Improving N utilization in dairy cows and especially reducing N output in excreta is desirable due to global concerns of agricultural contribution of N to environmental pollution, particularly as ammonia. Data from five N balance experiments were used to develop a dynamic model that was evaluated with independent data. Model predictions of feces, urine, and milk outputs were close to observed values. Statistical analysis showed that 96% of mean square prediction error for feces and urine N output predictions was due to random variation. However, the model tends to overpredict milk N output, especially at higher N intake levels. Evaluation of model predictions for independent experimental observations from Agricultural Development Advisory Service at Bridgets (U.K.) showed good agreement between predicted and observed urine N output (95% due to random variation). However, there was a slight underprediction for fecal N output (14% mean square prediction error due to bias) and overprediction of milk N output (22% of mean square prediction error due to bias). The model predictions of N outputs in excreta were sensitive to changes in energy concentration of the diet. Dietary protein degradability had only a small influence on predicted fecal N output. However, the model was sensitive in its predictions of urine N when protein degradability was varied. Application of the model to assess reduction in ammonia emissions from dairy cows showed that increasing the energy concentration could potentially reduce ammonia emissions by up to 25% per cow. Similarly, reducing CP concentration in the diet to about 16% could reduce ammonia production by 20% and lower degradability of CP to match microbial requirement by 19% per cow. The model is a first step toward a mechanistic approach of nutrient modeling, and it is a valuable method for predicting N excretions and estimating N emissions from dairy systems.

Since estimated dietary selenium intake in the UK has declined steadily from around 60 µg day −1 in 1975 to 34 µg day −1 in 1997, there is a need to increase selenium intake from staple foods such as milk and milk products. An experiment was therefore done to investigate the relationship between dietary source and concentration of selenium and the selenium content of bovine milk. In a 3 × 3 factorial design, 90 mid-lactation Holstein dairy cows were supplemented over 8 weeks with either sodium selenite (S), a chelated selenium product (Selenium Metasolate) (C) or a selenium yeast (Sel-Plex) (Y) at three different dietary inclusion levels of 0.38 (L), 0.76 (M) and 1.14 (H) mg kg −1 dry matter (DM). Significant increases in milk selenium concentration were observed for all three sources with increasing inclusion level in the diet, but Y gave a much greater response (up to +65 µg l −1 ) than the other two sources of selenium (S and C up to +4 and +6 µg l −1 respectively). The Y source also resulted in a substantially higher apparent efficiency of transfer of selenium from diet to milk than S or C. Feeding Y at the lowest dietary concentration, and thus within the maximum level permitted under EU regulations, resulted in milk with a selenium concentration of 28 µg l −1 . If the selenium concentration of milk in the UK was increased to this value, it would, at current consumption rates, provide an extra 8.7 µg selenium day −1 , or 11 and 14% of daily recommended national intake for men and women respectively.

-With the aim of reducing the degree of saturation and increasing the C18:1 cis fatty acid content of milk fat, the effects of feeding high levels of whole cracked rapeseed to dairy cows was investigated together with the effect of increasing dietary intake of vitamin E on the vitamin E content of milk. Using a 3 × 3 factorial design, 90 Holstein dairy cows were fed one of three levels of whole cracked rapeseed (0 (ZR), 134 (MR) and 270 g·kg -1 diet dry matter (DM) (HR)) in combination with one of three intakes of supplementary vitamin E (0 (ZE), 2 (ME) and 4 g·cow -1 ·d -1 (HE)). Supplementing with up to almost 2 kg·d -1 of rapeseed oil (diet HR) significantly (P < 0.001) increased C18:1cis in milk fat, from 181 (ZR) to over 400 g·kg -1 (HR) of total milk fatty acids. Concentrations of C18:0, C18:2 and C18:3 fatty acids were also increased (P < 0.001) but by a much lesser degree, and the saturated fatty acids C4:0 to C16:0 decreased substantially. Vitamin E supplementation increased (P < 0.01) milk vitamin E concentrations from 1.29 (ZE) to 1.68 mg·kg -1 whole milk (HE). Thus substantial changes in milk fat composition with potentially beneficial effects on human health were achieved and without any adverse effects on milk taste. However, these improvements must be offset against the substantial reductions (P < 0.001) observed in voluntary feed DM consumption (ZR, 20.6; HR, 15.2 kg DM·d -1 ), milk yield (ZR, 22.9; HR, 13.2 kg·d -1 ) and milk fat concentration (ZR, 42.1; HR, 33.4 g·kg -1 ) which would not be commercially sustainable unless a considerable premium was paid for this modified milk. It seems likely that the optimum dose of dietary rapeseed is lower than used in this study. Résumé -Augmentation de la concentration en acide oléique et en vitamine E du lait de bovin en incorporant dans la ration des graines de colza entières et de la vitamine E. Afin de réduire le degré de saturation des acides gras et d'augmenter la teneur en C18:1 cis du lait, les effets d'une alimentation avec des niveaux élevés de graines de colza concassées entières ont été étudiés chez des vaches laitières, en relation avec les effets de l'augmentation de l'ingestion de la vitamine E sur la teneur en vitamine E du lait. Au total, 90 vaches laitières de race Holstein ont été alimentées selon un schéma factoriel 3 × 3 avec trois niveaux de graines de colza concassées (0 (ZR), 134 (MR) et 270 g·kg -1 de matière sèche (HR)) et trois teneurs en vitamine E (0 (ZE), 2 (ME) et 4 g·vache -1 ·j -1 (HE)). La supplé-mentation avec près de 2 kg·j -1 d'huile de graines de colza (régime HR) a augmenté significativement (P < 0,001) la concentration du C18:1 cis du lait, de 181 (ZR) à plus de 400 g·kg -1 d'acides gras totaux du lait (HR). Les concentrations en C18:0, C18:2 et C18:3 ont été également augmentées (P < 0,001), mais dans des proportions moindres. Dans le même temps, la concentration des acides gras saturés C4:0 à C16:0 a diminué sensiblement. La supplémentation en vitamine E a augmenté les concentrations en vitamine E du lait ...

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This study investigated the effects on embryo growth and survival rate of feeding heavily-fertilised spring grass, containing high levels of quickly-degradable nitrogen, to pregnant cows. Forty-eight lactating Holstein cows between 2 and 8 weeks pregnant were turned-out, after a one-week transition period onto high- or low-nitrate pasture and fed a high- or low-concentrate supplement. Cows grazing the High nitrate pasture had significantly higher milk and plasma urea concentrations than cows grazing the Control pasture, while cows which were fed less concentrate had a notably higher plasma ammonia. However, there was no evidence that an increased quickly-degradable nitrogen (QDN) intake from pasture affected embryo survival or growth from 20 days onwards. This suggests that the impact of turnout on fertility mainly affects ovulation, fertilisation and/or the early embryo.