Sweeter grass does not lead to more milk
Contrary to general expectations, the characteristics of different varieties of perennial ryegrass such as sugar content do not influence the feed intake of grazing dairy cows. Moreover cell wall degradability characteristics were not different among perennial ryegrass varieties. Research carried out by the Palestinian researcher Hassan Z. H. Taweel at Wageningen University, Netherlands, shows that an increased dry matter intake can be achieved by gaining more insight into the regulatory mechanisms behind the maximum use of rumen capacity. Taweel defended his doctoral thesis Perennial Ryegrass for Dairy Cows: Grazing Behaviour, Intake, Rumen Function and Performance at Wageningen University.
In highly productive grazing dairy stock, dry matter intake (and consequently protein and energy intake) is a limiting factor in milk production. To augment milk production, grazing dairy cattle are therefore generally given supplements of feed concentrates and energy-rich maize silage.
In his study in Wageningen, researcher Taweel examined eating motivation as well as the capacity and rate of digestion in the rumen of dairy cattle. The first hypothesis was that eating motivation is strongly related to taste, and that tastiness is primarily determined by sugar, i.e., water-soluble carbohydrates (WSC) content. The second hypothesis was that plants with more easily digestible cell walls pass through the rumen faster, and the expectation was that this increased the processing capacity and hence grass intake.
Taweel carried out experiments feeding various ryegrass varieties to dairy cows under stall-feeding and grazing conditions. in each experiment, a single variety of ryegrass was available ad libitum for periods of two weeks. The varieties were all commercially available and selected for their varying WSC content. The researcher also examined differences in cell wall degradability of the ryegrass varieties.
No significant differences in grass intake or milk yield due to different grass varieties were observed. The experiment concluded that WSC levels did not affect the voluntary feed intake of the animals, and that relative differences in degradability of cell walls were very small and offered few perspectives to improve intake. Varieties with a high sugar content did result in reduced ammonia content in the rumen and lower urea levels in milk, which is interesting for N-surplus reduction issues in dairy farms. Recent research at Wageningen University (in partnership with NIZO food research) also showed that cows eating fresh grass produce milk with a healthier composition of fatty acids than cows that are fed on silage.
Follow-up research by Taweel into grazing periods during the day and rumen capacity also yielded groundbreaking results. Grazing cows feed during three main periods in a day - early in the morning, around midday and at sunset - during each of which they continuously graze for more than an hour. Interestingly, however, the evening grazing went on for a much longer time than the others. This led Taweel to study rumen fill, which is considered one of the factors that end a grazing period. He found that the rumen was entirely full only at midnight. This implied that, at other moments during the day, cows stopped grazing long before reaching maximum rumen capacity.
Further research will be necessary to find out the reason for this behaviour. If cows would use their full maximum potential of rumen fill during the morning and afternoon grazing periods, this could result in higher intake and therefore absorption of energy from fresh grass, and, hence, in increased milk production.
Jac Niessen | alfa
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