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The effect of stocking rate and cow breed on resumption of cyclicity, blood indicators of energy status, uterine health and reproductive parameters in pasture-based dairy systems
Identifying the optimum stocking rate (SR) and cow breed for pasture-based systems is essential to maximise output/ha without compromising reproductive performance. The objective of this study was to compare the performance of two different breeds (Holstein Friesian, HF, and Jersey crossbreds, JEX; n = 69 per breed) on one of three different SR (Low: 2.5 cows/ha; Medium: 2.9 cows/ha; and High: 3.3 cows/ha; n = 46 per treatment). The study was carried out over 3 consecutive years. Milk samples were collected three times per week from parturition until week 5 post AI for progesterone analysis. Ten blood samples/cow/year were collected (weeks -2, 1, 2, 3, 4, 6, 8, 10, 14, and 18 relative to parturition) to determine circulating concentrations of glucose, non-esterified fatty acids (NEFA), β-hydroxybutyrate (β-HBA) and insulin-like growth factor-1 (IGF1). Uterine cytology samples were collected on Week 6 after parturition from 252 cows during Years 2 and 3 of the study to determine the proportion of cows with sub-clinical endometritis. The fertility performance of each herd was monitored during a 12-week breeding season across three years of the study.
Days to resumption of cyclicity was not affected by SR or breed (25.2, 24.5, and 25.1 ± 1.2 d for Low, Med and High SR; 24.7, vs. 25.2 ± 1.7 d for HF vs. JEX, respectively, P>0.05). Mean plasma glucose (72.9, 71.9, and 72.4 ± 0.9 mg/dl), BHBA (0.61 ±, 0.61, and 0.62 ± 0.01, mmol/l), NEFA (0.46, 0.48, and 0.47 ± 0.01 mmol/l) and IGF1 (93.7, 92.9, and 98.9 ± 4.3, mmol/l) concentrations were not affected by SR (Low, Med and High, respectively; P>0.05). Mean glucose concentrations were greater in JEX than HF (73.5 ± 0.7 vs. 71.3 ± 0.8, mg/dl; P< 0.05, respectively), but concentrations of NEFA, β-HBA and IGF1 were not affected by breed (P>0.05). Neither SR nor breed affected the proportion of cows with sub-clinical endometritis at Week 6 postpartum (SR: 0.15, 0.14 and 0.27 of cows for low, medium and high SR, respectively; Breed: 0.18 and 0.18 of HF and JEX cows, respectively) or 42d pregnancy rate and final in-calf rate (all P>0.05). In conclusion, under the conditions of this study, there was no major effect of increased SR or cow breed on reproductive performance. It is important that farm SR allows nutritional requirements to be met and that cows are genetically suited to seasonal-calving pasture based systems.
Keywords: breed, fertility, stocking rate