Discipline: nutrition/feeding; Keywords: Digestion, energy nutrients, metabolism, milk production, reproduction.
The energy burden on the high-yielding dairy cow is phenomenal. As the diet needs to be a concentrate-roughage mixed diet because of the energy requirement on the one hand and effective rumen function on the other, it becomes a fine balancing act in diet formulation. The roughage component often delays outflow from the rumen which results in the intake of the cow not attaining the level necessary to meet her requirements. Apart from lower than expected milk yield, the resulting energy deficit may impair the next conception and metabolic diseases such as ketosis may occur. To overcome the intake-induced energy deficit without decreasing the roughage component, researchers have evaluated starch and lipid (fat) addition to the diet. This has been met with variable success and therefore the authors cited below have decided to review the present knowledge on the subject. The review considers the effects of starch and fat before and after calving on metabolism, energy balance, milk production and reproduction.
The transition of the cow from a pregnant non-lactating state to a non-pregnant lactating state induces physiological changes, which alters the metabolic and endocrine (hormonal) status to redirect body energy towards the udder to manufacture milk. Overfeeding high starch and fat levels during the dry period after calving may result in cows failing to adapt to the negative energy balance because of major liver and rumen dysfunction. Alternatively, keeping dry cows on high-forage/low-energy diets adjusts dry matter intake to optimize rumen function and decrease the severity of the negative energy balance during transition. These pre-calving biological improvements in cows have real benefits such as fewer post-calving health complications (e.g. milk fever, ketosis, mastitis, metritis), decreased body condition loss and improved reproductive outcomes in the subsequent lactation.
By adding dietary starch and/or fat to the diet after calving will increase milk yield. In addition, milk protein will increase with glucogenic (starch) diets, but will decrease with lipogenic (fat added) diets. Conversely, milk fat will usually increase after feeding lipogenic diets, but will decrease with glucogenic diets. Glucogenic and lipogenic nutrients can affect the cow’s metabolism and its energy balance status positively, as observed with changes to blood non-esterified fatty acids (NEFA), β-hydroxybutyrate (BHB), glucose, amino acids, insulin, insulin-like growth factor-I (IGF-I), growth hormone (GH), gonadotropin hormones and progesterone (P4) levels. These metabolites (NEFA, BHB, glucose, amino acids) and hormones (insulin, IGF-I, GH, P4) will affect follicle development, ovulation, conception and pregnancy success. On the negative side, feeding a starch-based diet to the lactating cow can result in acidosis, but will increase glucose and insulin levels, while decreasing NEFA and BHB levels. Furthermore, a diet which increases insulin will favour an early resumption of ovarian activity, but may have adverse effects on the quality of the oocytes. In contrast, keeping dairy cows on a fat-based diet will elevate NEFA and BHB levels and decrease glucose and insulin levels. Additionally, a lipogenic diet will increase blood P4 levels and improve the quality of the oocytes.
These findings suggest that reproductive performances in dairy cows can be enhanced by feeding an insulin promoting diet until the resumption of the ovarian cycle and then switching to a lipogenic diet from the mating period onwards. However, since long-term field studies on fertility are limited and the reproduction process in dairy cows is multi-factorial, further research is needed on the pre- and post-calving effects of starch and fat, as well as their combination, on reproduction parameters and thus to draw conclusions on reproductive performance.
B. A. Useni, C. J. C. Muller & C. W. Cruywagen, 2018. Pre- and postpartum effects of starch and fat in dairy cows: A review. S. Afr. J. Anim. Sci. 48 (3), 413-426.