A.M. van Vuuren

Influence of declining level of feed intake and varying the proportion of starch in the concentrate on rumen ingesta quantity, composition and kinetics of ingesta turnover in dairy cows

Five dairy cows fitted with large-diameter rumen cannulae were fed diets of two-thirds pelleted concentrate that contained 5 proportions of starch (8, 14, 20, 26 or 32%). Measurements were made over one complete lactation to determine the effects of intake level and proportion of starch in the concentrate on rumen content of soluble and dry matter (DM) components as well as the kinetics of rumen neutral detergent (ND) fibre digestion and passage. Intake declined from 19.44 to 4.86 kg organic matter (OM) per day at equal intervals. Rumen content of total ingesta, non-DM ingesta and all measured components of DM ingesta declined (P <0.05) as level of intake declined, although the form of the reductions differed. Rumen pool sizes of ammonia and all volatile fatty acids (VFAs) either did (P <0.05), or tended to (P <0.10), decline as level of intake declined. Rumen passage rate of ND fibre declined linearly (P <0.05), but tended (P <0.10) to decline at an increasing rate as level of intake declined. Rumen rate of ND fibre digestion increased linearly (P <0.05), but tended (P <0.10) to increase at a decreasing rate as intake level declined. Rate of digestion of ND fibre in the rumen was closely related to rumen pH and rumen bacterial OM (RBOM) as a proportion of rumen ND fibre (RNDF); rate of digestion of ND fibre increasing as pH and RBOM/RNDF increased.

Influence of declining level of feed intake and varying the proportion of starch in the concentrate on rumen fermentation in dairy cows

Five dairy cows fitted with large rumen cannulae were fed diets of 23 pelleted concentrate containing five graded levels of starch between 8 and 32%. Intake declined at five equal intervals from 19.4 kg organic matter per day in early lactation to 4.9 kg in the dry period before parturition. Measurements were made on average values and range during the day for pH and concentrations of ammonia (NH3), D,L-lactic acid (HL), total volatile fatty acids (t-VFA), acetic acid (HAc), propionic acid (HP), butyric acid (HB), and the minor VFAs valeric (HV), caproic (HC), isobutyric (IHB), 2-methylbutyric (2MHB) and 3-methylbutyric (3MHB). Decreased level of intake significantly (P<0.05) increased average pH (from 6.0 at the highest to 6.6 at the lowest level of intake) and decreased average concentrations of HL and of t-VFA (from 131 to 93 mmol L−1). Of the minor VFAs only HV and HC significantly decreased. Range during the day was decreased for pH (from 1.07 to 0.5) and for the concentrations of NH3, HL, t-VFA (from 61 to 35 mmol L−1), HV and HC. Decreasing the level of starch significantly (P<0.05) decreased concentrations of HP, HV, IHB, and 3MHB. Range during the day was decreased for pH and HB and increased for the concentrations of 2MHB and 3MHB. It is concluded that changes in level of feed intake are quantitatively more important in rumen fermentation than increasing levels of starch in the concentrate.

Effect of the level of feed intake on nitrogen entering the small intestine of dairy cows

The effect of the level of feed intake on the quantity of protein entering the small intestine was studied in re-entrant cannulated dairy cows fed mixed diets with varying N contents. The degradation of dietary protein in the forestomachs was estimated by two methods, one based on the use of diaminopimelic acid (DAPA) as a microbial marker, the other based on regression techniques. At the higher level of intake, the flow of N into the small intestine as a proportion of the N ingested was higher than at the lower level of intake. This difference could be explained by a decreased degradation of dietary N at the higher level of feed intake. This was thought to be the result of an increased rate of passage of digesta through the forestomachs at a higher level of feed intake.

Influence of declining level of feed intake and varying the proportion of starch in the concentrate on milk production and whole tract digestibility in dairy cows

Five dairy cows fitted with a large diameter rumen cannula were fed diets of two-thirds pelleted concentrate that contained 5 graded levels of starch from 8 to 32%. Measurements were made over one lactation to determine the effects of diet intake level and proportion of starch in the concentrate on milk production and whole tract digestibility. Intake declined from 19.44 to 4.86 kg organic matter (OM) per day at equal intervals. Milk production declined linearly (P <0.05 at about 0.7 kg week−1, from 25.4 kg day−1, as level of feed intake declined. Fat production declined linearly (P <0.05) as the proportion of starch in the concentrate increased, reflecting a linear (P <0.05) decline in milk fat proportion. The apparent digestion of all dietary components, except starch, increased (P <0.05) as the level of intake declined, although the form of the change differed. The extent of the increase in neutral detergent (ND) fibre digestion was large, almost 14 percentage units. The apparent digestion of OM and nitrogen (N) increased linearly (P <0.05) as the proportion of starch in the concentrate increased. The estimated true digestibility of N and non-ND fibre OM declined at an increasing rate (P <0.05) as level of intake increased and the proportion of starch in the concentrate increased. However, the extent of change for OM was not large in either case (6 and 5 percentage units respectively).

