With the recent abolition of milk quotas in the European Union, there has been a large increase in milk production in Ireland driven by an increase in cow numbers and milk yield per cow. Dairy production systems in Ireland are primarily pasture-based and factors such as sward type and cow genotype can affect the efficiency and profitability of these systems. Pasture-based production systems typically require highly fertile, healthy and robust cows with greater emphasis on milk solids production as opposed to milk yield. The aim of this study was to assess the productivity of three cow genotypes when grazing tetraploid or diploid perennial ryegrass (Lolium perenne L.; PRG) sown with and without white clover (Trifolium repens L.). Four grazing treatments were compared for this study; tetraploid PRGonly swards, diploid PRG-only swards, tetraploid PRG with white clover swards and diploid PRG with white clover swards. Three cow genotypes were analysed; Holstein-Friesian (HF), Jersey × HF (JEX) and a 3-way cross consisting of 50% Norwegian Red, 25% Jersey and 25% HF (3WAY). Thirty cows (ten of each genotype) were assigned to each grazing treatment and swards were rotationally grazed after calving in spring at a stocking rate of 2.75 cows/ha and a nitrogen fertiliser rate of 250 kg/ha annually over four years (2014 to 2017). Milk production did not differ between the two ploidies over this four year study, but cows grazing the PRG-white clover treatments had significantly greater milk yields (+ 596 kg/cow per year) and milk solid (kg fat + protein; MS) yields (+ 48 kg/cow per year) compared with cows grazing the PRG-only treatments, resulting in increased milk (+ 1,954 kg/ha) and MS (+ 156 kg/ha) yields per ha. Ploidy did not affect the white clover content of the swards, with swards having an average white clover content of 23% over the four years. The PRG-white clover swards also produced an additional 1,205 kg DM/ha herbage on average over the four year period. This additional herbage was harvested in summer and used in spring when there was lower pasture availability on PRG-white clover swards due to lower overwinter growth compared to PRG-only swards. Holstein-Friesian cows produced higher total milk yields compared to JEX and 3WAY cows (5,718 vs. 5,476 and 5,365 kg/cow, respectively; P < 0.001). However, JEX and 3WAY had higher milk fat and protein content (4.86% and 4.75% and 3.87% and 3.88%, respectively) compared to HF (4.52% and 3.72%: P < 0.001), resulting in similar MS yield for JEX and HF, (469 and 460 kg/cow) and slightly lower (P = 0.003) MS from 3WAY (453 kg/cow) compared to JEX. Reproductive performance did not differ significantly between the three genotypes with similar 24 day submission rates, six-week pregnancy rates and overall pregnancy rates. Bodyweight (BW) was significantly different (P < 0.001) between all three genotypes with HF being the heaviest followed by 3WAY and JEX (530, 499 and 478 kg, respectively) and 3WAY cows had a higher body condition score throughout lactation (P < 0.001) compared to HF and JEX. Perennial ryegrass ploidy had no impact on dry-matter intake (DMI) however, significant increases in DMI (+ 0.5 kg DM/cow per day) were observed from cows grazing PRG-white clover swards compared to PRG-only swards. Dry-matter intakes differed significantly between genotypes (17.2, 17.0 and 16.7 kg DM/cow per day for HF, JEX and 3WAY cows, respectively) which consequently affected production efficiencies. Jersey x Holstein-Friesian had the greatest total DMI/100 kg BW (3.63 k), 3WAY were intermediate (3.45 kg) and HF were lowest (3.36 kg). The above biological results supplied data for modelling the economic performance of six different production systems (two sward types (PRG-only and PRG-white clover) with three genotypes (HF, JEX and 3WAY)) using the Moorepark Dairy Systems Model (stochastic budgetary simulation model). The analysis showed that adding white clover to the PRG swards increased profitability by €305/ha in a fixed land scenario with a milk price of 29c/l, across cow genotype. In the same fixed land scenario, JEX cows were most profitable (€2,606/ha), followed by 3WAY (€2,492/ha) and HF (€2,468/ha). The system that produced the highest net profit was JEX cows grazing PRG-white clover swards (€2,751/ha). Although white clover is generally used in combination with reduced nitrogen fertiliser use, this thesis provides evidence that including white clover in either tetraploid or diploid PRG swards, combined with high levels of nitrogen fertiliser can be effectively managed to increase milk production per cow and per ha, however the environmental impacts and the persistency of white clover require further investigation. This thesis also showed that all three genotypes were suitable for spring-calving, pasture-based milk production systems as they had similar MS production, reproductive performance and functional traits however, there are still production efficiency benefits to be gained from crossbreeding.
|Date of Award||Dec 2020|
|Sponsors||Teagasc Walsh Scholarship|
|Supervisor||Trevor Gilliland (Supervisor) & Andy Meharg (Supervisor)|
Student thesis: Doctoral Thesis › Doctor of Philosophy