No abstract

A trial was designed to study the relationships of dietary fiber and protein percentage and source to fecal consistency in lactating cattle. Thirty Holstein cows were assigned randomly to one of six TMR through four 21-d periods. The TMR were formulated to contain 17 or 25% ADF and CP of 15 or 22% with soybean meal supplementation or 22% with a combination of corn gluten and soybean meals. Two forage combinations were corn silage with or without alfalfa. Fecal consistency was evaluated using a four-point visual observation scale. Lower dietary fiber reduced fecal pH, score, NDF, and ADF but increased fecal DM and starch. A higher percentage of soybean meal lowered fecal DM and fecal score. Forage source affected fecal DM, NDF, ADF, and starch, but not pH or score. Prediction of fecal score from dietary components and cow parameters resulted in dietary DM percentage and 4% FCM as the most related variables. Accurate prediction of fecal consistency score from dietary and cow parameters was not possible.

Milk urea nitrogen (MUN) is correlated with N balance, N intake, and dietary N content, and thus is a good indicator of proper feeding management with respect to protein. It is commonly used to monitor feeding programs to achieve environmental goals; however, genetic diversity also exists among cows. It was hypothesized that phenotypic diversity among cows could bias feed management decisions when monitoring tools do not consider genetic diversity associated with MUN. The objective of the work was to evaluate the effect of cow and herd variation on MUN. Data from 2 previously published research trials and a field trial were subjected to multivariate regression analyses using a mixed model. Analyses of the research trial data showed that MUN concentrations could be predicted equally well from diet composition, milk yield, and milk components regardless of whether dry matter intake was included in the regression model. This indicated that cow and herd variation could be accurately estimated from field trial data when feed intake was not known. Milk urea N was correlated with dietary protein and neutral detergent fiber content, milk yield, milk protein content, and days in milk for both data sets. Cow was a highly significant determinant of MUN regardless of the data set used, and herd trended to significance for the field trial data. When all other variables were held constant, a percentage unit change in dietary protein concentration resulted in a 1.1mg/dL change in MUN. Least squares means estimates of MUN concentrations across herds ranged from a low of 13.6 mg/dL to a high of 17.3 mg/dL. If the observed MUN for the high herd were caused solely by high crude protein feeding, then the herd would have to reduce dietary protein to a concentration of 12.8% of dry matter to achieve a MUN concentration of 12 mg/dL, likely resulting in lost milk production. If the observed phenotypic variation is due to genetic differences among cows, genetic choices could result in herds that exceed target values for MUN when adhering to best management practices, which is consistent with the trend for differences in MUN among herds.

Twelve Holstein cows and 12 Jersey cows were used in six 4 x 4 Latin squares to investigate the effects of the degradability of dietary protein and supplemental dietary fat on milk components. Dietary dry matter contained 16% crude protein with two concentrations of ruminally undegradable protein (RUP) obtained by substituting blood meal for a portion of the soybean meal. Treatments were 1) 29% RUP, 0% added fat; 2) 29% RUP, 2.7% added fat (Ca soaps of fatty acids); 3) 41% RUP, 0% added fat; and 4) 41% RUP, 2.7% added fat. The dry matter of the total mixed ration fed at 1000 and 1400 h consisted of 30% corn silage, 29% alfalfa haylage, and 41% concentrate. Supplemental dietary fat depressed dry matter intake by 6.2%. Plasma urea N was greater at 0700 and 1600 h for Jerseys fed diets containing added fat and greater at 0700 h for Holsteins fed diets containing 41% RUP than for Holsteins fed 0% added fat and 29% RUP. When averaged across both breeds, milk production increased 7.1%, and production of 4% fat-corrected milk by Jerseys increased 8.4%, in response to added dietary fat. Milk protein was reduced when Holstein diets contained 41% RUP. Milk protein content was reduced 7.1 and 3.9%, and milk urea N was increased 4.9 and 8.5%, by added fat and 41% RUP in both breeds, respectively. Added fat reduced the concentration, but not the yield, of milk components. Substitution of blood meal decreased the concentration and yield of milk protein and casein N.

Lead factors are used in computerized ration formulation programs developed at Virginia Tech to increase milk production above a herd or group average for which total mixed rations are formulated for group feeding. These lead factors theoretically increase milk production for which the ration is formulated so 83% of the cows in a group will receive adequate or more than adequate nutrients from the ration. Two methods of calculating lead factors produced similar results. When test-day Dairy Herd Improvement records within a herd were not grouped by production, the average lead factor was 1.31 for one method [lead factor = (mean milk yield + one standard deviation)/mean milk yield] and 1.32 for the other [lead factor = (milk yield of 83rd percentile cow)/mean milk yield]. Grouping test-day records in each herd by milk production resulted in smaller lead factors for each group compared with lead factors for ungrouped herds. Changes of percentage of cows in each group in a herd resulted in different lead factors. Generally, groups with greater proportion of cows had larger lead factors. Season, herd size, and herd milk production slightly affected lead factors, but little improvement of estimation was gained by considering these variables.