Understanding and mitigating the impacts of major dietary changes on dairy cows

© 2019 Dr Victoria Marie RussoFour experiments were conducted to investigate the effects of major dietary changes on ruminal pH, ruminal fluid composition, eating behaviour, feed intake and milk production of dairy cows. The impacts of both diet composition and management strategies were evaluated. The initial experiment investigated the impact of early adaptation when instigating a complete dietary change from one forage to another at calving, as is common practice in Irish dairy farming. Three weeks prior to their expected calving date, 14 spring calving dairy cows were assigned to one of two treatments: pasture silage pre-partum followed by fresh cut perennial ryegrass (PRG) post-partum, or fresh PRG both pre and post-partum. There were no differences in dry matter intake (DMI), body condition score, energy balance or milk yield and composition between the treatments. The results of the initial experiment suggested that early adaptation to avoid a major dietary change at calving did not result in health or production benefits. This was speculated to be due to the similarities of the two diets, creating little challenge for the rumen to adapt. The second experiment focused on a more challenging dietary change, incorporating a large amount of concentrate into a forage-only diet. Thirty-two lactating dairy cows were initially fed 100% lucerne hay cubes, wheat was then gradually substituted in until it comprised 40% of total dry matter (DM) and lucerne hay cubes, the remainder. Wheat was substituted for lucerne cubes via one of four strategies, (1) in six small increments (each 6.7% of total DM) over 6 days; (2) in six small increments (each 6.7% of total DM) over 11 days; (3) in three large increments (each 13.3% of total DM) over 6 days; or (4) in three large increments (each 13.3% of total DM) over 11 days. The 6-day strategies are considered rapid for the dairy industry yet none of the treatments resulted in ruminal fluid pH values that would have compromised ruminal function, nor were there differences in DMI or energy corrected milk (ECM) yields. Furthermore, there were no differences between ruminal fluid volatile fatty acid (VFA), lactate or ammonia concentrations. It is speculated that the properties of the lucerne cubes helped the ruminal contents resist the pronounced declines in pH often seen with the fermentation of large amounts of wheat. These results suggested that changes to rumen function are driven not only by the characteristics of the concentrate being introduced but also by those of the forage. The third experiment aimed to investigate the role of forages in grain adaptation. Twenty-eight lactating dairy cows were fed either PRG hay or lucerne hay and wheat was gradually substituted for forage in three equal increments, over 6 or 11 days, until wheat made up 40% of DM (~ 8 kg DM/cow per day). The results varied significantly with forage type. Cows fed lucerne hay ate more, produced more ECM and had lower ruminal pH values. Furthermore, of the cows fed lucerne hay, those adapted to wheat in the shorter time frame (6 days) exhibited significantly lower mean ruminal pH values. Despite the ruminal pH of these cows declining to levels typically considered low, none of their other measured parameters indicated compromised fermentation or acidosis. Rather, it was these same cows that had the greatest ECM yields, producing an average of 1.5 kg ECM/cow per day more than their 11-day counterparts. The 6-day adaptation strategy allowed for a rapid increase in metabolisable energy, while the hay promoted adequate buffering within the rumen. No difference was seen between adaptation strategies when PRG hay was fed. This was due to the higher metabolisable energy concentration and lower intake of the PRG hay resulting in a less pronounced increase in metabolisable energy intake with the wheat substitution. The greater intakes of cows fed the lucerne hay likely contributed to their greater ECM yields and lower ruminal pH values. However, both forages allowed the rumen contents to resist the large declines in ruminal pH that are typically seen during rapid grain adaptation. The final experiment aimed to further evaluate the role that forage plays in ruminal, behavioural and production responses to the incorporation of large amounts of wheat grain into the diet. Sixteen dairy cows in early lactation were fed a forage only diet of either lucerne hay, PRG hay or one of two cultivars of fresh PRG pasture (cultivar Bealey or Base) for three weeks. The forage-only diet was then supplemented with crushed wheat grain at a rate of 8 kg DM/cow per day, with no adaptation period. Wheat comprised between 32 and 43% of total DMI and was fed over two meals, followed by forage, for one day only. Feeding fresh pasture resulted in lower ruminal pH values, with pH remaining below 6.0 for longer each day. Following supplementation of wheat, cows fed pasture exhibited ruminal fluid pH levels associated with sub-acute ruminal acidosis. Hay created a ruminal environment that was better able to cope with the influx of acid produced as wheat was digested. A combination of increased ruminating time and a decreased rate of fermentation are likely responsible for the higher ruminal fluid pH values. The ruminal environment of cows fed lucerne hay remained most stable throughout the grain challenge, with ruminal fluid spending the least amount of time below pH 6.0. Reducing the introductory time for concentrates into a dairy cow’s diet means an ability to rapidly increase the energy content of a diet, resulting in milk production benefits. However, this thesis highlights the importance of forage choice when determining introduction strategies. Traditional, gradual adaptation strategies must still be employed when feeding highly digestible fresh forages. However, more aggressive adaptation strategies can be implemented when hays are used as the base forage. In situations where high energy grains are substituted for a low energy, high fibre basal forage, rapid introduction can have milk production benefits over gradual strategies