As part of an investigation of factors responsible for a previously reported lower efficiency of carcass lean gain in steers offered grass silage diets, 16 Simmental × Friesian steers (515 (s.e. 6·4) kg) were offered perennial ryegrass silage ad libitum (C0) or silage plus rolled barley at 200 (C20), 400 (C40) or 600 (C60) g/kg total diet dry matter (DM). Barley-supplemented diets were intake-restricted to provide equal DM and metabolizable energy (ME) intakes to those offered C0. Eight steers were selected at random to determine the ME contents of the diets by open-circuit respiration calorimetry. The other eight steers were offered the same diets and were blood-sampled at 20- to 60-min intervals, for 10 h, to monitor changes in the concentrations of a number of nutritionally related plasma metabolites and hormones. Estimated ME intakes in these steers were 85·7, 83·1, 84·4 and 86·2 (s.e. 0·91) MJ/day from diets C0, C20, C40 and C60 respectively. Rumen-fistulated Hereford × Friesian steers provided 24-h rumen data for the same diets offered at equal amounts of ME per kg metabolic live weight.Mean 24-h plasma concentrations of insulin and insulin-like growth factor 1 (IGF-1) were linearly and positively related (P < 0·01 and P < 0·001 respectively) and glucagon quadratically related (P < 0·05) to the proportion of barley in the diet. Plasma insulin increased after feeding on all diets but concentrations on diets C40 and C60 were significantly higher than those on C0 and C20 at all post-feeding sampling times up to 9 h after feeding. Plasma IGF-1 concentrations increased above pre-feeding levels following feeding of the higher barley diets (C40 and C60; P = 0·053) but remained unchanged in steers offered C0 and C20. Mean plasma concentrations of glucose were unaffected by diet but those of β-hydroxybutyrate (BOHB) and urea were positively and negatively related respectively (both P < 0·001) to the proportion of barley in the diet. Plasma BOHB and urea concentrations also changed with time after feeding (P < 0·001). Amongst the rumen parameters measured (pH; ammonia and volatile fatty acid concentrations and proportions) only the mean 24-h concentrations and proportions of butyrate were positively related to the proportion of barley in the diet (P = 0·051 and P < 0·05 respectively). All rumen parameters were affected by time after feeding (acetate, P < 0·01; others, P < 0·001) but there was no interaction between treatment and time for any parameter.

In an investigation of factors responsible for the lower efficiency of carcass lean gain seen previously in steers offered grass silage diets 18 Simmental × British Friesian steers (361 (s.e. 5-35) kg) were offered one of three diets: a perennial ryegrass silage ad libitum and alone (S) or supplemented with rolled barley at 300 g/kg of total dry matter (SC) or supplemented as described but restricted (SCr) in quantity so as to provide the same dry matter (DM) and digestible energy (DE) intakes as for S. Eating (Eb), ruminating (Rb), standing (Sb) or lying (Lb) behaviour was quantified during four 24-h periods of manual observation. Eb was noted in more detail in a second experiment using computerized Calan-Broadbent gates and load cells to monitor times and rates of eating. Blood was taken via temporary indwelling jugular catheters at 30 to 60 min intervals on each of 4 days 1 month apart. Rumen fluid was sampled hourly for three 24-h periods from three rumen-cannulated steers given the same three diets in a separate 3 x 3 change-over design experiment.Steers offered the restricted diet SCr ate most of their food in one extended meal within 6 h of feeding while two peak eating periods (morning and evening) were observed in steers offered the other two diets. Steers offered SCr spent more time in Sb (P < 0.05), and less time in Eb (P < 0·001) and Rb (P < 0·05) activities than did animals offered the two diets ad libitum (SC and S). Mean 24 h insulin-like growth factor-1 (IGF-1) concentrations and postprandial insulin concentrations were significantly higher with diet SCr than with diet S (P < 0·001) despite equal daily DM and DE intakes from each. Insulin appearance in the jugular vein reflected the pattern of food intake on all treatments. Rumen fermentation characteristics were largely unaffected by diet. Mean 24 h rumen volatile fatty acid, pH and ammonia concentrations did not differ between diets but post-prandial rumen pH tended to be lower in animals offered the SC and SCr diets.Differences in patterns of food intake between animals offered food ad libitum and at a restricted level are likely to determine patterns of nutrient absorption and the secretion of nutritionally regulated splanchnic hormones. The higher proportions of Sb and Rb activities in steers offered the restricted diet represent an energy cost to these animals while the higher plasma IGF-1 and insulin concentrations also seen in these animals may collectively influence the partitioning of nutrients to the peripheral tissues and contribute to the increased efficiency of carcass lean deposition previously shown in animals offered such diets.

