Graduation Research Allard Levers

This graduation thesis examined the relationship between the level of starch and different feed ingredients in combination with protein efficiency and urea excretion in dairy cattle. Increasing societal and political pressure to further reduce nitrogen emissions and ammonia provided the motivation for carrying out this study.

The data used in this research were obtained from 108 dairy farms in the Netherlands and Belgium. Ration analyses and milk production data were collected between February and May 2025. The average protein efficiency in the dataset was 31.8%, and the average milk urea level was 19.6 mg/l.

The study showed that starch is not only an important energy source but also enhances protein utilization through biochemical mechanisms in the rumen and the mammary gland. Starch supplementation can positively influence the mammary gland by activating the regulatory mechanism mTOR (mechanistic Target Of Rapamycin). Conversely, shortages of energy—and thus starch—can activate the Integrated Stress Response (ISR), which negatively affects protein synthesis (Toerien et al., 2010).

Data analysis revealed that rations with a high proportion of starch are positively associated with protein efficiency (P < 0.01). When corrected for crude protein content in the ration, this relationship explained 55% of the variance.

Analysis of individual feed ingredients showed that maize silage had a positive relationship with protein efficiency (P < 0.05), while barley and wheat (after correction) showed a negative effect. No significant relationship was found for CCM or MKS and soda grain. With regard to milk urea levels, the proportions of CCM, MKS, and barley in the ration showed a clear reducing effect.

The effects on protein efficiency and milk urea can be explained by glucose supply derived from starch. Maize silage provides a relatively stable glucose supply, whereas barley and wheat contain starch that ferments more rapidly, potentially leading to greater protein losses.

Recommendations to improve protein efficiency include optimizing the ratio of crude protein to starch in the ration, prioritizing maize silage, and exercising caution with high inclusion levels of rapidly fermentable grains. To reduce milk urea levels, it is important to consider not only starch but also crude protein and all other energy-rich nutrients. For future research, it is recommended to analyze data at the individual cow level and to investigate the effects of different starch sources on the mTOR and ISR signaling pathways in mammary gland metabolism.