Effects of dietary starch and fatty acid supplementation on milk production and metabolic responses during the immediate postpartum period in dairy cows.

Dairy cows experience negative energy balance (NEB) during the immediate post-calving period due to reduced dry matter intake (DMI) and increased energy demands for milk production. As a result of NEB, cows compensate by mobilizing body energy reserves by increasing the flow of non-esterified fatty acids (NEFA) to the liver, peripheral tissues, and mammary gland for milk fat synthesis. However, excessive adipose tissue mobilization may impair production and reproduction performance, elevate the risk of developing metabolic disorders, and compromise immune function. The strategy then often is to increase starch in the diet, but the response is often a reduction in milk fat. If fatty acids (FA) are increased, milk fat may remain intact, but DMI and energy metabolism may be adversely affected. However, recent results suggest that this may depend on FA composition. The question then arises what will happen if both starch and FA with specific FAs are supplemented. In the study cited, therefore, the objective was to determine the effects of dietary starch and FA supplementation on milk production of early-lactation. It was hypothesized: (1) that increasing dietary starch in the immediate postpartum period would increase milk yield and reduce milk fat content, and (2) that FA supplementation would increase milk fat yield and milk fat content, thereby mitigating any reduction in milk fat with the higher dietary starch diet.

In the study, 60 multi-lactation Holstein cows were used in a randomized complete block design and assigned to one of four treatments in a 2 × 2 factorial arrangement. The following treatment diets were fed from day one to 23 post-calving (fresh period): (1) a diet containing 22% DM starch and no supplemental FA (LS); (2) a diet containing 28% DM starch and no supplemental FA (HS); (3) a diet containing 22% DM starch and a calcium salt containing 70% C16:0 and 20% cis-9 C18:1 (LS+FA); and (4) a diet containing 28% DM starch and a calcium salt containing 70% C16:0 and 20% cis-9 C18:1 (HS+FA). The starch concentration of the treatment diets was altered by partially replacing maize meal with soyhulls and soybean meal. The FA supplement was added at 2.6% of diet DM to deliver ~2.0% of FA and replaced soyhulls in the FA-supplemented diets. During the carryover period (24 to 44 days post-calving), all cows were offered a common diet (27.8% starch and 3.05% FA of DM) to evaluate the carryover effects of the treatment diets.

The high starch diet increased the yields of 3.5% FCM by 4.6 kg/day and milk fat by 0.17 kg/day compared with LS. The LS+FA treatment increased milk fat yield by 0.18 kg/day and tended to increase 3.5% FCM yield by 3.2 kg/day compared with LS. Dietary starch and FA supplementation interacted with time to affect DMI, plasma β-hydroxibutyrate (BHB), and tended to interact to affect the energy-corrected milk (ECM) yield and milk lactose content. The starch × FA interaction affected the yields of milk, milk fat, milk lactose, 3.5% fat-corrected milk (FCM), ECM, and tended to affect milk protein yield and feed efficiency (FCM/DMI). Based on pairwise comparisons among treatment combinations, HS increased milk and lactose yields compared with the other treatments. The HS and LS+FA diets increased milk fat yield compared with LS, and HS increased 3.5% FCM and ECM yields compared with LS, whereas LS+FA tended to increase 3.5% FCM yield compared with LS.

In conclusion: The lack of a synergistic effect between starch and FA supplementation likely reflects reduced DMI observed with FA supplementation at 2.0% of diet DM and potential adverse effects on fibre digestibility and rumen bio-hydrogenation when a high-starch diet and a FA supplement were combined. Importantly, neither high starch nor FA supplementation exacerbated body weight loss. Overall, increasing dietary starch increased milk and milk fat yields, whereas FA supplementation improved milk fat yield only under low-starch conditions.