Lower Omega-6-Omega-3 Ratio Increased Milk Production and Had Limited Effects on Early Pregnancy Development in Dairy Cattle

Abstract

The aim of this study was to evaluate the effect of the linoleic-α-linolenic acid ratio (LA:ALA) on cyclicity, oocyte quality, early pregnancy parameters, milk yield, and composition. Holstein cows were randomized to a 6:1-LA:ALA diet (Low-OMG3: n = 3 pens; 11 primiparous, 14 multiparous) or a 2:1-LA:ALA diet (High-OMG3: n = 3 pens; 10 primiparous, 14 multiparous). Diets were isocaloric and isonitrogenous and fed between 15 and 140 days in milk (DIM). Data were compared using linear mixed models. As expected, omega-3 concentrations in milk and blood increased in the High- compared to Low-OMG3 cows. No effect of diet was observed on cyclicity by 45DIM or oocyte quality at 50DIM. High-OMG3 cows had larger corpus luteum size (11-32 d post-timed artificial insemination [TAI]) and greater blood flow (32-60 d post-TAI) than Low-OMG3 cows. However, there was no effect of diet on progesterone, pregnancy-associated glycoproteins, or conceptus size. High-OMG3 cows produced more milk throughout the study, had greater lactose, and tended to have greater protein yield at 50DIM. In conclusion, decreasing the LA:ALA dietary ratio in lactating dairy cows did not provide evidence of effects on cyclicity, oocyte quality, or other early pregnancy-related parameters, but affected corpus luteum size and blood flow, enhanced milk production, and partially increased protein and lactose yields.

Keywords: cattle; dairy; fatty acids; omega-3; pregnancy; reproduction.

Figure 1

Experimental design of the study: (A) Phase 1 comprised the voluntary waiting period: forty-nine lactating dairy cows were enrolled in weekly cohorts at approximately 15 days in milk (DIM). Cows were randomized into receiving a diet with a 6:1-LA:ALA ratio (Low-OMG3: n = 3 pens; 11 primiparous, 14 multiparous) or a 2:1-LA:ALA ratio (High-OMG3: n = 3 pens; 10 primiparous, 14 multiparous). Diets were isocaloric and isonitrogenous. From 21 to 45 ± 3 DIM, cows were evaluated twice weekly by ultrasound to assess cyclicity resumption post-partum by the presence of the corpus luteum (CL). At 45 ± 3 DIM, all cows received 100 μg of gonadorelin acetate and were submitted to ovum pick-up (OPU) at 48 ± 3 DIM for oocyte collection. Subsequently, cows were enrolled at 55 ± 3 DIM into a synchronization protocol for the TAI first service at 75 ± 3 DIM. The protocol consisted of 500 mcg of cloprostenol sodium, followed 3 days later by 100 mcg of gonadorelin acetate, and seven days later by an Ovsynch-56 (with two full doses of cloprostenol). Blood and milk samples were also collected at 15, 50, 75, 110, and 140 DIM to determine the plasma and milk fatty acid profiles and milk components. Milk weights were recorded at each milking. (B) Phase 2 comprised the pregnancy development assessments. Briefly, weekly blood samples were collected to assess progesterone (P4) and pregnancy-associated glycoprotein (PAG) concentrations from 11 days post-TAI until pregnancy diagnosis at 32 days post-TAI. Ultrasound examinations were performed at the same time to measure CL diameter and blood flow. If a cow was diagnosed pregnant at 32 days post-TAI, weekly blood samples were collected until 60 days post-TAI. From 32 to 60 days post-TAI, weekly ultrasound examinations included conceptus and amniotic vesicle size. Additionally, body condition score was evaluated (1 to 5 units) at 21 days before expected calving, calving, 21 days after calving, OPU, TAI, and 30 and 60 days of gestation.

Figure 2

Estimated marginal means ± SEM for omega-6–omega-3 ratio (top) and omega-3 concentration and yield (bottom) in plasma (right) and milk (left). Low-OMG3 and High-OMG3 are depicted in green and pink colors, respectively, whereas multiparous and primiparous cows are depicted by a solid or a dashed line, respectively. Data were obtained from Holstein cows receiving a diet with a 6:1-LA:ALA ratio (Low-OMG3; green; n = 3 pens, 25 cows) or a 2:1-LA:ALA ratio (High-OMG3; pink; n = 3 pens, 24 cows). Diets were isocaloric and isonitrogenous. Statistical analyses were performed within sample type (milk or plasma). Statistical differences (p ≤ 0.05) were obtained from Tukey multiple comparison testing within a given DIM and sample type (milk or plasma). An asterisk indicates the interaction between two variables. For the omega-6–omega-3 ratio in plasma (A), High-OMG3 was different than Low-OMG3 on days 50, 75, 110, and 140, but the difference increased as DIM progressed. For the omega-3 concentration in plasma (B), the Low- and High-OMG3 groups were different from 75 DIM onwards. Moreover, the Low- and High-OMG3 groups differed in the omega-6–omega-3 ratio (C) and the omega-3 yield (D) in milk samples at 50 DIM.

Figure 3

Estimated marginal means ± SEM for (A) milk production in kg from cows receiving a diet with a 6:1-LA:ALA ratio (Low-OMG3; green; n = 3 pens, 25 cows) or a 2:1-LA:ALA ratio (High-OMG3; pink; n = 3 pens, 24 cows). Diets were isocaloric and isonitrogenous. An asterisk indicates the interaction between two variables. The average milk production during the study period was greater in the High-OMG3 cows (50.7 ± 1.2 kg/d) than in Low-OMG3 cows (49.3 ± 1.2 kg/d). (B) Depicts least squares means ± SEM for protein yield (kg) for Low- and High-OMG3 cows from samples collected at 50, 75, 110, and 140 DIM. High-OMG3 cows tended to have greater protein yield compared to Low-OMG3 cows only at 50 DIM. (C) Depicts least squares means ± SEM for lactose yield (kg) for Low- and High-OMG3 cows from samples collected at 50, 75, 110, and 140. High-OMG3 cows had greater protein yield compared to Low-OMG3 cows only at 50 DIM.

Figure 4

Examples of total lipid content and mitochondria abundance in oocytes from cows receiving a diet with a 6:1-LA:ALA ratio (Low-OMG3) or a 2:1-LA:ALA ratio (High-OMG3). Diets were isocaloric and isonitrogenous. Mature oocyte lipid content was determined by Nile Red staining, and mitochondria abundance was measured using Mitotracker^TM stain. The overlay image is the composite of channels, including brightfield.

Figure 5

Estimated marginal means ± SEM for pregnancy-related variables from cows receiving a diet with a 6:1-LA:ALA ratio (Low-OMG3; green solid line; n = 3 pens, 18 cows) or a 2:1-LA:ALA ratio (High-OMG3; pink dashed line; n = 3 pens, 10 cows). Diets were isocaloric and isonitrogenous, fed from 11 to 60 days post-TAI. (A) Progesterone (P4), progesterone concentrations in ng/mL; (B) Corpus luteum (CL) volume in mm³; (C) CL blood flow in percentage; (D) Pregnancy-associated glycoprotein (PAG) concentrations in ng/mL; (E) Embryo length in mm; and (F) Vesicle volume in mm³. The only parameter in which diet had an effect was the CL blood flow, in which High-OMG3 cows had greater CL blood flow percentage than the Low-OMG3 cows after day 50 post-TAI. An asterisk indicates the interaction between two variables.