Relationships among total mixed ration nutritional components and reproductive performance in high-producing dairy herds

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TL;DR

This study analyzed TMR composition from 48 high-producing Wisconsin dairy herds and found that higher dietary nonfiber carbohydrates (NFC) were strongly associated with lower pregnancy per AI and fewer cows pregnant by 150 days in milk, while higher neutral detergent fiber (NDF) was positively associated with fertility at first service. Crude protein and rumen-degradable protein showed no detectable effect on reproduction, but dietary fat was positively correlated with first-service pregnancy. These findings suggest that managing NFC and NDF levels in early-lactation diets may be a practical lever to improve reproductive outcomes in commercial dairy operations.

Full summary

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**Background:** Reproductive efficiency is a key driver of dairy herd profitability, yet the relationship between total mixed ration (TMR) composition and fertility in high-producing cows has not been systematically characterized. Previous work has focused on isolated nutritional manipulations (e.g., specific fatty acids, methionine, acidogenic diets), but large-scale, multi-herd analyses linking routine TMR components to reproductive outcomes are lacking. This study aimed to identify which macronutrient fractions in the TMR are associated with reproductive performance across well-managed commercial dairy farms.

**Methods:** Data were collected from 48 commercial dairy farms in Wisconsin, all with >100 lactating Holstein cows (range 143–2,717) housed in freestall facilities. Nutritional information was provided by each farm's nutrition consultant and included complete dietary ingredient lists and nutrient compositions for the high-production cow pens (post 21–30 DIM), covering the main breeding period from the end of the voluntary waiting period through ~150 DIM. A total of 64 diet ingredients were identified. Mean values for each farm were calculated for CP, RDP, RUP, NDF, NFC, starch, and fat (all as % of DM). Reproductive data were extracted from Dairy Comp 305 or PCDART software by a single technician, excluding “do not breed” cows. Key reproductive measures included percentage of TAI use, service rate (SR), overall P/AI, P/AI at first service, 21-d pregnancy rate (PR), days open (DOPN), and percentage of cows pregnant by 150 DIM (PREG150). Statistical analyses used SAS 9.4. Correlations were tested with the CORR procedure, logistic regressions with GLIMMIX (Gaussian distribution, Kenward-Roger df adjustment). Tertile analyses were performed for NDF and NFC. Significance was set at P ≤ 0.05; tendencies at 0.05 < P ≤ 0.10.

**Key Results:** Average daily milk production was 38.9 ± 0.60 kg/d (range 30.0–50.4). The voluntary waiting period averaged 65 DIM (range 40–85). TAI use for first service averaged 80% (range 25–100). Overall P/AI was 36.1% (range 22–49); primiparous cows had 19% greater fertility than multiparous (40.4 vs. 34.0%). P/AI at first service averaged 39.7% (range 20–51). The 21-d PR averaged 20.3% (range 10–42), and PREG150 averaged 52% (range 30–75). Overall DOPN was 129 d (range 96–189).

There was no correlation between any dietary component and herd average milk production. The strongest and most consistent associations were with NFC: NFC was negatively correlated with P/AI at first service (r = −0.51; P = 0.0005), overall P/AI (r = −0.48; P = 0.001), and PREG150 (r = −0.33; P = 0.03). Starch showed a similar negative trend with P/AI at first service (r = −0.35; P = 0.05) but not with other measures. Conversely, NDF was positively correlated with P/AI at first service (r = 0.34; P = 0.03) and showed numerical but non-significant positive associations with overall P/AI (r = 0.25; P = 0.11) and PREG150 (r = 0.11; P = 0.48). Dietary fat was positively correlated with P/AI at first service (r = 0.34; P = 0.02). No significant associations were found for CP, RDP, or RUP with any reproductive measure. Tertile analysis confirmed these patterns: herds in the highest NFC tertile had P/AI at first service of approximately 33% versus 43% in the lowest tertile (>20% relative reduction).

**Clinical Implications:** This herd-level analysis suggests that high dietary NFC in early lactation is associated with substantially reduced fertility, while higher NDF may be beneficial. The negative NFC effect may be mediated through mechanisms such as increased insulin disrupting oocyte and embryo quality, reduced DMI, upregulation of inflammatory pathways, or subacute ruminal acidosis. The positive fat association may reflect both its energy contribution when NFC is reduced and direct beneficial effects of unsaturated fatty acids on reproductive tissues. The lack of protein effects likely reflects adequate formulation in modern diets. Importantly, the study's observational design precludes causal inference; numerous unmeasured confounders (e.g., cow comfort, stocking density, BCS dynamics, forage quality, feeding consistency) could influence results. Nonetheless, these findings provide a strong rationale for controlled experimental studies testing whether reducing NFC and increasing NDF (while maintaining energy density through fat) can improve reproductive performance without compromising milk yield.