Determining the Minimum Fibre Requirement for Feedlot Cattle

Background

High grain diets improve feed conversion efficiency but contribute to abnormal behavior (eating bedding and fences), acidosis and liver abscesses, which are offset with antibiotics. A better understanding of fibre requirements may help find ways to maintain efficiencies while reducing the health, welfare and antibiotic use concerns. Research has not defined whether these issues are related to absolute fibre levels, forage:concentrate ratio, or specific fibre fraction(s). Fibre requirements may also vary with digestibility, fibre particle size, starch source, processing, etc.

Objectives

• To evaluate criteria that may affect the fibre requirement for feedlot cattle, and improve the ability to predict ruminal pH in feedlot cattle using dietary parameters.

What They Did

Four finishing diets were fed once daily in a small pen study (360 crossbred yearling steers, 15 head per pen, six pens per diet). Each diet contained 90% concentrate (primarily barley, dry-rolled to a 66% processing index). The 10% roughage portion used different silage crops (AC Ranger barley vs. CDC Landmark wheat) and chop lengths (half vs. three quarter inch) to produce different physically effective fiber levels, but all other nutrient levels were identical among the four diets. Steers were finished for 100 days, then slaughtered, and carcass data was collected. 

What They Learned

Silage crop, chop length and their combination did not have any meaningful impacts on feed intake, growth rate or feed:gain. Some interesting differences showed up in the carcass traits, though. 

Silage source mattered. Barley silage produced significantly heavier carcass weights (827 vs. 818 lbs) but slightly more livers with minor abscesses (17 vs. 12%) than wheat silage. 

Physically effective fiber mattered. Long chop lengths (higher physically effective fiber) produced significantly higher dressing percentages (59.4 vs. 59.0%), more AAA grades (78 vs. 68%) and fewer AA grades (22 vs. 32%) than short chop lengths (lower physically effective fiber), regardless of whether it was barley or wheat silage. 

Physically effective fiber was particularly important with wheat silage. Long chop length (both wheat or barley silage) and short chop length barley silage produced significantly fatter carcasses (55 vs. 63% yield grade 2 and 33 vs. 22% yield grade 3) and fewer severe liver abscesses (21 vs. 29%) than short chop length wheat silage. 

What It Means

Barley silage photos provided by Tara Mulhern Davidson 

Silage is more than an inconvenient dietary necessity in finishing rations. Silage is a small proportion of the finishing diet, but subtle differences may have real impacts. In this case, the higher physically effective fiber level didn’t impair animal performance, but it did improve carcass grades for both barley and wheat silage. 

AC Ranger barley silage outperformed CDC Landmark wheat silage in this study, but that doesn’t mean that any barley silage is better than all wheat silage. There’s probably as much variation between varieties of the same crop as between different crops. The economic advantage of wheat silage, particularly with respect to lodging and its wider harvest window, may also outweigh the feeding benefits of barley silage in some situations. 

Good timing, good weather, good luck and good technique likely have more impact on silage quality than the crop or chop length used. But if conditions allow cereal silage to be harvested at the optimal 30-40% dry matter, consider using a longer chop length due to the grading benefits. In contrast, if silage is too dry at harvest time, longer chop lengths will make it harder to pack and will reduce silage quality. If silage is harvested late, it’s better to chop more finely. More mature crops will have higher fiber levels, and that will also make for more physically effective fiber.