Effect of early exposure to mixed rations differing in forage particle size on feed sorting of dairy calves.

Feed sorting of dairy cattle is influenced by dietary forage particle size. The objective of this study was to determine the effect of early exposure to rations differing in forage particle size on development of feed sorting in dairy calves. Twenty Holstein bull calves were exposed for 8 wk to 1 of 2 mixed rations containing (on a dry-matter basis) 90% crumb starter concentrate and either (1) 10% coarsely chopped (3- to 4-cm) grass hay (CRS; n=10) or (2) 10% finely ground (2-mm) grass hay (FN; n=10), both offered ad libitum. Calves received 8L of milk replacer/d (1.2 kg of dry matter/d), with the amount progressively reduced after 5 wk, to facilitate weaning by the end of wk 7. At the beginning of wk 9, all calves received the CRS diet and were followed for 3 wk. Intake was recorded daily and calves were weighed twice per week. Samples of fresh feed and orts were taken on d 1 to 4 of wk 9 and 11 for analysis of feed sorting. Sorting of the ration was assessed through analysis of nutrient intake. Actual intake of each nutrient was expressed as a percentage of predicted intake of that nutrient, based on the concentration in the fresh sample. Daily dry matter intake (DMI) was similar between treatments after transition to the common CRS ration (3.20 kg/d, standard error=0.25 kg/d). However, feed efficiency was subject to a treatment-by-week interaction, with calves previously fed the FN diet having an initially greater gain-to-feed ratio than those fed the CRS diet [in wk 9, 0.60 vs. 0.47 kg of average daily gain (ADG)/kg of DMI] and similar feed efficiency in the following weeks (in wk 10, 0.43 vs. 0.43 kg of ADG/kg of DMI). A corresponding tendency was observed for ADG and body weight to evolve differently, depending on treatment, with calves previously fed the FN diet having greater ADG initially (in wk 9, 1.60 vs. 1.32 kg/d) but similar ADG to those fed the CRS diet in the following weeks (in wk 10, 1.39 vs. 1.33 kg/d and in wk 11, 1.32 vs. 1.31 kg/d). Calves previously fed the FN diet consumed less neutral detergent fiber as a percentage of predicted intake and tended to consume less acid detergent fiber and more nonfiber carbohydrates, as a percentage of predicted intakes, than calves previously fed the CRS diet. Given the nutrient compositions of hay and concentrate, this indicates that calves previously fed the FN diet were sorting for concentrate. These results indicate that the pattern and extent of feed sorting may be affected by early experience with rations differing in forage particle size.