Dairy cattle farming in temperate regions often relies on grass herbage (GH)-based diets but the effect of several grass management options on enteric CH4 emission has not been fully investigated yet. We investigated the combined effect of N fertilization rate and length of regrowth period of GH (predominantly ryegrass) on CH4 emission from lactating dairy cows. In a randomized block design, 28 lactating Holstein-Friesian dairy cows received a basal diet of GH and compound feed [85:15; dry matter (DM) basis]. Treatments consisted of GH cut after 3 or 5 weeks of regrowth, after receiving either a low (20kg of N/ha) or a high (90kg of N/ha) fertilization rate after initial cut. Feed intake, digestibility, milk production and composition, N and energy balance, and CH4 emission were measured during a 5-d period in climate respiration chambers after an adaptation to the diet for 12d. Cows were restricted-fed during measurements and mean DM intake was 15.0±0.16kg/d. Herbage crude protein content varied between 76 and 161g/kg of DM, and sugar content between 186 and 303g/kg of DM. Fat- and protein-corrected milk (FPCM) and feed digestibility increased with increased N fertilization rates and a shorter regrowth interval. Increasing the N fertilization rate increased daily CH4 emission per cow (+10%) and per unit of DM intake (+9%), tended to increase the fraction of gross energy intake emitted as CH4 (+7%), and (partly because of the low crude protein content for the low fertilized GH) only numerically reduced CH4 per unit of FPCM. The longer regrowth interval increased CH4 emission per unit of FPCM (+14%) compared with the shorter regrowth interval, but did not affect CH4 emission expressed in any other unit. With increasing N fertilization CH4 emission decreased per unit of digestible neutral detergent fiber intake (-13%) but not per unit of digestible organic matter intake. There was no interaction of the effect of N fertilization rate and regrowth interval on CH4 emission, but effects of N fertilization were generally most distinct with GH of 5 wk regrowth. The present results suggest that altering grass quality through an increase of N fertilization and a shorter regrowth interval can reduce CH4 emission in zero-grazing dairy cows, depending on the unit in which it is expressed. The larger amount of CH4 produced per day and cow with the more intensively managed GH is compensated by a higher feed digestibility and FPCM yield.

Grass silage is typically fed to dairy cows in temperate regions. However, in vivo information on methane (CH 4 ) emission from grass silage of varying quality is limited. We evaluated the effect of two rates of nitrogen (N) fertilisation of grassland (low fertilisation (LF), 65 kg of N/ha; and high fertilisation (HF), 150 kg of N/ha) and of three stages of maturity of grass at cutting: early maturity (EM; 28 days of regrowth), mid maturity (MM; 41 days of regrowth) and late maturity (LM; 62 days of regrowth) on CH 4 production by lactating dairy cows. In a randomised block design, 54 lactating Holstein-Friesian dairy cows (168 ± 11 days in milk; mean ± standard error of mean) received grass silage (mainly ryegrass) and compound feed at 80 : 20 on dry matter basis. Cows were adapted to the diet for 12 days and CH 4 production was measured in climate respiration chambers for 5 days. Dry matter intake (DMI; 14.9 ± 0.56 kg/day) decreased with increasing N fertilisation and grass maturity. Production of fat-and proteincorrected milk (FPCM; 24.0 ± 1.57 kg/day) decreased with advancing grass maturity but was not affected by N fertilisation. Apparent total-tract feed digestibility decreased with advancing grass maturity but was unaffected by N fertilisation except for an increase and decrease in N and fat digestibility with increasing N fertilisation, respectively. Total CH 4 production per cow (347 ± 13.6 g/day) decreased with increasing N fertilisation by 4% and grass maturity by 6%. The smaller CH 4 production with advancing grass maturity was offset by a smaller FPCM and lower feed digestibility. As a result, with advancing grass maturity CH 4 emission intensity increased per units of FPCM (15.0 ± 1.00 g CH 4 /kg) by 31% and digestible organic matter intake (33.1 ± 0.78 g CH 4 /kg) by 15%. In addition, emission intensity increased per units of DMI (23.5 ± 0.43 g CH 4 /kg) by 7% and gross energy intake (7.0 ± 0.14% CH 4 ) by 9%, implying an increased loss of dietary energy with advancing grass maturity. Rate of N fertilisation had no effect on CH 4 emissions per units of FPCM, DMI and gross energy intake. These results suggest that despite a lower absolute daily CH 4 production with a higher N fertilisation rate, CH 4 emission intensity remains unchanged. A significant reduction of CH 4 emission intensity can be achieved by feeding dairy cows silage of grass harvested at an earlier stage of maturity.

