Our study compared methane (CH4) emissions from lactating dairy cows measured using the sulfur hexafluoride (SF6) tracer and open-circuit respiration chamber techniques. The study was conducted using 16 lactating Holstein-Friesian cows. In each chamber, the cow was fitted with the SF6 tracer apparatus to measure total CH4 emissions, including emissions from the rectum. Fresh ryegrass pasture was harvested daily and fed ad libitum to each cow with a supplement of 5 kg of grain/d. The CH4 emissions measured using the SF6 tracer technique were similar to those using the chamber technique: 331 vs. 322 g of CH4/d per cow. The accuracy of the SF6 tracer technique was indicated by considering the ratio of the CH4 emission measured using the SF6 tracer to the emission measured using the chamber for each cow on each day. The calculated ratio of 102.3% (SE = 1.51) was not different from 100%. A higher variability within cow between days was found for the SF6 tracer technique [coefficient of variation (CV) = 6.1%] than for the chamber technique (CV = 4.3%). The variability among cows was substantially higher than within cows, and was higher for the SF6 technique (CV = 19.6%) than for the chamber technique (CV = 17.8%). Our CH4 emission data were compared with whole-animal chamber studies conducted in Canada and Ireland. In the Canadian study the SF6 technique did not measure CH4 emissions from the rectum and emissions were 8% lower than those measured using the chamber, indicating that emissions from the rectum may be greater than previously measured (1%). The relationship between CH4 emission and dry matter intake was examined for our data and for that reported in the Canadian study. There was a difference in the slopes of the regressions derived from our data and that from Canada; 17.1 vs. 20.8 g of CH4/kg of dry matter intake. A difference between the 2 locations was expected based on the difference in diet composition for these 2 studies. The SF6 tracer technique is reasonably accurate for inventory purposes and for evaluating the effects of mitigation strategies on CH4 emissions.

. 2009. Potential use of Acacia mearnsii condensed tannins to reduce methane emissions and nitrogen excretion from grazing dairy cows. Can. J. Anim. Sci. 89: 241Á251. We measured the effect of condensed tannins (CT) extracted from the bark of the Black Wattle tree (Acacia mearnsii) on the milk production, methane emissions, nitrogen (N) balance and energy partitioning of lactating dairy cattle. Sixty lactating cows, approximately 32 d in milk grazing ryegrass pasture supplemented with 5 kg d(1 cracked triticale grain, were allocated to three treatments: Control, Tannin 1 (163 g CT d( 1 ) or Tannin 2 (326 g CT d(1 initially, reduced to 244 g d (1 CT by day 17). Cows were dosed twice daily after milking for 5 wk with the powdered CT extract (mixed 1:1 with water). Low and high CT supplementation reduced (PB0.05) methane emissions by 14 and 29%, respectively (about 10 and 22% on an estimated dry matter intake basis). However, milk production was also reduced by the CT (P B0.05), especially at the high dose rate. Milk yields were 33.0, 31.8 and 29.8 kg cow(1 d (1 . Tannin 2 also caused a 19% decline in fat yield and a 7% decline in protein yield, but protein and lactose contents of milk were not affected by CT supplementation. After the initial 5-wk period, five cows representative of each treatment group were moved to metabolism facilities to determine effects of CT on energy digestion and N balance over 6 d. The energy digestibility was reduced (P B0.05) from 76.9 (Control) to 70.9 (Tannin 1) and 66.0% (Tannin 2) and the percentage of feed N lost to urine was reduced (PB0.05) from 39 to 26% and 22% for the respective treatments. The CT also caused a reduction (PB0.05) in intake during the metabolism study, effectively increasing CT as a percentage of intake. Although CT can be used to reduce methane and urinary N losses from cows fed pastures with a high crude protein (CP) concentration, reduced milk yield in this study suggested the dietary concentration was too high. If CT are to be considered as a means for lowering methane emissions further research is needed to define impacts of lower doses of A. mearnsii CT on methane production and cow productivity. Dairy producers will be reluctant to adopt feeding practices that compromise profitability.