Effect of reducing nitrogen fertilizer on grassland on grass intake, digestibility and milk production of dairy cows

To quantify the effect of a reduction in nitrogen fertilizer on grass intake and animal performance, four zero-grazing experiments were carried out, two in spring/early summer and two in late summer. Grass was fertilized at three levels of N fertilizer, 450, 300 and 150 kg/ha per year and harvested daily at dry matter yields between 1500 and 2000 kg/ha. Grass was fed ad libitum to three groups of 12 dairy cows in mid lactation. Reducing fertilizer N decreased crude protein content and in-vitro digestibility, but increased sugar content in grass. Overall, in the spring experiments, a reduction in N fertilization from 450 to 150 kg/ha per year did not affect grass intake. In one of the experiments carried out in spring, net energy intake of cows offered 150N grass was lower, resulting in lower milk yields. In late summer, cows consumed less 150N grass and produced less yields of milk, fat and protein compared to the other treatments. Except for milk production differences in S92, a reduction of N fertilizer from 450 to 300 kg/ha per year did not affect intake or milk production.

Energy metabolism of dairy cows fed on grass

Production performance of grass-fed dairy cows is often lower than expected from the estimated energy supply. To explain the overestimation of the energy content of grass for dairy cows, data from energy balance trials from three different laboratories (Wageningen, Lelystad and Hillsborough) were collected. The trials in Wageningen and Lelystad were carried out in the 1970s and those in Hillsborough in the 1990s. Regression analyses were carried out with the complete data set as well as per laboratory to identify differences per laboratory. Average net maintenance requirements per kg3/4 (NEm) were 0.573 MJ, whereas the efficiency of metabolizable energy utilization for lactation (k1) was 0.777. When NEm was fixed at the presently used value of 0.293 MJ/kg3/4, k1 was 0.60. Between-laboratory NEm varied between 0.294 (Lelystad) and 0.786 MJ/kg3/4 (Hillsborough), whereas k1 varied between 0.57 (Lelystad) and 0.84 (Hillsborough). For Wageningen and Hillsborough, NEm was high, whilst k1 was also high. With the intercept fixed at 0.293 MJ/kg3/4, efficiency varied between 0.53 (Hillsborough) and 0.62 (Wageningen). The k1 and NEm are interrelated. Based on these data we surmise that the maintenance requirements for grass-fed dairy cows are 10% higher than presently assumed, with no change in k1.

Intestinal digestibility of rumen undegraded protein of formaldehyde-treated feedstuffs measured by mobile bag and in vitro technique

Samples of lupines, peas, rapeseed meal and field beans were ground to pass through a 1 mm screen, treated with 0, 10, 20, 30 and 40 g formaldehyde kg−1 crude protein (CP) and incubated in situ for 12 h in the rumen of four cows, surgically prepared with both rumen and duodenal (T-piece) cannulae. In the dried residues, CP digestibility was determined, using either the mobile bag technique or an enzymatic (pepsin-pancreatin) in vitro method. Treatment with 10 and 20 g formaldehyde kg−1 CP was effective in reducing rumen CP degradability of lupines, peas, rapeseed meal and field beans. In these samples the decline in rumen degradability caused by the formaldehyde treatment, was nearly completely compensated for by intestinal digestion. Intestinal and enzymatic CP digestibility of rapeseed meal treated with 30 and 40 g formaldehyde kg−1 CP was highly reduced. The mobile bag digestibility values were significantly (P < 0.001) correlated with the in vitro values, when the deviating values of formaldehyde-treated rapeseed meal were not taken into account.

Sources of variation of the in situ nylon bag technique

Samples of a totally mixed ration and of soybean hulls were incubated in nylon bags in the rumen of four cows. Incubation times were 3, 6, 12, 24, 48 and 280 h. The 3- to 48-h incubations and determination of solubility were repeated in seven periods, whereas 280-h incubation was repeated in three periods. From these incubations, rates of disappearance of dry matter (DM), nitrogen (N) and neutral detergent fibre (NDF) were calculated. Statistical analyses for the following variances were calculated: between cows (Vc), between periods within cows (Vp) and between bags within periods within cows (Vb) only for DM, disappearance at incubation times of 24, 48 and 280 h. In the soluble fractions, Vp was generally higher than Vb, which might be explained by the bag treatment not being sufficiently standardized before and after washing. For rumen incubations, Vc was smaller than Vp or Vp + b. In most cases, variances were smaller at longer incubation times, except for Vc at 24- and 48-h incubations. The obtained data give a basis for selecting the numbers of bags, periods and cows in nylon bag experiments.