Physically effective fiber is needed by dairy cattle to prevent ruminal acidosis. This study aimed to examine the effects of different sources of physically effective fiber on the populations of fibrolytic bacteria and methanogens. Five ruminally cannulated Holstein cows were each fed five diets differing in physically effective fiber sources over 15 weeks (21 days/period) in a Latin Square design: (1) 44.1% corn silage, (2) 34.0% corn silage plus 11.5% alfalfa hay, (3) 34.0% corn silage plus 5.1% wheat straw, (4) 36.1% corn silage plus 10.1% wheat straw, and (5) 34.0% corn silage plus 5.5% corn stover. The impact of the physically effective fiber sources on total bacteria and archaea were examined using denaturing gradient gel electrophoresis. Specific real-time PCR assays were used to quantify total bacteria, total archaea, the genus Butyrivibrio, Fibrobacter succinogenes, Ruminococcus albus, Ruminococcus flavefaciens and three uncultured rumen bacteria that were identified from adhering ruminal fractions in a previous study. No significant differences were observed among the different sources of physical effective fiber with respect to the microbial populations quantified. Any of the physically effective fiber sources may be fed to dairy cattle without negative impact on the ruminal microbial community.Keywords: denaturing gradient gel electrophoresis, physically effective fiber, real-time PCR, ruminal microbial community ImplicationsPhysically effective fiber is needed by high-producing dairy cattle for normal rumen function and milk production. This study showed that corn silage, alfalfa hay, corn stover and wheat straw, when used as physically effective fiber, did not result in differences in rumen microbial communities or abundance of known fibrolytic bacteria and total archaea. Our results corroborated the suitability of corn stover, which is widely and abundantly available and inexpensive, as a source of physically effective fiber.

Studies have shown that reducing energy intake by restricting dry matter intake (DMI) at a constant forage: concentrate (F:C) ratio is more effective at increasing carcass lean and reducing carcass fat content than is reducing energy intake by increasing the F:C ratio (1). Research at this Institute has also shown that, per megajoule of digestible energy (DE), diets restricted in this manner are 45 % more efficient at producing carcass lean.The mechanism by which these methods of restriction result in these differences in carcass composition has previously been assumed to be that of rumen fermentation. More recently however, the significance of rumen fermentation in controlling the carcass composition of beef cattle has been questioned (2,3).The aim of this experiment was to examine the effect of decreasing the F:C ratio, at constant DE and DMI, on both rumen and endocrinological parameters, in particular the hormones insulin and insulin like growth factor-1 (IGF-1), in finishing beef steers.

Steen (1992) demonstrated that grass silage diets sustained lower carcass lean gain per MJ digestible energy (DE) intake than diets of low forage: concentrate ratios (F:CR), at equal levels of DE intake. Jugular concentrations of IGF-1, insulin and glucagon were significantly elevated in animals offered the latter diets while rumen VFA were unaffected (Thorp et al1994). Both IGF-1 and insulin stimulate protein synthesis (Abdul-Razzaq and Bickerstaffe 1989, Douglas et al1991) and it is generally accepted that insulin and glucagon influence carbohydrate metabolism in the ruminant. Plasma propionate and butyrate stimulate insulin and glucagon secretion (Harmon 1991). This experiment was part of a series designed to investigate the control of lean and fat deposition in beef cattle and aimed to determine concentrations of IGF-1, insulin, glucagon and VFA appearing across the splanchnic tissues.