Fractional passage rates form a fundamental element within modern feed evaluation systems for ruminants but knowledge on feed type and feed component specific passage rates are largely lacking. This study describes the use of carbon stable isotopes ((13)C) to assess component-specific passage kinetics of 6 intrinsically (13)C-labeled corn silages varying in quality (2 cultivars×3 maturity stages) in a 6×6 Latin square design using 6 rumen-fistulated lactating dairy cows. An increase in maturity increased starch and decreased neutral detergent fiber and acid detergent fiber contents of corn silages. Passage kinetics were assessed for an external (chromium mordanted fiber; Cr-NDF) and an internal marker ((13)C isotopes) collected in feces and omasal digesta. The best fit was obtained with a deterministic multicompartmental model compared with stochastic Gn and GnG1 models with increasing order of age dependency (n=1 to 5) for both sampling sites. The Cr-NDF marker yielded higher rumen fractional passage rates (K1) than did (13)C in the dry matter ((13)CDM) in feces (0.042/h vs. 0.023/h). Omasal marker excretion patterns support the conclusions based on conventional fecal marker excretions. Component-specific passage was assessed for acid detergent fiber ((13)CADF) in feces and for starch ((13)CST) in omasal digesta. The fractional passage rate based on fecal (13)CDM and (13)CADF did not differ. Omasal (13)CST provided higher K1 values (0.042/h) than omasal (13)CDM (0.034/h) but lower values than omasal Cr-NDF (0.051/h). Fractional passage rates from the proximal colon-cecum (K2) based on fecal marker concentrations showed trends similar to K1, with Cr-NDF providing a value (0.425/h) more than twice as high as that of (13)CDM (0.179/h) and (13)CADF (0.128/h). Total mean retention time in the gastrointestinal tract was approximately double for (13)CDM (64.1h) and (13)CADF (77.6h) compared with Cr-NDF (36.4h). Corn silage quality did not affect any of the estimated passage kinetic parameters. In situ fractional degradation rates did not differ among corn silages, except for a decreased fractional degradation rate of starch with advancing maturity. Results indicate that isotope labeling allows assessment of component-specific passage kinetics of carbohydrate fractions in corn silage.

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Mastitis is a highly prevalent disease, which negatively affects cow performance, profitability, welfare, and longevity. The objectives of this study were (1) to quantify the impact of the first instance of mastitis, at different stages of lactation, on production and economic performance, and (2) to further quantify the impact of the first instance of mastitis when only cows that remain in the herd for at least 100 d in milk (DIM) and those that remain for 305 DIM are included in the analysis. A retrospective longitudinal study was conducted using data from existing animal health record files and Dairy Herd Improvement records. After editing based on selected inclusion criteria and completeness of health records, data consisted of records from first-lactation Holstein cows, from 120 herds, that calved for the first time between 2003 and 2014, inclusive. Mastitic cows were assigned to 1 of 4 groups based on when in the lactation the first event of mastitis occurred: transition (1-21 DIM), early lactation (22-100 DIM), mid lactation (101-200 DIM), or late lactation (201+ DIM). Mid-lactation and late-lactation mastitic cows were also stratified by cumulative milk yield before the mastitis event. Healthy cows (i.e., no recorded mastitis event) were randomly assigned for each lactation stage, with mid-lactation healthy and late-lactation healthy cows similarly stratified. Production performance (cumulative milk, fat, and protein yield) and economic performance [milk value, margin over feed cost (MOFC), and gross profit] were analyzed using a mixed model with herd as a random effect. Significant losses in cumulative milk yield (−382 to −989 kg) and correspondingly lower fat and protein yields were found in mastitic cows, with transition and late-lactation mastitic cows having the highest losses. Drops in production translated to significant reductions in cumulative milk value