different proportions of ryegrass-based pasture with FB for cows fed ad libitum in late lactation. The objective was to measure effects of FB on intake, digestion and urinary nitrogen (N) excretion using 16 Holstein–Friesian cows with a permanent rumen fistula. Before the 9-day indoor measurement period, cows were adapted to feeding 0%, 20%, 40% or 60% FB (bulbs and tops) with medium quality ryegrass-based pasture (191 g CP and 517 g neutral detergent fibre (aNDF)/kg DM; DM digestibility 0.634) over 2 weeks. However, it became apparent that a diet of 60% FB was detrimental to cow health, with two cows developing acidosis. The allocation of FB was reduced and the actual intakes were 23% and 45% of DM intake during the trial. Feeding FB substantially reduced N intake and urinary N excretion. The lower level (23%) of FB resulted in greater DM digestibility, albeit with a small reduction in N and aNDF digestibility. However, the higher level (45%) of FB reduced N and aNDF digestibility by 7.1 and 12.1 percentage units respectively, relative to pasture. Daily creatinine excretion was lower than published measurements from cattle fed dry diets, and circadian variation in urinary N:creatinine ratios suggests a need for caution when attempting to predict daily urinary N excretion from spot samples.

Context Fodder beet (FB) is a popular feed for dairy cows in temperate climates due to its high yields, high digestibility, low nitrogen (N) content in the dry matter (DM) and convenience of feeding (grazing in situ). However, the risk of ruminal acidosis requires research to design feeding regimes that capture these benefits without compromising animal health. Aims To understand aspects of rumen function when FB is offered in conditions representative of practical feeding in temperate pastoral systems. Methods Two indoor experiments were undertaken; one with cows in late lactation fed fresh perennial ryegrass with three proportions of FB (0, 0.23 and 0.45) and another with non-lactating cows fed pasture silage with 0.65 FB or barley straw with 0.86 FB. Measurements included rumen pH, short-chain fatty acid (SCFA) and ammonia concentrations determined at 2-h intervals, as well as daily individual cow intakes, estimates of microbial growth and rumen dynamics. Key results The inclusion of 0, 0.23 and 0.45 FB with fresh pasture in the 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) of lactating cows. The non-lactating cow ration comprising 0.86 FB with straw was inappropriate and resulted in low intakes and insufficient dietary N. Microbial growth was approximately one-third of that in cows fed pasture silage with 0.65 FB. The ruminal pH reached lower values in all treatments where FB was offered. 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 non-lactating cows. The amount of FB in the diet affected the extent of the circadian changes in molar proportions of SCFA. Conclusions Based on the results presented here, feeding fresh FB to dairy cows should not exceed ~0.4 of their DMI with pasture (late lactation), or ~0.6 of their intake with silage (non-lactating). Implications These findings could support evidence-based recommendations for FB use, considering its effects on aspects of rumen function, such as microbial protein synthesis and pH.

An over-wintering diet for dry cows comprising about 65% fodder beet with 35% pasture silage provided adequate nutrition, although there was some risk of acidosis. In contrast, the diet containing about 85% fodder beet with barley straw resulted in lower DM intakes, poor rumen function, negative N balance so that both nutrition and welfare were compromised.

Forages and forage mixtures with greater tolerance of dry conditions than perennial ryegrass (Lolium perenne L.) are desirable for dairy farming in New Zealand, and a low urinary nitrogen (N) excretion is desirable to lessen pollution of waterways and ground water, and nitrous oxide emissions. Measurements were undertaken with late-lactation Holstein-Friesian cows (5/treatment) fed tall fescue (Festuca arundinacea Schreb.) with a substantial incursion of weed grasses, as either a sole diet (Grass), or with lucerne (Medicago sativa L.) or lucerne and plantain (Plantago lancelota L.), in metabolism stalls. Approximate ratios (DM basis) of Grass with lucerne (GL) were 55:45, and Grass with lucerne and plantain (GLP) were 55:25:20. Measurements included intakes, production, digestion, rumen function, and urinary excretion, including the circadian patterns of metabolite excretion with a focus on nitrogenous aspects. The DM intakes (kg/day) of cows fed Grass, GL and GLP were 14.9, 12.7 and 15.0 (P = 0.006), and DM digestibility (%) was 58.0, 59.8 and 61.9 (P = 0.006), respectively. Milk yields (kg/day) were Grass, 9.0; GL, 8.7 and GLP, 11.7 (P = 0.003) but composition was not affected by diet. Rumen digesta weight was greatest in cows fed Grass, averaging 23.4% of liveweight after morning feeding. The microbial growth (g/kg organic matter digested) was 16.8 in cows fed Grass and ~24.0 in the other diets. Dietary crude protein concentrations (g/100 g DM) were Grass, 15.4, GL, 20.1 and GLP, 18.3 and urinary N excretion (g/day) was lowest with the Grass diet but urinary N concentration was lowest from cows fed GLP. Daily creatinine excretion was unaffected by diet but there was a 2-fold range in excretion rates within 24 h and values from cows fed fresh forages are lower than published values from cows fed dry diets. Results showed that supplementation of poor quality pasture with lucerne or lucerne with plantain had minor effects on digestibility, and measurements of urinary N suggest a need for caution when predicting urinary N excretion from spot urine sampling in grazing trials.