Cows chew all day long

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Ruminants have the amazing ability to convert plant matter without nutritional value for humans to highly valued foods such as meat and milk. The vast majority of Norway’s farmland is only suitable for grass production. We are usually not aware of how dependent we are on the rumen, with its bacteria, protozoa and fungi all working hard to utilise nutrients in the ingested plant matter.

Grass that is harvested at an early stage of maturity contains considerably more energy and protein than grass harvested at a“normal” harvesting time. Such nutritious grass can result in a very high feed intake by ruminants; not only during grazing, but also during the indoor season. When animals are fed roughage that is both tasty and has a high nutrient content they can express their performance potential at a lower concentrate consumption than when fed roughage of a “poor” or “normal” quality. Previous studies indicated that finely chopped grass silage to (2-3 cm median particle length) increases the intake of “normal” quality silage. We thus wished to study how such chopping affects the intake of nutrient-rich silage that had been harvested much earlier than normal. We were slightly worried that finely chopped silage could even have adverse effects, since optimal rumen function depends on extensive chewing and the production of 100-200 litres of saliva daily.

Silage that was harvested very early (1-2 June 2005) or near the “normal” cutting date (16 June) was fed to the cows at three different chop lengths: long (17 cm), coarsely chopped (5.5. cm) or finely chopped (2.4 cm) and fed to the cows ad libitum. The cows were also fed a daily concentrate ration of 6 kg, mainly consisting of domestically grown ingredients (grains and rape). Six cows participated in the trial, and all of them were subjected to all feed regimes. Daily dry matter (DM) intake was 17.2 and 15.4 kg for the early and “normal” harvesting dates, respectively, and thus the effect of early cutting was 1.8 kg DM on average for all three silage chop lengths. On average for the two cutting dates, the cows ate 15.8, 16.2 and 17.1 kg DM for the long, medium and short chop lengths, respectively. There were no significant interactions between cutting date and silage chop length, which means that the effect of chopping was practically the same for young, nutrient-rich silage as for silage harvested at the “usual” time. Feeding unchopped silage harvested at the “normal” time resulted in a feed intake of 20 kg DM, whereas finely chopped, early cut silage resulted in a feed intake of 23.2 kg DM, a high level at a concentrate ration of merely 6 kg.

Cows given the “normal” long silage spent 7.3 hours per day eating (feed intake) and 9.3 hours ruminating. Thus, they spent 16.5 out of 24 hours chewing. Chewing is especially stimulated by the feed’s fibre contents (NDF). The time spent chewing pr kg NDF was about the same for grass harvested early (low NDF contents) as for grass with higher fibre contents (“normal” cutting date). However, chewing time per kg NDF decreased when the silage was finely chopped. Eating time per kg NDF was especially reduced, but also the time spent ruminating decreased slightly. Chopping the forage thus enabled the cows to eat more silage per unit of time, and although they spent somewhat less time eating, their total feed intake increased. More feed thus had to be ruminated, so the total time spent ruminating did not change in spite of a slight reduction of ruminating time per kg NDF.

The cow’s digestion was checked by collecting and analysing the feed, manure and urine. As expected, the percentage of feed that was digested was significantly higher when the cows were fed silage harvested early (79.1 % of DM) than when fed silage harvested at the “normal” time (73.3 % of DM). However, for both harvesting dates, there were no differences in digestibility between the three studied chop lengths. Chopping early-cut silage to 2 cm thus did not impair rumen function or feed digestibility. When the feed is well-preserved and silage quality is good the cows eat a lot, it seems that fine-chopping is purely beneficial. For poorly-fermented, less tasty silage, however, one must be aware of the risk of having too little structure in the rumen. Furthermore, the chop length should not be reduced below an average length of 2 cm. International literature indicates 2 cm as being the optimal length. Note that the effect of chop length on silage intake was much more pronounced for the 2.4 cm fraction than for the 5.5 cm chop length, see figure below.

Chop length and silage intake, kg DM

Lang - Long
Middels - Medium
Kort - Short
Tidlig høstetid - Early cutting date
Middels høstetid - Average cutting date

Cows have to chew a lot, and they actually spend most of the day doing so. However, by providing them with feed that takes less time to eat we can improve the utilisation of Norway’s grassland resources. In the winter of 2008, the project “More and better forage as a basis for Norwegian meat and milk production” will conduct a trials with dairy cows, in which we will study silage harvesting dates, chop lengths and two types of feed concentrates. The project is also conducting trials with early-cut silage as feed for bulls, sheep, lambs and dairy goats.

Published: 03.11.07
Updated: 14.05.09
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