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...

63The grass plant is comprised of leaf, pseudostem, true stem (including inflorescence) 64 and dead. These components differ in digestibility and variations in their relative 65 proportions can impact sward quality. The objective of this study was to determine the 66 change in the proportion and organic matter digestibility (OMD) of leaf, pseudostem, 67 true stem and dead components of four perennial ryegrass cultivars (two tetraploids: 68

The production performance of herbage‐fed animals is affected by herbage voluntary dry‐matter intake (VDMI) and organic matter digestibility. The objective of this study was to determine the effect of herbage mass (HM) on in vivo herbage voluntary intake and digestibility in sheep. The three HM treatments were as follows: 1,100 kg dry matter (DM)/ha (low, L), 2,300 kg DM/ha (medium, M) and 3,700 kg DM/ha (high, H). The study was a Latin square design, repeated on two occasions in 2012: 24 May to 20 July (summer) and 21 July to 5 October (autumn). Twelve Texel wether sheep (individually housed) were offered fresh cut perennial ryegrass herbage for ad libitum consumption. Using the total faecal collection method, the digestibility of dry matter, organic matter, neutral detergent fibre and acid detergent fibre was determined. In summer, L had a similar VDMI to M and both had a higher VDMI than H. In autumn, L had a higher VDMI than both M and H. For dry‐matter digestibility and organic matter digestibility, there was no significant difference between L and M, which were both higher than H. For every 1% increase in neutral detergent fibre digestibility, VDMI increased by 0.03 kg. In the zero‐grazing scenario examined, offering low (1,100 kg DM/ha) HM swards enabled animals to achieve high intakes of highly digestible herbage, which should ensure high animal production performance.