This study was designed to evaluate the respective influences of stage of lactation (SOL) and time of year on the seasonal variation in milk composition for pasture-fed dairy cows in New Zealand. Four herds of ∼20 Friesian cows were used, one herd calving in a 6 week period beginning in each of January, April, July and October. Cows grazed rye-grass–white clover pasture only, except during June when all cows received supplementary pasture silage. Milk samples were collected from each cow in milk on four occasions during the year (September, December, March and June), to give a total of three samples per cow (early, mid and late lactation; about 30, 120 and 210 d respectively after calving). Samples were analysed for a detailed range of components. Concentrations of many milk components (e.g. total protein, fat, casein and whey protein) increased as lactation progressed; the extent of these increases depended on the time of year. These results indicated that spreading calving throughout the year would lessen seasonal variations in the gross composition of milk supplied to factories, leading to a more even distribution of product yield across the year. Despite this, variations in some important manufacturing properties were affected by time of year but not by SOL. Ratios of protein[ratio ]fat and casein: whey protein were not significantly affected by SOL, but were affected by time of year. The solid fat content of milk was also affected by time of year. Seasonal variations in the manufacturing properties of milk may be reduced but not eliminated by changing the time of calving.

New Zealand Holstein-Friesian cows (n = 4) were used to quantify the importance of endogenous synthesis of cis-9, trans-11 conjugated linoleic acid (CLA) via Delta(9)-desaturase in cows fed a fresh pasture diet. The experiment was a 4 x 4 Latin square design with treatments arranged in a 2 x 2 factorial. Treatments lasted 4 d and were pasture only, pasture plus sterculic oil, pasture plus sunflower oil, and pasture plus sunflower oil plus sterculic oil. Abomasal infusion of sterculic oil inhibited Delta(9)-desaturase and decreased the concentration of cis-9, trans-11 CLA in milk fat by 70%. Using the changes in cis-9 10:1, cis-9 12:1 and cis-9 14:1 to correct for incomplete inhibition of Delta(9)-desaturase, a minimum estimate of 91% of cis-9, trans-11 CLA in milk fat was produced endogenously in cows fed fresh pasture. Dietary supplementation of a pasture diet with sunflower oil increased the proportion of long chain fatty acids in milk fat; however, the increase in vaccenic acid concentration was small (18%) and there was no increase in cis-9, trans-11 CLA concentration. Overall, results show that endogenous synthesis is responsible for more than 91% of the cis-9, trans-11 CLA secreted in milk fat of cows fed fresh pasture. However, the failure of plant oil supplements to increase the concentration of cis-9, trans-11 CLA in milk fat from pasture-fed cows requires further investigation.

The effects of somatic cell count and stage of lactation on the yield and quality of Cheddar cheese were investigated. Cheese was manufactured in a pilotscale factory using milk of low bulk milk cell count (BMCC) from herds in early (LE) and late (LL) lactation, and milk of high BMCC from herds in early (HE) and late (HL) lactation. The deleterious effect of an elevated BMCC on product yield and quality in late lactation was clear. Cheese made from LL milk was significantly superior to that made from HL milk for most yield and quality characteristics measured.Stage of lactation also affected cheese yield and quality, as evidenced by the lower recovery of fat and poorer flavour score for cheese from LL milk compared with that manufactured from LE milk. The observed differences could be explained largely by differences in raw milk composition. We conclude that the effect of stage of lactation was magnified by an elevated BMCC, and that many of the problems encountered when processing late season milk could be overcome by containing mastitis at this time.Bulk milk cell count (BMCC) is a measure of the number of somatic cells in bulk milk and is a commonly used and reliable indicator of the incidence of subclinicalmastitis within a dairy herd (Eberhart et al. 1982). Such subclinical mastitis is usually caused by pathogenic bacteria colonizing the teat cistern (Anon. 1971), and the resulting inflammation can have marked effects on gross milk composition. Damage sustained by the mammary epithelium as a result of mastitis leads to the reduced synthesis and secretion of milk components synthesized de novo. At the same time, the tight junctions between the mammary epithelial cells often rupture, allowing the free influx of serum components from the extracellular fluid into the milk (Kitchen, 1981). These components include a range of hydrolytic enzymes which further modify milk composition via the breakdown of casein and fat (Grieve & Kitchen, 1985). Thus, during mastitis there are generally decreases in concentrations of milk fat, lactose and casein, and increases in concentrations of whey proteins (Auldist et al. 1995).The precise effects of BMCC on the yield and quality of dairy products are less well known. Some researchers have observed the poor coagulating properties of t Present address and correspondence: Dairying