Interpretive Summary 67 DAIRY COW DIGESTIBILITY 68 3Beecher 69 70This study aimed to measure if differences existed among dairy cow genotypes in 71 gastrointestinal tract size, digestibility and selected rumen microbial populations. Jersey and 72JerseyHolstein-Friesian cows had proportionally larger gastrointestinal tract weight than 73Holstein-Friesian cows. Jersey cows had a superior total tract digestibility and lower relative 74 abundance of Ruminococcus flavefaciens in the rumen than Holstein-Friesian cows. These 75 differences could contribute to the production efficiency differences among genotypes 76 previously reported. 77The superior milk production efficiency of Jersey (JE) and JerseyHolstein-Friesian 80 (JEHF) cows compared with Holstein-Friesian (HF) has been widely published. The 81 biological differences among dairy cow genotypes, which could contribute to the milk 82 production efficiency differences, have not however been as widely studied. A series of 83 component studies were conducted using cows sourced from a longer-term genotype 84 comparison study (JE, JEHF and HF). The objectives were to: (i) determine if differences 85 exist among genotypes regarding gastrointestinal tract (GIT) weight, (ii) assess and quantify 86 if the genotypes tested differ in their ability to digest perennial ryegrass, and (iii) examine the 87 relative abundance of specific rumen microbial populations potentially relating to feed 88 digestibility. Over 3 yr the GIT weight was obtained from 33 HF, 35 JE and 27 JEHF non-89 lactating cows post-slaughter. During the dry period the cows were offered a perennial 90 ryegrass silage diet at maintenance level. The unadjusted GIT weight was heavier for the HF 91 than for the JE and JEHF. When expressed as a proportion of bodyweight (BW) the JE and 92 JEHF had a heavier GIT weight than the HF. In vivo digestibility was evaluated on 16 each 93 of JE, JEHF and HF lactating dairy cows. Cows were individually stalled allowing for the 94 total collection of feces and were offered freshly cut grass twice daily. During this time daily 95 milk yield, BW and dry matter intake (DMI) were greater for HF and JEHF than for JE. 96Milk fat and protein concentration ranked oppositely. Daily milk solids yield did not differ 97 among the 3 genotypes. Intake capacity, expressed as DMI/BW, tended to be different among 98 treatments, with the JE having the greatest DMI/BW, the HF the lowest and the JEHF cows 99 were intermediate. Production efficiency, expressed as milk solids/DMI, was higher for the JE 100 than HF and JEHF. Digestive efficiency, expressed as digestibility of dry matter, organic 101 matter, N, neutral detergent fibre and acid detergent fibre, was higher for JE than HF. In 102 grazing cows (n=15 per genotype) samples of rumen fluid, collected using a transesophageal 103 5 sampling device, were analyzed to determine the relative abundance of rumen microbial 104 populations of cellulolytic bacteria, protozoa and fungi. These are critically important for 105 fermentation of...

An experiment was undertaken to investigate the effect of white clover inclusion in grass swards (GWc) compared with grass-only (GO) swards receiving high nitrogen fertilization and subjected to frequent and tight grazing on herbage and dairy cow productivity and enteric methane (CH4) emissions. Thirty cows were allocated to graze either a GO or GWc sward (n=15) from April 17 to October 31, 2011. Fresh herbage [16 kg of dry matter (DM)/cow] and 1 kg of concentrate/cow were offered daily. Herbage DM intake (DMI) was estimated on 3 occasions (May, July, and September) during which 17 kg of DM/cow per day was offered (and concentrate supplementation was withdrawn). In September, an additional 5 cows were added to each sward treatment (n=20) and individual CH4 emissions were estimated using the sulfur hexafluoride (SF6) technique. Annual clover proportion (± SE) in the GWc swards was 0.20 ± 0.011. Swards had similar pregrazing herbage mass (1,800 ± 96 kg of DM/ha) and herbage production (13,110 ± 80 kg of DM/ha). The GWc swards tended to have lower DM and NDF contents but greater CP content than GO swards, but only significant differences were observed in the last part of the grazing season. Cows had similar milk and milk solids yields (19.4 ± 0.59 and 1.49 ± 0.049 kg/d, respectively) and similar milk composition. Cows also had similar DMI in the 3 measurement periods (16.0 ± 0.70 kg DM/cow per d). Similar sward and animal performance was observed during the CH4 estimation period, but GWc swards had 7.4% less NDF than GO swards. Cows had similar daily and per-unit-of-output CH4 emissions (357.1 ± 13.6g of CH4/cow per day, 26.3 ± 1.14 g of CH4/kg of milk, and 312.3 ± 11.5 g of CH4/kg of milk solids) but cows grazing GWc swards had 11.9% lower CH4 emissions per unit of feed intake than cows grazing GO swards due to the numerically lower CH4 per cow per day and a tendency for the GWc cows to have greater DMI compared with the GO cows. As a conclusion, under the conditions of this study, sward clover content in the GWc swards was not sufficient to improve overall sward herbage production and quality, or dairy cow productivity. Although GWc cows had a tendency to consume more and emitted less CH4 per unit of feed intake than GO cows, no difference was observed in daily or per-unit-of-output CH4 emissions.

In grass–legume swards, biologically fixed nitrogen (N) from the legume can support the N requirements of the grass, but legume N fixation is suppressed by additional fertilizer N application. This study sought to identify a fertilizer N application rate that maximizes herbage and N yields, N fixation and apparent N transfer from white clover to companion grasses under intensive grazing at a site with high soil‐N status. During a 3‐year period (2011–2013), swards of perennial ryegrass and of perennial ryegrass–white clover, receiving up to 240 kg N ha−1 year−1, were compared using isotope dilution and N‐difference methods. The presence of white clover increased herbage and N yields by 12–44% and 26–72%, respectively. Applications of N fertilizer reduced sward white clover content, but the effect was less at below 120 kg N ha−1. The proportion of N derived from the atmospheric N fixation was 25–70%. Nitrogen fixation ranged from 25 to 142 kg N ha−1 measured using the isotope dilution method in 2012 and from 52 to 291 kg N ha−1 using the N‐difference method across all years. Fertilizer N application reduced the percentage and yield of fixed N. Transfer of N from white clover to grass was not confirmed, but there was an increased N content in grass and soil‐N levels. Under intensive grazing, the maximum applied N rate that optimized herbage and N yields with minimal effect on white clover content and fixation rates was 60–120 kg N ha−1.