A comparative study into dietary strategies for optimising the efficiency of dairy cow performance

The abolishment of the European Union (EU) milk production quota in 2015 has led to rapid expansion of the dairy industry in Ireland. However, where access to additional land is limited, farmers must explore alternative or additional feeding strategies to improve milk production and enhance production efficiencies in an environmentally sustainable manner. With an increased emphasis on milk solids production (MS), the incorporation of white clover into the sward or supplementing dairy cows with total mixed ration (TMR) or partial mixed ration (PMR- pasture supplemented with TMR), may be viable options to improve animal performance over a grass-only diet. The aim of this thesis was to identify dietary strategies to optimise cow efficiency in temperate regions where grazed grass is generally the cheapest and most predominantly fed forage for milk production. Four feeding systems were compared as part of this study; cows grazing outdoors on perennial ryegrass (Lolium perenne L.; PRG) only swards receiving 250 kg nitrogen (N)/ha (GRASS), grass-white clover (Trifolium repens L.) swards receiving 150 kg N/ha (CLOVER), cows housed indoors and fed a total mixed ration in a 100% confinement system (TMR), and cows fed a combination of pasture and TMR (partial mixed ration; PMR). After calving each year, cows were assigned to one of the treatment groups based on calving date, lactation number, 2 week pre-experimental milk yield, and 2 week pre-experimental MS (kg of fat and protein) yield. Cows remained assigned to their treatment groups for the entire lactation in each year. The grazing treatments were stocked at 2.74 cows/ha. Both the GRASS and CLOVER treatment groups grew 14.1 t dry matter (DM)/ha despite an extra 100 kg N fertiliser being applied on the GRASS treatment. At an average white clover content of 21% over the duration of the study, the CLOVER cows produced an additional 18 kg of MS, which translated into an extra 51 kg MS/ha compared with the GRASS cows. The TMR system supported the greatest daily and cumulative milk and MS yield compared with the pasture-based diets. In terms of reproductive performance, the majority of parameters were not influenced by feed system with similar conception rate to first service, 21-day submission rate, conception rate to second service, and overall pregnancy rate. However, the CLOVER cows required six days less between calving and conception (93 days) compared with the GRASS and TMR cows who both required 99 days. Additionally, a greater proportion of CLOVER and TMR cows were pregnant to their first service. Bodyweight (BW) was significantly greater for the TMR fed cows compared with the pasture-based cows; the TMR cows were 29 kg heavier over the entire lactation (553.4 vs. 524.7 kg). Average daily milk and MS yield was greatest for the TMR cows (24.7 and 2.05 kg/cow, respectively), intermediate for the CLOVER cows (20.3 and 1.68 kg/cow, respectively), and lowest for the GRASS cows (19.1 and 1.56 kg/cow, respectively). This is a reflection of the total dry matter intake (TDMI; DMI) being significantly different between treatments. On average, the TMR consumed 2.30 kg/cow greater than the CLOVER cows, which consumed 1.03 kg/cow greater than the GRASS cows. Similarly, cows consuming the TMR diet, had a greater energy intake in comparison with the pasture-fed cows. Consequently, this influenced the production efficiencies. The TMR cows had greater amounts of energy available for milk production after maintenance and also had greater total energy available to produce 1 kg MS compared with the GRASS and CLOVER treatments. When the PMR system was incorporated into the study in 2020, the daily milk and MS production and cumulative totals (24.35, 2.09 and 6701, 584 kg/cow, respectively) were intermediate between the fully nutritionally balanced TMR system (25.60, 2.25 and 6988, 629 kg/cow, respectively) and the GRASS system (20.54, 1.77 and 5770, 526 kg/cow, respectively) over two full lactations, despite a similar DMI for the PMR and TMR (20.90 kg/cow). A similar trend to that for milk production efficiencies was also observed for DMI per 100 kg BW, fat and protein corrected milk (FPCM) per 100 kg per 100 kg BW, and MS per 100 kg BW. However, the PMR cows produced the lowest quantities of MS per kg total DMI which suggests that energy was being diverted from milk production towards walking and grazing behaviour when a combined pasture and TMR feeding system is implemented. While throughout this study, the TMR system supported the greatest milk and MS yield over the pasture-based diets, a consequence of this was significantly greater daily enteric methane (CH4) emissions and gross energy intake lost as CH4. The TMR cows emitted, on average over the five days of each of the three measurement periods (May, July, and September), 610.0 g CH4/cow, the CLOVER cows emitted 436.9 g CH4/cow, while the GRASS cows emitted 404.5 g CH4/cow. Additionally, the TMR feeding system supported the greatest gross energy intake losses as CH4, the CLOVER was intermediate and the GRASS was lowest (9.0 vs. 7.7% vs. 7.4%, respectively). The GRASS and CLOVER cows emitted similar quantities of CH4 in the May and July measurement periods, while in September the CLOVER cows emitted 67.56 g/cow per day more CH4 compared to the GRASS group. Nonetheless, both pasture-based treatments emitted a similar amount of CH4 when corrected for DMI (30.32 g/kg DMI). This research PhD study proves that milk production and its associated efficiencies can be improved by feeding a TMR or PMR, or by incorporating white clover into the sward compared that achieved by a pasture-only diet. Nonetheless, in temperate regions, grazed grass remains the cheapest feed source for milk production, any increases in animal performance obtained through alternative feeding strategies must be environmentally and economically viable.