Eight multiparous Holstein±Friesian dairy cows in late lactation were used to investigate the potential of using perennial ryegrass with a high concentration of watersoluble carbohydrate (WSC) to increase the ef®ciency of milk production. After a pretreatment period on a common pasture, the cows were each given ad libitum access to one of two varieties of zero-grazed grass continuously for 3 weeks. Treatments were: high sugar (HS), an experimental perennial ryegrass variety bred to contain high concentrations of WSC; or control, a standard variety of perennial ryegrass (cv. AberElan) containing typical concentrations of WSC. The two grass varieties were matched in terms of heading date. All animals also received 4 kg day ±1 standard dairy concentrate. Grass dry matter (DM) intake was not signi®cantly different between treatments (11á6 vs. 10á7 kg DM day ±1 ; s.e.d. 0á95 for HS and control diets respectively), although DM digestibility was higher on the HS diet (0á71 vs. 0á64 g g ±1 DM; s.e.d. 0á23; P < 0á01) leading to higher digestible DM intakes for that diet. Milk yield from animals offered the HS diet was higher (15á3 vs. 12á6 kg day ±1 ; s.e.d. 0á87; P < 0á05) and, although milk constituent concentrations were unaffected by treatment, milk protein yields were signi®cantly increased on the HS diet. The partitioning of feed N was signi®cantly affected by diet, with more N from the HS diet being used for milk production (0á30 vs. 0á23 g milk N g ±1 feed N; s.e.d. 0á012; P < 0á01) and less being excreted in urine (0á25 vs. 0á35; s.e.d. 0á020; P < 0á01). In a separate experiment, using the same grasses harvested earlier in the season, the fractional rate of DM degradation, measured by in situ and gas production techniques, was higher for the HS grass than for the control. It is concluded that increased digestible DM intakes of the HS grass led to increased milk yields, whereas increased ef®ciency of utilization of the HS grass in the rumen resulted in the more ef®cient use of feed N for milk production and reduced N excretion.

When silage is exposed to air on opening the silo, or after its removal from the silo, fermentation acids and other substrates are oxidized by aerobic bacteria, yeasts and moulds. The aerobic stability of silage is a key factor in ensuring that silage provides well‐preserved nutrients to the animal with minimal amounts of mould spores and toxins. In this paper, key findings and recent developments are reviewed, and findings of recent research are integrated in terms of four themes: (i) the most significant biochemical and microbiological factors, (ii) physical and management factors, (iii) type of additive and (iv) silo sealing. The development of yeasts and moulds during plant growth, and during field wilting or storage, and the concentration of undissociated acetic acid in silage are important microbiological and biochemical factors affecting aerobic stability. Silage density and porosity are key physical factors that affect the rate of ingress of oxygen into the silage mass during the feed‐out period. A target for potential silage aerobic stability is 7 d including time in the feed trough. To achieve this target, speed of harvest should be coordinated with packing tractor weight to achieve a minimum silage density by the time of feed‐out of 210 kg DM m−3, maximum proportional porosity of 0·4 and a rate of silage removal, which matches or exceeds the depth of air penetration into the silo. The use of additives to increase aerobic stability is advisable when there is the risk of these objectives not being met. Novel microbial approaches to solving the problem of silage aerobic deterioration are needed.

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